Recent Advancement of CoagulationâFlocculation and Its Application

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Recent advancement of coagulation-flocculation and its application in wastewater treatment Chee Yang Teh, Pretty Mori Budiman, Katrina Pui Yee Shak, and Ta Yeong Wu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b04703 • Publication Date (Web): 04 Mar 2016 Downloaded from http://pubs.acs.org on March 5, 2016

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Industrial & Engineering Chemistry Research

Recent advancement of coagulation-flocculation and its application in wastewater treatment

Chee Yang Teha, Pretty Mori Budimana, Katrina Pui Yee Shaka, Ta Yeong Wu*,a

a

Chemical Engineering Discipline, School of Engineering, Monash University, Jalan Lagoon

Selatan, 47500 Bandar Sunway, Selangor Darul Ehsan, Malaysia.

*

Corresponding author: Ta Yeong Wu

E-mail addresses: [email protected]; [email protected] Tel: +60 3 55146258 Fax: +60 3 55146207

Abstract

Increasing environmental awareness coupled with more stringent regulation standards has triggered various industries to challenge themselves in seeking for appropriate wastewater treatment technologies. Coagulation-flocculation process is regarded as one of the most important and widely used treatment processes of industrial wastewaters due to its simplicity and effectiveness. This paper provides a critical review on recent studies of coagulation-flocculation treatment process of various industrial wastewaters. The limitations and challenges for coagulation-flocculation process such as the toxicity and health hazard possessed by inorganic coagulants, production of large amount of toxic sludge, ineffectiveness in removing heavy metals and emerging contaminants, increase in effluent color, inefficient pollutant removal using natural coagulants and complexity of scaling up procedure are presented. In addition, an overview on the

ACS Paragon Plus Environment


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influence of process parameters on treatment efficiency is included in this review. Finally, this review concludes with recommendations for improvements and new directions for this longestablished process.

Keywords: challenge; coagulant; coagulation-flocculation; flocculant; improvement; primary treatment


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1.

Introduction

One of the constant concerns in wastewater treatment facilities is to comply and cope with the new and changing regulations. With tougher standards, individual point source dischargers must use the best available technology to control pollution levels of the effluent which is discharged into the environment 1. In industries, highly variable and complex nature of the wastewaters result in biological or any other single stage treatment insufficient to meet regulation standards. For complete removal of target contaminants, a combination of several treatment methods is usually required, knowing that each method has its own advantages and disadvantages

2

. The

combination of processes such as adsorption, physicochemical oxidation, biological oxidation, stripping and membrane separation have been extensively studied in water and wastewater treatments to produce clean water, which is suited for reuse applications and safe disposal into the water streams 3.

Coagulation-flocculation process is one of the widely used methods for purification of urban and industrial wastewaters

4,5

. It was reported that the Egyptians were using aluminum

sulfate (alum) to cause suspended particles to settle in water as early as 1500 BC 6. Although the early Romans were also familiar with alum, it was only until 77 AD that its utilization as a coagulant in water treatment has been mentioned 7. Today, the coagulation-flocculation of water is implemented with the aim to agglomerate fine particles and colloids into larger particles for reducing turbidity, natural organic matter as well as other soluble organic and inorganic pollutants in the wastewater 6. This process is comprised of two distinct stages: (1) rapid mixing of dispersed coagulant into water/wastewater to be treated via violent agitation, and (2) flocculation for agglomeration of small particles into well-defined flocs via gentle agitation 8. Finally, the flocs are allowed to settle and then removed as sludge while the treated water/wastewater (supernatant) is transferred into subsequent treatment process or for discharge into watercourse.



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Due to its easy operation, relatively simple design and low energy consumption, coagulationflocculation has been successfully employed in different type of industries

9,10

. Moreover,

coagulation-flocculation can be used as a pre-treatment, post-treatment, or even as the main treatment of wastewater due to the versatility of the treatment process 11.

This review paper aimed to give an overview understanding of coagulation-flocculation process in wastewater treatment, from its fundamentals, to a summary of recent works using this process. As this review emphasized on the remediation of industrial wastewater, discussion on the treatment of natural waters into potable water will not be included in this work. Readers interested on coagulation-flocculation treatment to produce potable water are referred to some excellent reviews on this topic 12–14. Challenges and drawbacks of this treatment process together with possible improvements for enhancing the overall treatment efficiency were also discussed in this paper.

2.

Brief overview on coagulation and flocculation process: Destabilization mechanisms

Generally, coagulation and flocculation in water/wastewater treatment involves the addition of chemicals to alter the physical state of dissolved and suspended solids to facilitate its removal via subsequent sedimentation process

15

. Number of literatures used the term coagulation and

flocculation interchangeably and ambiguously. However, coagulation is generally defined as destabilization of suspension, giving rise to aggregations. On the other hand, flocculation describes the process in which the destabilized particles are induced to make contact for formation of larger aggregates 7,16.

In water, the behaviour of colloidal particles is strongly influenced by their electrokinetic charge. Each particle carries a like charge and nearly all colloidal impurities in water are


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negatively charged. For hydrophilic colloids, these primary charges are mainly contributed by polar groups such as carboxylic and amine group 17. Due to the size of colloidal particles (about 0.01 to 1µm), attractive body forces between particles are lesser than the repelling forces of the electrical charges 18. Thus, these particles tend to remain discrete and dispersed in the suspension. In order to remove colloidal particles by settling, it is practical only if they settle rapidly in the order of several hundreds of millimeters per hour 17.

[Insert Figure 1 here]

In a solution, the primary charge of the colloids attracts ions of the opposite charges, also known as counter-ions. These ions, held by electrostatic and van der Waals forces, will form a compact layer (Stern layer) around the primary charge as shown in Figure 1 18. The counter-ions attached to the surface will in turn attract their own counter-ions (the co-ions of the primary charge), forming the diffuse layer. Unlike the charges in Stern layer, only a part of the diffused layer will move along with colloid by shearing at the shear plane 17. The potential at this surface of shear is often called as zeta potential and is measured in wastewater treatment operations by means of a zeta-potential meter. The measured zeta potential gives a good approximation of the surface charge of the colloidal particle 4. For colloids in the water, zeta potential between -5 and 40 mV is usually acquired due to the presence of its charged groups 6.


The stability of colloidal particles can be explained in a quantitative manner by using the DLVO Theory (named after Derjaguin, Landau, Verwery and Overbeek). This theory involves the estimations of attraction energy (van der Waals forces) and the energy of repulsion (overlapping of electrical double layers) in terms of distance between particles as shown in Figure



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2 7. The net interaction energy, also known as the energy barrier, is the difference between the repulsion and attraction forces. Approaching particles would have to overcome this large energy barrier to come into contact. Under normal conditions, as the barrier height is usually much larger than average thermal energy of particles, it is almost impossible for colliding particles to surmount the barrier and the suspension will remain stable 16. In the case when the kinetic energy of the particles is large enough to overcome the energy barrier, the particles will coalesce 7.

Four mechanisms which can bring upon particle destabilization are double layer compression, charge neutralization, colloid entrapment and intraparticle bridging 17. Depending on their mode of action, the additive used for the process may be termed as coagulants or flocculants. In industrial applications, vast majority of the additives fall into one of the two categories, namely hydrolyzing metal coagulants or polymeric flocculants

16

. The coagulation

mechanism is determined by the coagulant selection, dosage, water/wastewater characteristics, as well as treatment techniques such as design of mixing devices

6,7

. Double-layer compression

mechanism involved the reduction in the double layer around the colloidal particle by the change in ionic strength induced from addition of an indifferent electrolyte, which resulted in destabilization of colloid 19. Under stable condition in which the concentration of counter-ions is low, colloid particles are unable to get close to each other because of their thick electric double layer

17

. However, as the concentration of counter-ions increases via the addition of salts, the

diffuse layer becomes thinner and particles can approach closer before experiencing repulsion. This effect is shown in Figure 2 when the repulsion curve compresses towards the left. van der Waals attraction may exceed the double layer repulsion due to additions of counter-ion, leading to coagulation of the particles

16,17

. It is important to note that the added ions retain their own

identities and does not adsorb to the particles. For example, monovalent counter-ions (Na+) compress the electrical double layer and reduce the electrophoretic mobility but do not reverse the charge of kaolinite particles 20.


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In industry, double layer compression is not a feasible coagulation mechanism to be used for wastewater treatment due to the massive amount of salt needed

18

. It is unlikely that a

sufficient increase in ionic strength would give a practical destabilization method. On the other hand, absorption of counter-ions on the surface of the particles is much promising. This destabilization method is called charge neutralization and are often achieved by adsorption of mononuclear and polynuclear metal hydrolysis species or polyelectrolytes on the surface of the particles 18. Hydrolysing coagulants are found to be effective in neutralizing the negative surface charge of many types of particles, including bacteria and clays

21

. The most commonly used

metal coagulants for this purpose can be categorized into aluminum coagulants (alum, aluminum chloride and sodium aluminate) and iron coagulants (ferric chloride, ferric sulfate, ferrous sulfate and ferric chloride sulfate)

22

. Furthermore, continuous studies on the aquatic chemistry and

behavior of simple Al salt leads to the development of highly efficient pre-polymerized inorganic coagulants

22,23

. Pre-polymerized coagulants such as polyaluminum chloride (PAC),

polyaluminum sulfate (PAS) and polyaluminum chloro-sulfate (PACS) have been used extensively worldwide during the last two decades

23

. These coagulants are able to function

efficiently over a wide range of pH, temperature and colloid concentration ranges as compared to the conventional coagulants 22,23. Due to the precise nature of charge-neutralizing species, charge reversal which can lead to restabilization will occur at high dosages 16. Hence, quantification of zeta potential is essential to monitor the degree of charge neutralization and to determine the optimum dosage required for a near-zero net charge of the particles 14.

For a practical water treatment operation, it is favorable to add metal coagulants at a dosage higher than the solubility of the amorphous hydroxide precipitates

16

. In this case,

particles may provide the nucleation site for precipitation to occur and this phenomenon leads to entrapment of the particle as the precipitate forms.

The settling hydrous precipitates then



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enmeshes with other colloidal particles in water and aids its removal 6,17. This method often leads to faster aggregation as compared to charge neutralization by giving larger and stronger flocs 16. One of the major advantages of colloid entrapment is its least dependence on the nature of the impurity particles to be removed, whether the particle is consisted of bacteria, clays, oxides or others. However, production of large amount of sludge remains a problem for this mechanism 16.

The next mechanism of coagulation is interparticle bridging, usually achieved by an addition of water soluble polymers (cationic, anionic or non-ionic). This mechanism requires the polymer chain to be adsorbed on the particle surface, either by chemical bonding or by mere physical attachment 17. Only several segments of the polymer chain are attached, while the bulk of the chain extends to the surrounding solution for contact and adherence with other particles 24. Bridged particles then intertwine with other bridged particles during mixing and produce flocs 18. Although the flocs are fast settling and have the ability to withstand high shear, they may not readily reform when the flocs are broken. Linear and high molecular weight polymer is found to be very effective coagulant in this mechanism as compared to the branched or cross-linked structures with the same molecular weight 16.

Supplementary to destabilization, flocculation is required as the next step to induce aggregation and settling of large agglomerates from destabilized particles. The overall process of flocculation is divided into two stages known as (1) perikinetic (microscale) and (2) orthokinetic (macroscale) flocculation. The first stage of flocculation occurs immediately after destabilization and throughout random thermal agitation of fluid molecules known as Brownian motion. In this case, the rate of flocculation is dependent on the temperature and concentration of particles as presented in the theory of Smoluchowski

25

. Normally, perikinetic flocculation occurs

automatically. The floc formed during perikinetic flocculation has poor settling characteristics as the process is limited by floc size 7 and only capable of removing minute particles.


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Unlike perikinetic flocculation, orthokinetic flocculation in the second stage induces contact of particles through bulk fluid motion (gentle motion of fluid). In general, the bulk movement can be prompted by mechanical agitation to induce velocity gradients in the liquid for improved contact between particles. Orthokinetic action can be applied using conventional jar test, under controlled shear in a Couette apparatus or capillary flow 26. The rate of flocculation based on orthokinetic action is dependent on the particle nature (i.e. size and concentration) and velocity shear gradient of the fluid

27

. Based on a general approximation, orthokinetic floc

formation is predominant in a system with particles larger than 1µm in diameter and velocity gradients higher than 5 s-1 7. This can be explained by the rate of flocculation represented in terms of the rate of reduction of particle number concentration as each effective collisions leads to the net loss of a particle 28:

−

dn 16 2 = αn Ga 3 dt 3

(1)

Equation 1 is based on the assumption of spherical particles, radius a, in a laminar shear field with a uniform shear rate G. In addition, the particle number concentration is expressed as n. Although not practical to be applied in real-time coagulation and flocculation processes, it outlines the important parameters in orthokinetic flocculation. Evidently, there is a strong dependence on particle size (i.e. rate of flocculation depends on the cube of particle radius), which explains the importance of orthokinetic flocculation for larger particles

28

. Initial rate of

floc formation through orthokinetic action is typically proportional to the velocity shear gradient. Although orthokinetic action promotes further aggregation to aid settling of flocs, there is a limit to the velocity gradient applied as the ultimate floc size becomes smaller due to continuous breakdown of larger flocs 7. High velocity gradients initiate breakage in large flocs due to internal tension and/or surface sheer erosion 27. Similarly, residence time during orthokinetic flocculation



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can affect floc growth. Agitation with lower velocities requires longer mixing time to obtain an optimum floc size. However, the final floc size tends to be larger in this case. Evidently, countless coagulation and flocculation studies have reported the investigation of agitation speed and mixing time to achieve optimum floc growth for effective settling.

Both stages of flocculation are important depending on the nature of the particles. For example, perikinetic flocculation is important in the removal of viruses as aggregation of viruses (small in size; 85%) in a closed recirculating shrimp

culture tank could be achieved by using appropriate dosage of chitosan (40-50 mg/L) under pH range of 7-9 49.

Oily wastewater produced by vegetable oil processing plant represents an impact to the environment if it is released to the watercourses without undergoing sufficient treatment. One of


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the examples of oily wastewater is palm oil mill effluent (POME). POME is a fresh thick brownish colloidal mixture of water, oil and fine TSS, produced from crude palm oil production 50

. It is estimated that for each tonne of crude palm oil produced, 5-7.5 tonne of water are required

and more than 50% of this water may end up as POME 51–53. POME possesses a very high BOD3 (100 times as polluting as domestic sewage) and causes considerable environmental problems if the effluent is discharged untreated

54,55

. In a coagulation-flocculation study on treatment of

POME, the performance of chitosan, alum and PAC were evaluated. Results showed that chitosan yielded more than 95% removal of oil and SS at a significant lower dosage (0.5 g/L) as compared to alum (8 g/L) or PAC (6 g/L)

56

. Besides chitosan, M. oleifera seed was also found to be a

suitable coagulant for pre-treatment process of POME as demonstrated by Bhatia et al. 57. It was found that almost all the TSS and oil/grease were removed from the POME through coagulationflocculation process 57. Recent studies also showed the potential of unmodified starches to replace alum in the treatment of POME 24. Starch is an interesting material to be used as natural coagulant as it is one of the most abundant natural polymers in the world. In its crude form, starch consists of a mixture of two polymers of anhydroglucose units, amylase and amylopectin 58. Teh et al.

59

showed that among the investigated starches, rice starch gave the highest percentage TSS removal from POME (83.6%), followed by wheat starch (64.3%), corn starch (42.7%) and potato starch (25.7%). The effective bridging mechanism by rice starch was attributed to its significantly higher amylopectin molecular weight (~26.8x108) as compared to other starches. Using the recommended treatment conditions of dosage, initial pH, settling time and slow stirring speed at 2 g/L, pH 3, 5 min and 10 rpm, respectively, the use of rice starch yielded similar TSS removal and higher TP removal as compared to treatment using alum. Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA) revealed that flocs produced from the investigated treatments exhibited certain characteristics similar to the coagulant used. As shown in Figure 4, flocs produced by alum were rough and porous structures while bridging flocculation by rice starch gave a more compact, denser and larger structure of the flocs 59.



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A new effluent treatment scheme consisting of coagulation and anaerobic digestion of coagulated sludge to treat POME was also been reported

60

. The sludge produced from the

coagulation process using alum with cationic polyelectrolyte was tested for its digestibility in anaerobic digester. The anaerobic biomass did not exhibit inhibition when subjected to varying alum dosage in coagulated palm oil sludge because the digester performance was in conformity with the regular treatment process 60.

Even now, a number of physical and chemical treatment methods have been reported in literature to treat wastewater such as olive mill wastewater, which lacked appropriate treating technologies

61

. Olive mill effluent has been subjected to coagulation-flocculation as well as

direct flocculation (i.e. without prior coagulation) using various cationic and anionic polyelectrolytes in contrast to conventional coagulants and flocculants (e.g. lime and ferric chloride) 62–64

. Most of these polyelectrolytes were found to be more effective in terms of organic matter

reduction with effective TSS reduction. In addition, lesser generation of sludge and effective treatment at its ambient pH (typically slightly acidic) were reported with the use of polyelectrolytes 63. However, Sarika et al.

63

found that cost evaluation associated to the use of

various coagulants and flocculants showed that treatment with polyelectrolytes was more expensive and highlighted the need for optimum combined use of polyelectrolytes and conventional coagulants or flocculants to improve process economics. In a study led by Ginos et al. 62, the optimized combine use of inexpensive conventional coagulants/flocculants (e.g. lime or ferrous sulphate) and cationic polyelectrolytes led to satisfactory results with reduced concentration of organics (i.e. 10-40% COD removal and 30-80% total phenolic content removal) and phytotoxicity. In addition, combined use of calcium hydroxide and PDADMAC showed


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reductions of 43% in total solids, 27% in TSS, 56% in COD and 76% in phenols after treatment of olive mill wastewater in another study 64.

3.3

Food processing industry wastewater

Food processing industry is one of the world largest potable water users (about 9% of the water and wastewater treatment market sales) 65. Thus, the generation of large amount of wastewater containing high BOD, COD, TSS, nitrogen, phosphorus oil and grease are usually inevitable. Food processing wastewater has distinctive characteristics, depending on the type of feedstock, processes and product. Generally, it is nontoxic in nature with low hazardous chemical content 65,66

.

One of the examples of food industry wastewater is molasses, a byproduct of sugar production characterized by its dark brown color and high organic load. Melanoidins, a dark brown pigment formed by Maillard reaction between amino compounds and carbohydrates, are responsible for its color. The chemistry of melanoidins resembles humic substances possessing high molecular weight nitrogenous polymers and negative charges due to the dissociation of functional groups 67. Liang et al. 67 studied the coagulation-flocculation process to treat two types of melanoidins-based molasses effluent with different COD concentrations using ferric chloride and alum. They found that ferric chloride gave a higher melanoidin removal as compared to alum due to the higher affinity of ferric ion to the reaction sites of melanoidins. Their experiment also showed that the functional groups of melanoidins-dominant organics were more favorably amenable to removal by iron salts as compared to alum salts. A stoichiometric relationship between the organic concentration of molasses and coagulant demand for ferric chloride was found as a change in solution conditions was proportional to the organic content. The optimum dosage of ferric chloride expressed in terms of metal to organic carbon removal was found to be



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0.73-0.81g Fe3+/g COD

67

. The settleability of the flocs could also be significantly enhanced

through addition of cationic polyacrylamide in conjunction with the use of ferric chloride 68.

A study also reported the ineffectiveness of anaerobic-aerobic process in the treatment of food industry wastewater and this was mainly due to the presence of cleaning products (disinfectants and sanitizers) in the wastewater

4

. Coagulation-flocculation process is

recommended to control high pollutants loads, followed by biological system in order to minimize the effect of elevated organic loading and cleaning products contained in the food industry wastewater. Pavón-Silva et al.

4

showed that the best coagulant to pre-treat the food

industry wastewater was aluminum hydroxychloride (with a dosage between 0.5-1 mg/L) and the combination processes between coagulation-flocculation and biological treatment enabled an average efficiency of COD removal up to 99% per month in wastewater treatment plant 4.

The treatability of winery wastewater (white and red wine) using coagulation-flocculation process was also been conducted

69

. Winery wastewater is usually generated from equipments,

bottles washing and purges from cooling process. This wastewater contains low concentration of solids but very high degradable organic matter 70. It was found that among all other coagulants investigated (ferric sulfate, ferric chloride, alum, calcium hydroxide), calcium hydroxide and alum was the most effective coagulant in treatment of white and red wine wastewater, respectively. However, the use of coagulant alone failed to remove COD effectively (~30%) as compared to turbidity, TSS and volatile suspended solids (VSS) removals (~90%). Hence, integration treatments using biological process, long term aerated storage (LTAS) and coagulation-flocculation process which enabled the removal of turbidity, CODTotal, CODSoluble, TSS and VSS up to 96.6, 84.5 87.9, 99.1 and 98.7%, respectively was suggested 69.


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Recently, the application of carbon nanotubes (CNT), ferric chloride, and its combination of CNT and ferric chloride in treating brewery wastewater containing toxic chemicals were assessed 71. Coagulation treatment using ferric chloride alone resulted in 83% turbidity removal, whereas CNTs coagulation alone resulted in 56% turbidity removal. CNT has been known to possess good adsorption properties for removing different types of organic and inorganic pollutants as well as heavy metals. It was found that the magnitude of negative zeta potential decreased with increasing dosage of acid-functionalized CNT. Apart from removals by adsorption, this increase in zeta potential indicated that the coagulation of colloidal particles in brewery wastewater also occurred via charge neutralization by acid-functionalized CNTs. However, acid-functionalized CNTs coagulation alone resulted in only 56% turbidity removal as compared to 83% by using ferric chloride alone. Although promising, the combination between ferric chloride and acid-functionalized CNTs only resulted in slight improvement in turbidity removal and was not recommended due to the absence of synergetic effect 71.

3.4

Pulp and paper wastewater

Pulp and paper mills are one of the sources of industrial pollution worldwide. This industry consumes up to 60 m3 freshwater per tonne of paper produced and generates large quantities of heavily polluted wastewater

72

. Depending upon the type of pulping process, wastewater from

pulp and paper mills contains high amount of BOD and COD, TSS, lignin, tannins, as well as various toxic chemicals such as extractives (resin acids), chlorinated organics (measured as adsorbable organic halides), unsaturated fatty acids, diterpene alcohols, juvaniones and others 73. If the effluent is discharged untreated, it will cause an adverse effect to the environment as it is toxic to the aquatic, exhibits strong mutagenic effects and can lead to physiological impairment 74

. Among all the various treatment processes available for pulp and paper effluent treatment,

only a few techniques such as sedimentation, flotation, aerated lagoons, activated sludge and



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some physico-chemical treatment are commonly adopted 73.

Besides being used to remove initial COD, turbidity and lignin prior to biological treatments, coagulation-flocculation process has been employed as a tertiary treatment in pulp and paper mill 73,75. Chaudari et al. 76 studied the coagulation performance of aluminum chloride, PAC and copper sulfate in treating black liqour obtained from integrated kraft pulp and paper mill. Although COD and color removal using copper sulfate (74 and 76%, respectively) was lower than the best coagulant, PAC (83 and 92%, respectively), the presence of Cu in the supernatant could act as a good catalyst for wet oxidation in the subsequent stage 76. Recent study showed that combination of polydiallymethylammonium chloride (polyDADMAC) with polyacrylamide (PAM) as a flocculant to treat pulp and paper mill wastewater could successfully remove turbidity, TSS, and COD up to 80 96.8 and 98% respectively

77

. The use of various

cationic PAMs (Organopol WPB20, Organopol WPB40, and Superfloc 567) as flocculants in treating pulp and paper mill wastewater together with PAC were also promising

78

. When

Organopol WPB20 was used together with PAC, treatment process achieved 99% turbidity removal with COD and TSS removals of 60 and 92%, respectively 78.

Cassia obtusifolia (C. obtusifolia), a plant of the Leguminose family commonly found in Asia, was also been actively investigated for its coagulation properties 79,80. The seed gum derived from this weed plant has a structure of 1,4-β-D-mannopuranose unit with 1,6 linked α-Dgalactopyranose units, mannose to galactose ratio of 5:1 and molecular weight of 100,000300,000 g/mol 80. Its absorptive properties were attributed to the presence of metal chelating cishydroxyl groups in the chain 79. When tested on coagulation-flocculation of raw pulp and paper mill wastewater, C. obtusifolia obtained similar TSS (86.9%) and COD removals (36.2%) as compared to alum using its recommended treatment conditions 80.


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Rodrigues et al.

81

attempted a combined treatment of coagulation-flocculation followed

by heterogeneous photocatalysis to treat cellulose and paper industry effluent. In order for successful photocatalysis, COD value must be lower than 800 mg/L as high suspended material contents in wastewater will lead to light scattering effects 82. Hence, coagulation-flocculation was employed with ferric chloride as a coagulant and the COD was successfully reduced to about 514 mg/L (around 56% reduction) before photocatalytic process. They also reported that the addition of chitosan as an auxiliary coagulant did not significantly contribute to COD reduction. Nevertheless, chitosan was found to promote faster coagulation, better sedimentation and formation of more compact flocs, which is important in industrial scale applications

81

. Recent

studies to improve pulp and paper mill wastewater treatment also suggested that coupling the iron sulfate (Fe2(SO4)3) coagulation process with electro-coagulation using six iron plates as sacrificial electrodes could improve TSS and COD removal from 21 to 93% and 36 to 97%, respectively 83.

Polysilicate complex coagulant was also been tested for the treatment of papermaking wastewater. For instance, poly-silicic-cation (PSiC) coagulants were prepared from industrial wastes such as fly ash, pyrite slag and wasted sulfuric acid via synchronous-polymerization (PSiCs) and co-polymerization method (PSiCc)

84

. Results indicated that preparation methods

have the same effect on polymerization and conformation of PSiC, in which the coagulants consisted of complex compounds of Fe, Al, Si and other ions rather than a simple mixture of the raw materials. In terms of performance, although both coagulants exhibited similar turbidity and chroma removal from papermaking wastewater, PSiCs gave significant higher COD removal than PSiCs. IR analysis revealed the higher coagulants’s characteristic bonds such as Si-O-Al and SiO-Fe existed in PSiCs as compared to PSiCs which contributed to its better adsorption bridging ability 84.



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There are several studies aimed to improve the performance of wastewater treatment for actual existing pulp and paper mill. For example, proposed an integrated treatment method consisting of coagulation, oxidation with hydrogen peroxide and biological treatment using activated sludge for two different paper mills was proposed in Egypt (RAKTA and Al-Ahlia) 85. The use of ferric chloride as coagulant was found to be better than alum or lime. Although TSS and turbidity was significantly reduced after coagulation, the quality of the effluent was still unsafe to be discharged as high amount of tannin, lignin and dissolved metals were still present. On the other hand, Žarković et al.

72

presented a thorough case study on effluent treatment plant

for a paper mill located in Serbia. In their work, the existing physicochemical effluent treatment was viewed as a sufficient, stand-alone technique for the mill. Effective settleable solids and TSS reduction (97.1 and 85.8%, respectively) was observed even without the addition of coagulants. Even though the addition of alum and PAM further improved the treatment, the treated effluent still contained COD and BOD (841 and 395 mg/L, respectively) higher than the European norms for wastewater discharge. They suggested some interesting in-mill measures to reduce fresh water consumption by internal water system closure which indirectly reduced the wastewater generation. Besides, in order to meet the discharge limits, inclusion of a biological treatment was proposed although it might not be economically sustainable.

3.5

Tannery wastewater

For the past few decades, developing countries have witnessed a sharp increase in leather production due to the decline in such activities in the developed nation because of its increasingly stringent environmental pollution control requirements and high labour cost 86. About 18 billion ft2 of leather are produced annually around the world with an estimated trade value of approximately US$ 70 billion 87. However, tannery effluent is one of the most harmful threats to the environment, containing high level of salinity, organic loading, inorganic matter, dissolved


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and TSS, ammonia, organic nitrogen and specific pollutants such as sulphide, chromium and other toxic metal salt residues 88. On average, about 30-35 L of wastewater will be generated for every 1 kg of hide/skin processed. Besides the high water consumption, at least about 300 kg of chemicals are also added per tonne of hide to produce the required characteristics of the finished product. These chemicals include low biodegradable substances such as surfactant, acids, dyes, natural or synthetic tanning agents, sulfonated oils salts and other chemicals 87,88. In order to treat tannery wastewater, different methods such as coagulation-flocculation

86,89–96

, advanced

oxidation processes, biological treatment, ozonation and activated carbon adsorption have been employed 87.

An interesting study was presented by Haydar et al.

86

in which chemically enhanced

primary treatment (CEPT) was introduced to treat tannery wastewater produced from Saddiq Leather Works, which processes 18000 kg raw hides per day 86. Initially, the tannery is equipped with a primary treatment plant without the practice of using coagulants. However, after introducing coagulation-flocculation process they found that alum was preferred over ferric chloride and ferric sulfate as it did not produce black colored treated wastewater. Pre-settling of the wastewater before CEPT was also found to be ineffective as it only contribute slightly towards improving effluent quality and therefore, should be omitted to reduce capital cost. National effluent quality standards for municipal and liquid industrial effluents (Pakistan) were met for both TSS and chromium content after undergoing CEPT but a secondary treatment was needed to treat the remaining high COD content of 1120 mg/L 86.

In India, common effluent treatment plant (CETP) is usually adopted to treat tannery wastewater because small scale tanneries could not afford to have their own effluent treatment facilities. With more than 2081 tanneries in India, only 15% of the tanneries have installed their own effluent treatment plant 92. An investigation on the performance of CETP to treat a combined



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wastewater from 50 small scale tanneries showed that the treated effluent did not comply with prescribed standards due to insufficient addition of coagulant, thus causing overloading in the aerobic system. Improvements could be made by proper dosing of coagulants such as alum, lime, ferric chloride together with a small dose of polyelectrolyte as a coagulant aid 92.

Tannery industry also generates some quantity of solid wastes which comprise of organic substances from hides and skins as well as primary sludge due to the treatment of wastewater using coagulation-flocculation process. These solid wastes, when disposed in landfill, will result in production of tannery landfill leachate

96

. Gotvajn et al.

96

conducted treatment experiments

such as aerobic biological treatment, air stripping, adsorption to activated carbon, coagulationflocculation and advance oxidation process with Fe2+/H2O2 to determine the appropriate technique to effectively treat heavily polluted tannery landfill leachate. Their investigation on coagulation-flocculation revealed that ferric chloride was a better coagulant for the treatment as compared to alum. However, at ferric chloride dosage higher than 1000 mg/L, a formation of orange color (due to high ferric concentrations) should be taken into consideration. Toxicity test using Vibrio fischeri on the treated tannery landfill leachate showed a significant reduction of toxicity after undergoing coagulation-flocculation process 96.

Study conducted by Ayoub et al. 94 showed that the use of bittern alone as a coagulant in treating tannery wastewater could achieve 97% TSS removal, 45% COD removal, and 99.5% turbidity removal. By combining this treatment with adsorption process using coconut-shellbased granular activated carbon (AcquaSorb CS), increases in the pollutant removal were achieved with a TSS, COD, and turbidity removal of 98, 72, and 100% respectively. Moreover, another recent study also suggested that the use of immobilized (using sodium alginate) coagulants of ammonium aluminum sulfate (NH4Al(SO4)2), alum (Al2(SO4)2), calcium carbonate


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(CaCO3), and sodium citrate (Na3C6H5O7) for tannery wastewater treatment could result in the final COD value of 1800, 1775, 1825, and 1825 mg/L respectively 95.

3.6

Landfill leachate

In most countries, sanitary landfilling is one of the most widely accepted and commonly used methods to eliminate municipal solid waste (MSW) 97,98. Nevertheless, there is an urgent need to reduce MSW loadings as landfill sites are known to produce leachate that carries both organic and inorganic contaminants 99, such as ammonia-nitrogen, heavy metals, chlorinated organics and inorganic salts

97

. Landfill leachate is a very dark colored liquid generated primarily by the

percolation of rainwater through MSW. Leachates from different landfill may vary considerably in its chemical composition due to several factors such as type of solid wastes deposited, hydrogeology of landfill site, specific climate condition, moisture routing through the landfill, landfill age as well as design and operation of the landfill 100. The suitable treatment of leachates depends on the above mentioned characteristics.

Coagulation-flocculation is generally implemented to treat stabilized and old landfill leachates 97. When PAC or alum was examined as coagulants for leachate treatment, the relatively low COD removal efficiencies achieved (43.1% by PAC; 62.8% by alum) indicated that coagulation-flocculation should be employed as a pre/post treatment for this purpose

100

.

Nevertheless, the use of PAC was better towards improving the physical characteristics of leachate in comparison with alum as indicated by its higher turbidity, color and TSS removal (94, 90.7 and 92.2%, respectively) even though the dosage of alum was almost fivefold of the PAC dosage 100.



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Page 28 of 101

The performance of PAC prepared using granular aluminum metal, a common by-product in several aluminum-processing secondary industries instead of using Al2O3 which is commonly used by most industries was also tested on biologically pretreated landfill leachate

101

. This

preparation method could potentially lower the cost of PAC production by saving energy and time. It was found that both the laboratory-prepared and commercial PACs showed similar turbidity removal (max. removal of about 70%), with the former exhibited a slightly better performance within coagulant dosage range of 10-100 mg Al/L. This phenomenon could be due to the slightly higher basicity and Al13 content of the laboratory-prepared PAC as compared to the commercially available PAC 101. In another study, Tzoupanos et al. 102 evaluated the performance of a new composite coagulant, namely polyaluminum silicate chloride, in coagulationflocculation process as a post treatment of biologically pretreated stabilized leachates. Their results showed that the silica-based coagulant exhibited better coagulation performance especially in turbidity, UV254nm, and COD removals as compared to both alum and PAC. It was found that the addition of high dosage of silica-based coagulant in treatment up to 300 mg/L did not result in restabilization of colloids. However, alum with dosage higher than 200 mg/L caused deterioration of treated sample quality. Increasing the molecular weight of the composite by incorporating silica chains into the structure of PAC could enhance its tolerance against pH variation. Besides, this silica-based coagulant performed better under natural pH of raw stabilized or pretreated leachates (pH 7-9) 102.

Similar to the other real industrial wastewaters, the treatment of landfill leachate often involves a combination of physical, chemical and biological methods. Marañón et al.

103

evaluated the possibility of using coagulation-flocculation as a pre-treatment process for young leachate to prevent fouling in ultrafiltration membranes. It was observed that the permeate flow of a young coagulated leachate (dosage of 1 g aluminum polychloride (PAX)/L and initial pH 8.3), was enhanced up to 33% after 24 h ultrafiltration as compared to the untreated young leachate.


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Therefore, membrane fouling could be minimized if the leachate was subjected to coagulationflocculation pre-treatment prior to ultrafiltration process

103

. Recently, the combination of

coagulation and dissolved air flotation (DAF) process to treat landfill leachate was also been investigated. The flocs formed by charge neutralization using ferric chloride were attached to the bubbles during the DAF process to aid the removal of pollutant particles. Pressure and flowrate were found to be the least significant parameters in pollutant removal but were important to maximize the performance of DAF 104.

Generally, the use of coagulant aid helps enhance the overall treatment of wastewater using coagulation-flocculation process. However, several researchers reported the ineffectiveness of flocculants as a coagulant aid in treating landfill leachate. For example, Marañón et al.

98

applied different coagulants (ferric chloride, alum, PAX) and flocculants (anionic PAMs) to treat leachate but the removal efficiencies were hardly affected by the addition of flocculants.

3.7

Heavy metal ions in wastewater

Presence of heavy metals in water represents a major environmental hazard and their removals are highly essential. Once the heavy metals enter the food chain, they may accumulate in living tissues and pose serious health hazards to humans or any other living forms. This is because heavy metals are not biodegradable and are highly soluble in the aquatic environment

105–107

.

During the treatment of industrial wastewater, toxic heavy metals such as zinc, copper, nickel, mercury, cadmium, lead and chromium are seen as the cause of serious environmental concerns 107

. Besides environmental safety, new and efficient recovery methods are developed to cater for

the increasing demand of heavy metals and depletion of ores 106.



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Macromolecule heavy metal flocculant is a new type of flocculant which is able to remove both turbidity (by electrical neutralization plus bridging effect between particles) and soluble metal ions (coordination and chelation)

108

. A strong ligand for heavy metals, namely

mercapto-group, was introduced to bind with chitosan to produce flocculant for Cu2+ removal 108. In the presence of activating agent, chitosan was amidated by mercaptoacetic acid to form mercaptoacetyl chitosan (MAC). The use of chitosan or mercaptoacetic acid alone produced fine particles which settled slowly and were difficult to be separated from mother liquid when it was tested on wastewater containing Cu2+. However, the use of MAC enabled the formation of larger flocs and thus enhanced the heavy metal removal as a result of the combined action of coordination between –NH2 and Cu2+ as well as coordination/chelation between mercapto-group and Cu2+

108

. In another study, it was reported that chitosan carboxyalkylation enabled the

improvement of flocculant solubility in wider range of pH, the enhancement of chelating ability and the possibility to control the net-charge of the macromolecule by variations of substitution degree and pH

109

. Besides imparting polyampholyte properties to chitosan, the presence of β-

alanine fragment in the derivative structure also helped reduce the heavy metal ion concentration in wastewater

109

. Flocculants of Konjac glucomannan-graft-poly(acrylamide)-co-sodium

xanthate was also attempted to be used as a coagulant for removing heavy metal from plating wastewater 110.

Humic substance, which is a major component of natural organic matter, can bind with the other pollutants species in aqueous solution to facilitate its removal. For example, the binding of Pb2+ and Zn2+ with humic acid, which then followed by coagulating-flocculating with cationic polyelectrolyte, namely polyDADMAC, was attempted by Hankins et al.

111

. Interestingly,

increasing the heavy metal concentration led to a decrease in the required amount of PolyDADMAC for flocculation, because partial neutralization of the negative charge of humic acid by metal ions after complexation would lead to a lesser amount of PolyDADMAC needed


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for neutralization of the remaining charge. Furthermore, the adsorption of metal-humic acid complexes onto a thermosensitive polymer was also been examined

112

. When a model system

contained metal ions (Cu2+ and Cu3+), humic acid and thermosensitive polymer were heated above the lower critical solution temperature, flocs of hydrophobic polymer would be formed and metal ions were then successfully adsorbed onto the flocculated polymer 112.

Coagulation-flocculation has also been used as the last treatment stage in a novel combined process consisting of pre-oxidation, co-precipitation and adsorption for the treatment of industrial wastewater which contained high concentration of arsenic

113

. During absorption

process, although ferric and manganese binary oxide absorbent was able to quickly absorb the arsenic in wastewater, the settling rate of ferric and manganese binary oxide was very slow due to its colloid character. With an addition of 150 mg/L PAC, residual arsenic concentration decreased dramatically from about 0.13 to less than 0.05 mg/L after 30 min of operation only. In a subsequent pilot plant study, the combined treatment successfully removed 99.998% of arsenic in the wastewater, giving an average effluent arsenic concentration of 0.008 mg/L, which was lower than the discharge limit of 0.05 mg/L 113.

3.8

Other industrial wastewater

There are many successes of using coagulation-flocculation technique as a main treatment or pretreatment to treat various industrial wastewaters

10,112,114–138

. For example, the treatment of

purified terepthalic acid (PTA) wastewater using coagulation-flocculation process to reduce COD 124

. PTA wastewater contains aromatic compounds such as p-xylene, p-toluic acid, benzoic acid,

phthalic acid, terepthalic acid and others which are toxic to living organisms

123,124

. It was found

that COD removal using ferric chloride (75.5% with dosage of 3 g/L) was far more efficient as compared to removal using ferrous sulfate, alum and PAC (not more than 50% COD removal).



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By using ferric chloride with an addition of cationic PAM, the sludge from coagulated treated PTA wastewater had better filterability as compared to municipal sludges, thermochemical precipitation sludge and electrochemical sludge as shown by its lower specific cake resistance. This phenomenon enabled the sludge to be dewatered easily to reduce its volume and moisture content before disposal 124.

For the treatment of polyvinyl chloride (PVC) wastewater, aluminum and ferric ions were reported to be far more efficient as compared to calcium ions as coagulants 9. An addition of polyelectrolyte would increase the rate of settlement and improve the clarity of the supernatant. However, due to the fact that PVC is negatively charged, coagulant has to be added prior to an addition of polyelectrolyte because the negatively charged polyelectrolyte might cause repulsion between particles, resulting in cloudy coagulation 9. On the other hand, Durán et al. 117 proposed a treatment scheme to treat Integrated Gasification Combined Cycle (IGCC) power station effluent which included coagulation-flocculation process as a pre-treatment. They found that by using calcium chloride and a commercial anionic polyelectrolyte, almost all fluoride ions (99%) and TSS (92%) can be removed before the subsequent treatment processes 117.

There have been studies investigating the use of polystyrene sulfate (PSS) as an auxiliary agent of coagulation in the treatment of wastewater. For instance, polystyrene foam waste was utilized to produce PSS for removing phenol from coking plant effluent

119

. This effluent was

found to contain ammonia, cyanide, thiocyanate and many toxic organic contaminants such as mono phenol, organic nitrogen compounds and polycyclic aromatic hydrocarbons (PAHs). The PSS was prepared by crushing the polystyrene foam into smaller pieces before immersing them into a solution of 98% sulfuric acid so that the sulfonic groups could be attached to their polymer chains. When the PSS was introduced as a flocculant, flocs formed were less abundant and more stable, attributed to the attachment of flocs to the sulfonic groups of polymer chain. The reuse of


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the waste as a flocculant not only reduces the treatment cost, but also provides a sustainable effect to the environment 119.

Wastewater produced from biodiesel production contains high amount of oil and grease with a low content of nitrogen and phosphorus

128

. A comparison between the performance of

chemical coagulation with alum and electro-coagulation process for treating biodiesel wastewater was conducted by Ngamlerdpokin et al.

129

. For chemical coagulation, an addition of CaO was

found to significantly promote COD, BOD5 as well as oil and grease removal as it acted as a coagulant coupling with the pollutant molecules, leading to the formation of organic precipitated sludge. Besides more effective in removing BOD levels, chemical coagulation also operated at lower operating cost (1.11 USD/m3) as compared to electro-coagulation process (1.78 USD/m3) 129

. A novel approach to enhance the efficiency of DAF using acidification and coagulation in the

treatment of biodiesel wastewater was also been investigated

128

. Coagulants such as alum, PAC

and ferric chloride could be used to effectively remove oil and grease content (>90%) in wastewater before DAF stage. This was due to the charge difference between soap molecules in biodiesel wastewater and aluminum or ferric molecules. However, alum was deemed to be the best choice due to its significantly lower cost (10 and 263% cheaper as compared to PAC and ferric chloride, respectively) 128.

Coagulation-flocculation was also been used to treat wastewater containing high amount of surfactant

10,122

. Synthetic surfactant possesses many industrial applications including washing

and cleaning, development of textiles, cosmetic or medicinal products, plastics, foodstuff and others. However, when untreated surfactant is discharged as wastewater, it produces toxic foams which are harmful to both aquatics and human beings

122

. Surfactants are also used to enhance

highly polluted soil restoration, which in turn generates substantial toxic effluents containing potentially hazardous chemicals extracted from the contaminated site. Torres et al.


10

confirmed


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the feasibility of using coagulation-flocculation process to treat the wastewater generated from these contaminant soil using ferric chloride and Technifloc 998. The treated wastewater could be disposed or recycled to the washing process for reusing purpose 10.

El-Gohary et al.

121

investigated two treatment schemes, namely chemical coagulation

followed by precipitation (C/P) and chemical coagulation followed by DAF (C/DAF) on the treatment of personal care products (PCPs) wastewater. PCPs wastewater is characterized by its high organic compound content, containing contaminants such as detergent, oil and grease. As compared to the ferric chloride and ferrous sulfate, alum was found to be the best coagulant for both schemes. Even though similar removal efficiencies were observed, C/DAF was more economical as compared to C/P with 23.7% lower total annual cost (construction, chemical, and sludge treatment and disposal cost). Besides, C/DAF process produced a relatively concentrated sludge as a result of the float layer formation above the water surface during the flotation process where air bubbles were flown through the wastewater. The float layer was drained to increase the dry solids concentration. Using this treatment scheme, less amount of sludge was produced with high solids content 121.

Various combination of physicochemical (coagulation, Fenton process and ozonation) and biological (aerobic oxidation) treatment methods were evaluated by Klauson et al. 139 to treat hardwood soaking basin wastewater which contains large amount of wood and bark-originating water extractive materials. Coagulation using ferric sulphate resulted in more than 35 and 30% of COD and DOC, respectively due to the removal of suspended solids. Additionally, the lignin content in the wastewater was reduced by up to 86% after coagulation process. The authors also demonstrated the superior COD and lignin removal efficiencies when both coagulation and Fenton process was used together as compared to treatment using Fenton process only. Besides


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requiring lower reagent dose ratio, combining the two processes also reduced the consumption of hydrogen peroxide by over 3 times 139.

Coagulation-flocculation process was employed as a pre-treatment for petrol station wastewater prior to photocatalytic process using TiO2, ZnO and Nb2O5 in a study conducted by Ferrari-Lima et al.

140

. In the pretreatment process, an environmentally friendly tannin-based

coagulant (Tanfloc) was employed. Tannins are water-soluble polyphenol compounds obtained from secondary metabolites of plants such as bark and wood from Acacia, Castanea and Schinopsis

33,141

. In particularly, the tannin-based coagulant used was originated from Acacia

mearnsii de Wild bark which has undergone Mannich aminomethylation by reaction with an aldehyde and an amine. Hence, the resultant tannin polymer has high molecular weight and exhibits ampholytic character due to the presence of both cationic amines and anionic phenols on its chain

142

. In pretreatment of petrol station wastewater, Tanfloc not drastically reduced the pH

of effluent and gave satisfactory removal of COD (73%), TOC (52%) and turbidity (90%). Additionally, pH adjustment was also not needed and there is no harmful metal ions added into the system 140.

The use of physico-chemical treatment consisted of addition of hydrochloric acid, followed by pH neutralization and coagulation-flocculation using alum to disinfect wastewater from cholera treatment center (CTC) has been demonstrated by Sozzi et al.

143

. In their study,

thermotolerant coliforms, an indicator of the presence of fecal materials in water, were used as an index of disinfection efficacy. Although the study presented does not enumerate the pathogen Vibro cholera directly, evidences suggested that thermotolerant coliform could sufficiently used as an indicator of the presence of Vibro cholerae as they exist in high concentration in human feces. The high removal rate of disinfection (COD >99%; suspended solids >90%; turbidity >90%; thermotolerant coliforms >99.9%) achieved clearly showed that the proposed treatment



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approach may be a suitable and valuable option for onsite-disinfection of CTC wastewater generated in the course of cholera epidemics as well as a potential disinfection technique during the outbreaks of other infectious diseases. This treatment approach is a more reliable and cost effective alternative to the untested ‘super chlorination’ technique 143.

4.

Challenges in using coagulation-flocculation process in wastewater treatment

Previous section has shown that coagulation-flocculation has been widely investigated and was used as an essential pre-treatment, post-treatment, or as a main treatment process for various industrial effluents. However, there are still many challenges and drawbacks affecting the use of this process and were highlighted in the following section.

4.1

Toxicity of alum and polymeric-based coagulant

One of the major issues related to coagulation-flocculation process is the toxicity and health hazard possessed by inorganic coagulants, such as alum and polymeric-coagulants. The use of conventional coagulant, such as alum has drawn major concern about the increase of metal concentration in the water. Although the use of high dosages of alum in the wastewater treatment might reduce COD effectively, but it might also result in an increase of aluminum species present in the water which may lead to Alzheimer’s disease upon consumption 144. Hence, it is necessary to control the residue Al3+ content in the discharge to be under 200 and 50 µg/L for European and United States of America standard, respectively 23. As for drinking water, Guida et al.

145

stated

that the maximum contamination level of Al3+ should be around 0.2-1 mg/L. Therefore, the use of alum-based coagulants is limited by its dosage and is not preferable as suggested by many researchers 11,37,39,56,57,116,146–149. Furthermore, due to this health impact, the reuses of either treated wastewater or generated sludge are also not recommended 57,115,116.


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Despite the widespread use of polymers, their toxicity to aquatic organisms has not been extensively studied

150,151

. Although the active monomers in the polymer chain will bind with

suspended solids to form aggregates for rapid settling, overdosing has led to the release of polymers into the watercourses. Most polymers synthesized for application in coagulationflocculation process are derived from subunits identified as monomers. More often than not, these monomers are toxic in nature although the secondary polymers could be non-toxic. Unlike polymer molecules which tend to be large and relatively less soluble, monomers are small and easily

absorbed

to

interact

with

bodily

systems.

For

example,

ethylenimine,

diallyldimethylammonium chloride, trimethylolmelamine and epichlorohydrin with acrylamide pose significant concern relating to health and aquatic toxicity

152

. The occurrence of monomers

in systems is typically due to the presence of residual (unreacted) monomers as product contaminants in polymers. Majority of the monomers showed both acute and chronic exposure to humans and animals

152

. Existing aquatic toxicity information suggests that the anionic and non-

ionic polymers have relatively low toxicity to aquatic organisms, unlike its cationic counterparts which is at least 100 times more toxic

150,151

. In particular, significant amount of literature has

discussed the health implications and toxicity of acrylamide and epichlorohydrin being carcinogenic to animals and probably humans. Low concentrations of these two monomers is an important concern, especially in potable water as these monomers are probably the most significant monomers due to their carcinogenicity and extensive usage in many products. Study on the effects of two cationic acrylamide copolymers with different chain branching degree on aquatic organisms by Costa et al. 153 indicated that toxicity of polymer strongly depends on chain architecture. The more branched polymer was found to be significantly less toxic to both daphnids and the bivalves.



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Moreover, when cationic polymers are released to the water bodies, a reduction of oxygen transfer to the aquatic organisms was also observed

60,154

. United States Environmental

Protection Agency suggested that the toxic effect of cationic polymers would occur at the dosage of 2-3 mg/L in pure water

60,149

. The difficulty to measure the polymer concentration and its

adsorption rates in treatment facilities commonly resulted in inaccuracy to gauge the toxicity level in the treated effluent. Therefore, an addition of lower dosage of cationic polymers less than 2 mg/L is preferable in the treatment scheme. Furthermore, application of mineral additive as a coagulant will give rise to several environmental problems such as the high metal concentration in water as well as production of large volume of toxic sludge from the coagulation-flocculation process 151.

4.2

Sludge production

The production of sludge as a secondary pollutant from coagulation-flocculation process was also one of the challenges faced in treatment facilities. The generated sludge must be dewatered after the treatment process 76,124,155,156 and this will incur extra cost as a separate sludge handling unit is usually required. Generally, the operating cost associated with the sludge handling was often reported to be a significant part of the overall operating cost in the wastewater treatment plant 40. Oladoja et al. 38 recommended that a good sludge production should have a sludge volume index (SVI) less than 80 mg/g, in which case a very good sludge is characterized at 50 mg/g SVI. On the other hand, SVI number greater than 120 mg/g indicates a very poor sludge settling characteristic. Proper disposal, regeneration and reuse of sludge are recommended to deal with the generated sludge

157

. However, in some cases, the recovery of sludge is difficult due to the

fragility of the forming flocs

158

. For example, complications in recovering alum-induced

coagulation sludge were reported due to the volume of sludge, its fragility and inconsistency of flocs

4,86,89,154,159

. Furthermore, the reuse of sludge as fertilizers is not recommended due to the


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presence of metal residual in the dewatered sludge

116

or lacking of maturity of sludge which

could be used for agronomy purpose.

High production of sludge from coagulation-flocculation treatment has been reported by various authors. For example, the use of conventional inorganic coagulants, such as alum, ferric chloride and PAC, were also reported to produce high volume of metal concentrated sludge in various dye wastewater treatment

37,39,146,160

. In POME coagulation, the use of PAC and alum as

coagulants were not preferable due to the production of toxic sludge and the problems associated with sludge disposal. Gao et al.

161

also reported that although the coagulation performance of

composite inorganic-organic coagulant from polyferric chloride (PFC) and PolyDADMAC was reported to be higher than PFC or PolyDADMAC alone, the production of toxic sludge at the end of the treatment was found to be the highest among all. Therefore, further economic feasibility of this composite coagulant must be considered.

4.3

Inability to fully remove heavy metals and emerging contaminants

Coagulation-flocculation process was also found to be less effective in treating wastewater containing heavy metals or emerging contaminants. In the case of heavy metal wastewaters, a significant increase of heavy metals content in a scrap collecting company wastewater was reported to occur after coagulation-flocculation treatment using ferric chloride. Chys et al.

162

observed that coagulation-flocculation treatment using ferric chloride resulted in higher concentration of total arsenic, cadmium, chrome, and nickel in the treated wastewater as compared to the influent. No detailed explanation was provided, however, it could be seen from the result that coagulation-flocculation failed to reduce the heavy metal contamination in the wastewater.



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Inability to remove metal ions by coagulation-flocculation process alone was also reported by Shang et al.

163

. The application of PAC as a coagulant and PAM as a flocculant to

treat ceramic printing wastewater was able to remove 71 and 47.7% of chromium and iron content, respectively. However, these removals were found to be insufficient as the levels of metal ions contaminant were still at 4 and 6 mg/L for chromium and iron, respectively. Other treatment option such as heavy metal scavenger was suggested to be more effective as it could remove 82.3 and 76.2% chromium and iron, respectively. A detailed review on heavy metal removal by Fu and Wang

107

also stated that various coagulants, such as PAC, polyferricsulfate

(PFS) and PAM, were nearly unfeasible to remove heavy metal from wastewater directly. Therefore, it was suggested that other treatment techniques, such as electro-coagulation, might be coupled with coagulation-flocculation treatment for obtaining higher removal of heavy metal.

Ineffectiveness of coagulation-flocculation process in the removal of emerging contaminants has also been reported. Emerging contaminants are commonly derived from industrial wastewaters, such as pesticides, pharmaceuticals, personal care products, fuel additives, flame-retardants, plasticizers, and numerous other industrial pollutants

164

. When applied on

hospital wastewater which contains microorganisms, heavy metals, toxic chemicals, and radioactive elements, coagulation-flocculation process using alum was not favorable due to the increase of TSS at the end of the treatment

165

. Furthermore, the ineffectiveness of coagulation

process, using ferric chloride, for removal of anti-epileptic carbamazepine (CBZ), ibuprofen (IBP), and iodinated contrast media iopromide (IPM) content in the hospital wastewater was also observed as the result of the presence of heavy metals which may exert trivalent cations 165.

An application of ferric chloride or alum in the coagulation was reported to be only suitable as a pre-treatment stage to adsorption and ultrafiltration treatment in treating secondary effluent containing 11 different emerging contaminants

164

. Coagulation alone was only able to


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remove less than 3% emerging contaminants. When combined with ultrafiltration, the combined system did not improve significantly the removal of emerging contaminants as compared to ultrafiltration alone. Therefore, an integrated treatment with a sequence of ferric chloride coagulation, PAC adsorption, ultrafiltration, and granular activated carbon adsorption was suggested to be the optimum treatment option to remove emerging contaminants in the wastewater. More than 80% of each emerging contaminant, such as acetaminophen, metoprolol, caffeine, antipyrine, sulfamethoxazole, flumequine, ketorolac, atrazine, isoproturon, 2hydroxylbiphenyl, and diclofenac were able to be eliminated by the integrated treatment

164

.A

similar integrated treatment to treat veterinary pharmaceutical wastewater containing emerging contaminants, such as sulfamethoxazole, trimethoprim, ciprofloxacin, dexamethasone, and febantel has also been investigated

166

. Coagulation process was applied as a pre-treatment stage

to microfiltration, reverse osmosis, and nanofiltration treatment. Pre-treatment stage was required in a membrane treatment in order to prevent the irreversible membrane fouling in the later stage. The integrated treatment scheme could successfully remove 94-100% of the emerging contaminants in the wastewater.

Low to medium level of emerging contaminants removal was also observed in the coagulation-flocculation treatment of secondary wastewater effluents followed by lamellar clarifier from two full-scale Spanish wastewater treatment plants

167

. Depending on the type of

emerging contaminant, 0-50% removal of contaminant was achieved during the coagulationflocculation process. Besides, no detectable removal of Bisphenol A indicated that coagulationflocculation may not be the suitable treatment option. An integrated treatment consisted of coagulation-flocculation, lamellar clarifier, filtration, UV radiation, and chlorination, was suggested as a potential treatment scheme for emerging contaminants removal with 78 and 90% overall removal of diclofenac and ketoprofen compound, respectively 167.



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An application of integrated treatment scheme instead of individual treatment stage was also recommended to remove 17 emerging contaminant compounds contained in Mexico City’s wastewater, such as pharmaceuticals, personal care products, plasticizers, and others

168

.

Coagulation-flocculation was reported to remove plasticizers and 4-nonylphenol compounds better as compared to pharmaceuticals and other phenolic compounds due to the higher hydrophobicity of the components. Without an additional treatment stages, coagulationflocculation was unable to completely eliminate other emerging contaminants. A combination of aerobic biological treatment together with coagulation-flocculation and ultrafiltration was suggested as the potential treatment scheme 168.

4.4

Increase in effluent color

In some cases, it was reported that the coagulation-flocculation process using ferric chloride presented some considerable limitations related to the production of color in the effluent after treatment. For example, in the coagulation treatment of sewage water, the use of ferric chloride resulted in color generation at the end of the treatment, which is unacceptable according to environmental regulation

144

. The use of ferric chloride in the treatment of food industry

wastewater treatment could also result in an increase of effluent color and the flocs were found to be fragile and difficult to settle 4. Meanwhile, during the coagulation treatment for phosphorous removal, the use of iron salt as a coagulant was reported to be more effective as compared to alum 116. However, the production of yellow color (iron residue) in the treated effluent represents a major problem after the treatment process. In the treatment of dairy wastewater, Sarkar et al. 169 highlighted that the use of ferric chloride at higher dosage generated treated wastewater with orange color at the end of the treatment. Karthik et al.

123

also found that iron-based coagulants

contributed a persistent yellowish color in the treated wastewater, which may increase the oxygen requirement during the later biological treatment.


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4.5

Increase in COD level by using natural coagulants

For the past few years, the increasing interest of natural coagulants as an alternative to conventional coagulants has started to gain more attention due to their environmental friendly features and biodegradability. However, in some studies, the use of natural coagulants might not be effective in removing COD due to their organic properties which could increase the COD level of the wastewater. An application of M. oleifera as a natural coagulant in its crude extract form was reported to cause major problem related to the residual DOC that presented in the final effluent after the treatment 170. Due to this matter, the treated water was infeasible to be used as a drinking water. M. oleifera trees are also widely available in a number of tropical countries with different varieties possess varied coagulating properties that depend on geographical location, climate, altitude and soil characteristics 7. Therefore, purification of M. oleifera proteins was required to be done prior to coagulation treatment. On the other hand, Verma et al. 124 also found that some natural coagulants, such as guar gum, xanthan gum, sodium alginate, and carboxy methyl cellulose, were not effective for COD removal in the treatment of petrochemical wastewater containing PTA. With the initial COD value of 2776 mg/L, the natural coagulants were only able to remove COD with percentage removal ranging from 2.4%-18.6%. This range was found to be much lower as compared to the ferric chloride, which was able to remove 75.57% of initial COD. In the treatment of dairy wastewater, the use of carboxymethyl cellulose (Na-CMC), alginic acid, and chitosan as coagulants were also not suitable inability to form flocs in the entire pH range in comparison with inorganic coagulants (alum and ferric chloride)

169

.

Moreover, high molecular weight of the compounds was suspected to increase the total dissolved solid of the treated effluent as the coagulant dosage increased. Therefore, a proper dosage control would be required in the application of natural coagulants. Another problem related to the application of organic coagulant is the short life of coagulant storage due to their biodegradability



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149

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. Longer storage period of natural coagulant might also influence the efficiency of coagulation

process. For example, the coagulation efficiency of non-freeze-dried seeds of M. oleifera was found to decrease by 7% after the seeds were stored from 5 to 6 months

171

. Bratby 7 also stated

that fresh M. oleifera suspensions should be prepared fresh daily or kept refrigerated to avoid deterioration especially in hot climates.

4.6

Complex scaling up process

Finally, another challenge faced by the coagulation-flocculation process is the complexity of scaling-up the process. Various parameters of coagulation-flocculation process require precise control to ensure the efficiency of the process. Furthermore, each coagulation process is also found to be specific on the type of wastewater and coagulant involved in the wastewater treatment

159

. Thus, bench-scale experiment or trial and error testing is always required

beforehand to determine the most efficient coagulant and pH range for particular wastewater at certain dosage. Wong et al.

74

highlighted that optimization of coagulation-flocculation process

using polymeric-based coagulant in industrial practices were still very much dependent on trial and error basis due to the complex nature of the process and large variety of polyelectrolyte. In addition, Zahrim et al.

149

and Oladoja et al.

38

also suggested that reassessment of coagulation

process parameter was also necessary for dye wastewater treatment due to the fluctuation of effluent parameter for each batch. Several criteria, such as (1) effectiveness, (2) cost and reliability of supply, (3) sludge consideration, volume and characteristics, (4) compatibility with other downstream or upstream process, (5) environmental effects, and (6) labor and technology required in storage, feeding, and handling, could be used as guidelines in coagulant selection 156.

5.

Influence of process parameters on coagulation-flocculation efficiency


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Efficiency of coagulation-flocculation treatment is highly dependent on the process parameters of the process. Optimization of significant parameters, such as (1) pH, (2) coagulant or flocculant dosage, (3) settling time, (4) mixing parameters and (5) temperature, are necessary to ensure a more effective coagulation-flocculation performance. Other process parameters include initial contaminant concentration.

5.1

Effect of initial pH of wastewater

Unlike polymeric flocculants which are unfazed by pH changes, most coagulants and flocculants require pH adjustment for effective treatment 172. It has been widely known that the efficiency of most conventional coagulants is highly sensitive to the pH of effluents. In general, optimum pH in coagulation process is proven to be specific to the type of coagulant used in the treatment 173. A specific pH range is usually determined by the type of wastewater and coagulant used during coagulation process in order to achieve higher coagulation efficiency. Table 1 summarizes the optimum pH reported by various researchers in treating several industrial effluents 37,56,83,124,125,156,174,175

. The coagulation performance is inclined to decrease significantly mainly due

to the restabilization of colloids during the treatment at pH outside the effective pH range

125

.

Organic coagulants from a majority of plant seeds were found to be effective coagulants in effluents or water at lower pH. In accordance with an investigation led by Yongabi

176

, particles

are prone to coalesce into flocs at lower pH owing to the presence of positively loose charged particles to bind with negatively charged colloids. The destabilization of colloidal particles are made possible through the addition of chemicals, such as acid or alkali, which promoted electrostatic attraction due to the elimination of inter-particle forces by reducing surface charges during pH alteration. An examination of the electrokinetic behavior of certain wastewater, such as POME, based on the zeta potential showed reduction in repulsion between particles at lower pH as reported by Shak and Wu 79 and Kursun 177.



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On the other hand, most inorganic coagulants perform in pH ranging from pH 3 to 9. Optimum coagulation using conventional inorganic coagulants, such as hydrolyzing metal salts, hinges on the pH range which induces the formation of the most effective hydrolysis species of the coagulant essential for effective coagulation 2,178. The predominant charges during coagulation tend to be positive at lower pH and negative at higher pH. Metal salts derived from aluminum tend to be present in the form of positively charged Al species in the effluent with pH values less than 6, which is favorable for charge neutralization to take place apart from adsorption 179. At pH above 6, rapid formation of amorphous precipitates enhances particle entrapment through sweep floc

179

. This occurrence is applicable to treatment using ferric metal salts under similar

conditions as well. However, the sweep floc mechanism is unfavourable compared to mechanisms present in treatment using aluminum metal salts at pH below 6 as it requires higher coagulant dosage and produce higher amount of sludge. According to Bratby 7, the destabilization mechanism present during coagulation in the effective pH range varied with the coagulation dosage-colloid concentration relationship (Section 5.2). Usually, the destabilization mechanisms vary from adsorption, bridging, double layer repression, and precipitate enmeshment depending on the charge of the predominant species and coagulant dosage. Overall, appropriate pH control is crucial in any application of coagulation-flocculation treatment to ensure high efficiency of treatment process.

[Insert Table 1 here]

5.2

Effect of coagulant or flocculant dosage

Optimal coagulant dosage is established as a significant and critical factor for effective coagulation-flocculation performance. The optimal dosage varies with the molecular weight,


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ionic character, and ionic degree of the coagulant 79. The cost of coagulant or flocculant dosing and formation of sludge can be reduced if the optimal dosage of coagulant or flocculant is applied during the treatment of wastewater

180

. In general, inadequate or excess dosages of coagulants or

flocculants can lead to poor coagulation-flocculation performance. Excess dosing of coagulants either agitates the sedimentation process which causes either resuspension of aggregated particles or insignificant changes

180–182

. Overdosing of coagulants can be identified when the applied

dosage reaches an inflection point known as the critical coagulation concentration 7. At times, insignificant effect may be observed when coagulant dosage is applied beyond the critical coagulation concentration point as sufficient ionic strength derived from high concentration of counter-ions allows for double layer compression. Alternatively, restabilization may occur when bridge formation between adjacent particles is prevented by the lack of adsorption sites as most of the sites are occupied by polymeric species 7. In this case, particle restabilization is not preceded by diffuse layer compression as a predominant particle destabilization mechanism.

On the other hand, mechanisms responsible for particle destabilization are highly dependent on the coagulant dosage, especially for metal coagulants. In most cases, more than one mechanism may be present. In general, treatment using smaller dosage of metal salts tends to destabilize particles through ionic strength effects in terms of double layer repression by charged counter-ions 7. Conversely, sweep-floc coagulation is predominant during the application of higher dosages of metal salts. Unlike metal salts or cationic coagulants/flocculants, most plantbased organic coagulants or flocculants are anionic or non-ionic in nature, resulting in interparticle bridging becomes the plausible predominant mechanism in the treatment process. Although weaker in nature, an abundance of –OH groups along chains of galactomannan and galactan in the natural coagulants such as seed extracts offers many adsorption sites, which leads to interparticle bridging

80,182,183

. Besides, high molecular weight of polyelectrolytes also

contributes towards bridging mechanism as discussed by Teh et al.


13

and Shak and Wu

79

.


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According to Bazrafshan et al.

180

, lower dosage of natural coagulant was recommended for

treatment since it was economical and had lower organic matter load in the treated water. In addition, more concentrated natural coagulant would contribute towards increases in both color and turbidity in the treated water 13,184. Numerous studies have shown that the zeta potential could be used as an indication to determine the optimum coagulant or flocculant dosage needed for the treatment of wastewater. Optimum coagulant or flocculant dosage can be indicated by the isoelectric point of flocs (zero zeta potential). The presence of particle restabilisation due to excess coagulant dosage can be established when the flocs potential shifts from negative to positive potential

185

. In addition, Wei et al.

185

also revealed that relatively higher coagulant

dosage is capable of accelerating floc growth rate. The dosages required for effective treatment varies with the type of coagulants or flocculant, especially during charge neutralization which depends on the coagulant or flocculant valency (positive charges of corresponding hydrolysed species)

7,186

. Corresponding hydrolysates may require relatively higher dosage of coagulant

because it possesses relatively lesser positive charges to sustain efficient charge neutralization.

5.3

Effect of settling time

The strength and settling velocity of flocs formed during the coagulation-flocculation treatment affect the overall process efficiency. Yu et al.

187

reported that the efficiency of solid-liquid

separation process is highly influenced by the characteristics of flocs. The flocs act as a medium to transport suspended particles to the bottom of wastewater through settling and separation from the treated effluent. In the coagulation-flocculation process, production of small flocs is not ideal due to their fragility, lower settling velocity, and difficulty to separate them from the effluent 2,56,57

. The settling capability of flocs produced after coagulation-flocculation process depends

greatly on the type of coagulant or/and flocculant used in treatment and also, the type of wastewater. In general, flocculants are much more effective in rapid settling of flocs when


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compared to coagulants as clumping and formation of flocs from destabilized particles occurs during flocculation. Settling of flocs is highly dependent on the floc size generated during treatment. Flocculants are capable of producing larger floc size, which yields rapid settling. The adsorption of polymers and bridging involving long chain polymers during flocculation remain as the predominant mechanisms which aid in the formation of macroflocs from mechanical bridging of microflocs for rapid settling. For instance, Teh et al. 24 reported that treatment of POME using rice starch alone showed faster settling velocity of up to nine times (shorter settling time) compared to treatment using alum alone. Rapid settling of flocs could be attributed to the rapid aggregation of particles of sufficient size

24,188

. According to Teh et al.

24

, the high molecular

weight of rice starch could form larger and strong flocs to encourage faster settling velocity by attaching to particles present in the effluent. This occurrence showed that heavier flocs (aggregates) settle faster than dispersed particles owing to the effect of gravity

79

. However,

without prior settling time, treatment using rice starch alone contributed to an increase in effluent turbidity due to the presence of amylase content when compared to alum 24.

5.4

Effect of mixing

Stirring, also known as mixing, is a crucial step in water and wastewater treatment via coagulation-flocculation process. There are two types of mixing: fast and slow. The main purpose of fast mixing is to ensure uniform dispersion of coagulant into effluent for efficient treatment 189. Mixing is necessary especially in the treatment of wastewater with high turbidity as it enhances the adsorption and floc formation rates due to the increase probability of particle collision rate 190,191

. According to BinAhmed et al.

192

and Ayoub et al. 94, fast and slow mixing time has an

effect on the size and strength of formed flocs. Relatively shorter fast mixing time allowed for larger floc growth except the flocs were less shear resistant (less compact), whereas stronger but smaller flocs were formed in treatments with extended fast mixing time. This phenomenon could



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be explained by the presence of lower collision efficiency of small flocs due to extended high shear rate and floc breakage to a limiting size when fast mixing time is extended 187. However, an increase in floc size could be observed when the slow mixing phase was extended after a certain period although similar observation were present initially compared to flocs formed during fast mixing phase 94. Mixing time has an important role in ensuring successful collision efficiency by allowing ample time for aggregates to restructure into a more compact shear resistant floc when compared to mixing speed.

In addition, mixing speed plays a role as well apart from mixing time such that higher fast mixing speeds should be accompanied by shorter mixing times. In treatments using alum, the adsorption and destabilization mechanisms were enhanced due to fast hydrolysis of alum and formation of hydroxide precipitates when fast mixing speed was increased. Conversely, treatments using iron metal salts showed less efficient treatment as the hydroxide flocs were more vulnerable to erosion at high shear. However, mixing (shear) does not necessarily breakdown aggregates but re-organize particles into a more compact form of flocs 192,193. Unlike slow mixing time, floc size tends to decrease with higher slow mixing speed at a certain point as flocs tend to break instead of forming at higher slow mixing speed. Although increasing slow mixing speed above the optimum speed could increase collision rate for aggregation of larger flocs, sometimes it tends to cause more floc breakage by turbulent drag, bulgy deformation or flocs splitting 24,80,94.

5.5

Effect of temperature

Temperature has a severe effect on the coagulation and flocculation performance as it influences the particle transport and collision rate by varying density and viscosity of the suspension at different temperatures

172

. In addition, the effectiveness of each type of coagulant of flocculant

varies with temperature 7. According to Xu et al. 194, temperature could also affect the turbidity of


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raw water. For example, the increase of temperature during summer leads to high turbidity in raw water due to rapid algae growth

194

, which may require intensive coagulation-flocculation

treatment compared to treatment during other seasons. On the other hand, the initial discharge temperature of untreated effluent varies with the type of process involved prior to the production of wastewater. Usually, a number of wastewaters are discharged at high temperatures prior to treatment such that the possibility of treating the effluent without prior cooling can be economical and potentially reduce treatment time 79. According to a study led by Shak and Wu 79, an organic polymer known as C. obtusifolia was capable of reducing 93% TSS and 56% COD content in POME at high temperatures (up to 90 °C).

At lower temperatures, the performance of metal coagulants such as alum is less effective due to decreased hydrolysis and precipitation kinetics compared to readily hydrolyzed coagulants such as polyaluminum chloride. Based on Xiao et al.

195

, low temperature impedes the

aggregation rate of flocs (irregular and less compact flocs) and retards perikinetic collision. Similarly, poor formation and settling of flocs due to the convection currents generated from high heat 79 and an increase of kinetic energy at very high temperatures 35 as observed by Shak and Wu 79

. It was postulated that the effect of temperature was not as significant when adsorption is the

predominant mechanism during treatment compared to coagulation via enmeshment (high dosage of metal coagulant) 7,79.

6.

Wastewater sludge management

The management of sludge produced during wastewater treatment processes is becoming an issue of growing importance. Not only sludge treatment and disposal could account for up to 60% of the total operating costs in wastewater treatment plants, the huge volume of sludge produced could lead to serious environmental problems if it is not managed properly. During the course of



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coagulation-flocculation process of industrial wastewater, substantial amount of sludge which contain a wide range of organic and inorganic substances are produced. Typically, principle methods to remove moisture from sludge include thickening, conditioning, dewatering and drying 18

while digestion, incineration, ocean discharge, landfill, land application and composting are

some of the commonly used sludge stabilization or disposal methods 196.

Generally, the amount and characteristics of sludge produced during coagulationflocculation process depends on the coagulant/flocculant used and the operating conditions of the process 11. One of the ways to cope with sludge-associated problems is to minimize its generation during treatment. Bolto and Gregory

150

highlighted the improvement in sludge properties when

polymers are used in coagulation-flocculation process. Besides lowering the volume of sludge produced, the use of polymers also gives larger and denser flocs which lead to more rapid settling sludges and clearer supernatant. Dewatering characteristics of the sludges during centrifugation and filtration could also be improved by using polymers during treatment. For example, Haydar et al. 89 evaluated the efficiency and sludge volume produced by the use of eleven types of cationic polymers with different molecular weights and charge densities in tannery wastewater treatment. It was found that the treatment using cationic polymers with molecular weight of 4, 6 and 8 with charge density of 55, 40 and 40%, respectively at optimum dosage (20 mg/L) was able to meet effluent standards for TSS and chromium. By comparing the sludge volume produced from the treatment with alum, the use of cationic polymer C-496 (40% charge density, 6x106 Dalton) produced 65% lesser in sludge volume. Besides cost saving in sludge handling, economic analysis also showed a saving of 50% when cationic polymer was used as compared to alum 89. Cationic or anionic polymers could also be used in combination with alum for treatment of tannery wastewater. A study showed that although the removal efficiency values of turbidity, TSS, TCOD and chromium achieved by an addition of 5 mg/L of coagulant aid were comparable with the values obtained using alum alone, sludge reduction up to 60 and 70% could be obtained


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by using cationic and anionic polymer, respectively, which was advantageous to reduce the overall treatment cost

197

. Amuda and Amoo

154

also reported the use of ferric chloride together

with a non-ionic polyacrylamide in coagulation-flocculation treatment of beverage industrial wastewater resulted in 60% sludge reduction as compared to treatment using ferric chloride alone. The lesser and more compact sludge produced was due to bridging between small flocs by the polymer 154. By taking consideration on environmental safety, many studies have also looked into the potential use of natural polymers for this purpose. For instance, Szygula et al. 11 found that the charge neutralization and bridging mechanism exhibited by chitosan enabled almost complete removal of Acid Blue 92 dye while producing only small amounts of highly settleable sludge. In another study, the use of lignin-base cationic flocculant yielded lower sludge production (4-6%) in dye wastewater treatment as compared to PAC (8-15%) 39.

Valorization by converting sludge produced from coagulation-flocculation treatment of wastewater into useful by-product has been an important concept in sludge management. However, Section 4.2 highlighted the toxicity of sludge generated when conventional metal or synthetic coagulant and/or flocculant was employed in the process which prevented its further usage.

Alternatively,

many

studies

have

suggested

the

potential

use

of

natural

coagulants/flocculants to produce sludge which are more biodegradable and non-toxic. Teh et al. 24

proposed that the sludge produced from coagulation-flocculation treatment of POME using rice

starch as natural coagulant could be reused as substrate in agro-industries, either as fertilizer and/or animal feed. Due to the high organic content and non-toxicity of the sludge produced, its proper use in land application not only improves soil fertility, but also provides a cheaper alternative to chemical fertilizers. The POME sludge produced is also suitable to be reused as dietary substitute for pigs, poultry and small ruminents 24. The reuse of POME sludge when C. obtusifolia seed gum was used as natural coagulant was also proposed by Shak and Wu

173

. This

was due to the fact that C. obtusifolia contains minerals, amino acid and other nutrients which



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Page 54 of 101

could be useful supplements when used as organic fertilizer in agricultural application. However, wastewater sludge should be stabilized via composting, vermicomposting or combination of both technologies before the sludge could be reused as matured fertilizer 198,199. Chi and Cheng 200 also recommended the use of chitosan to replace inorganic coagulants in the treatment of wastewater from milk processing plant because the sludge produced could be used as fertilizer, animal supplement or gardening husbandries as well as resolving the existing shrimp-shell waste disposal problem. The environmental impact of reuse of organic sludge produced during coagulationflocculation has to be carefully assessed before its application.

A study also showed that the sludge produced from coagulation-flocculation of raw molasses wastewater using ferric chloride could also be reused directly or after melanoidin recovery 201. The sludge from the process was washed with aqueous solution at different pH to remove melanoidin content before solubilization by addition of a small amount of HCl. When the resulting solution was reused in the second series of flocculation experiments, the sludge exhibited its original coagulation ability without any detrimental effect on its performance as shown in Figure 5. This is promising because the sludge could be reused in successive cycles without the need of ferric ion addition and sludge disposal

201

. In another study, Yan et al.

202

showed that the use of fly-ash-based composite coagulant in the treatment of coal washing wastewater produced approximately 20 vol% of slurry even though the effluent produced was clear with low pollutant concentrations. The authors suggested the reuse of coal sludge as fuel after dewatering and drying as X-ray diffraction analysis showed that the sludge slurry have a high calorific value with potential applications in energy production 202. Composite, heating value and proximate analysis was also carried out on sludge obtained from coagulation-flocculation treatment of pulp and paper mill wastewater using both PAC and bagasse fly ash by Srivatava et al. 155. They found that the sludge produced has high amount of volatile matter and relatively high ash content with heating value approximately half of that of coal. However, the sludge could be


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dewatered by centri-clarifiers, dried, briquetted and incinerated to recover its energy content. The bottom ash produced after incineration could also be blended with organic manure as value added by-product 155.


7.

Conclusion, recommendations and future directions

Although with the development of new and improved treatment processes, coagulationflocculation still remains as an imperative process for treating industrial wastewater due to its simplicity in design and operation, low energy consumption and high versatility. Most of the studies summarized in this review paper reported the effectiveness of coagulation-flocculation process and it is believed that this technique could be applied either as a primary or/and tertiary treatment for most of the wastewaters. The efficacy of this treatment is greatly influenced by operating parameters such as coagulant/flocculant dosage, pH, stirring speed, stirring time, settling time, and in some cases, temperature. Hence, it is important to optimize the key process parameters before large-scale implementations. Integrating coagulation-flocculation process with different treatment methods is recommended towards increasing pollutants removal efficiencies in treatment of real industrial wastewater. Synergistic effects of the integrated treatment process were usually observed as coagulation-flocculation serves as a complement to other treatment process(es) such as membrane filtration, biological treatments, and even advanced oxidation process consisted of Fenton process, ozonation or photocatalysis. Therefore, it is a common practice to integrate coagulation-flocculation process with other treatment method to enhance wastewater treatment as summarized in Table 2.

[Insert Table 2 here]



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Performance improvement of the coagulation-flocculation process can be initiated through the introduction of coagulant or flocculant aids. Improvement in flocs characteristics, such as formation of denser and stronger flocs as well as increase of flocs settling velocity, were observed and reported by various researchers after the addition of flocculant aid 87,197. Additional advantages such as reduction of coagulant dosage, suitable application for broader pH range, and improvement in pollutant removal, were also obtained by using suitable flocculant aids

87

.

Currently, improving the overall process sustainability and viability has been the main aim in wastewater treatment and thus, research interests have been shifted towards the use of plant-based materials as a coagulant/flocculant. Although these materials are often meant as simple domestic point-of-use technology for developing countries, its non-toxic and biodegradable nature proved to be the edge over its inorganic counterparts which are often a source of concerns due to their potential toxicity. Despite its advantages, there are still many issues which hamper the development of these coagulants for commercialization such as the absence of mass plantation of the plants that affords bulk processing, perceived low-volume market and non-existent supportive regulation that stipulates the quality of processed coagulant extracts

33

. The discovery of new

coagulant extracts from plant species which are non-toxic and easily mass produced remained a crucial part for this study. Investigations into new and improved extraction and purification techniques for active coagulating agents are also important as it not only minimizes the increase in organic content in treated wastewater during its utilization, but also towards preparing the coagulants into a form which could be easily applied for industrial wastewater treatment purposes.

Recent years also showed the increase in studies on the development of hybrid materials such as PACS, PASiC, starch-g-PAM, cationic starch-chitosan crosslinking polymers and others due to its superior performance in wastewater treatment as compared to individual


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coagulant/flocculant. Significant enhancement of the aggregation ability could be achieved by introducing functional chemical groups or effective components into the original material through methods such as hydroxylation-prepolymerization, physical blending at ambient of elevated temperature, copolymerization and chemical grafting/crosslinking

213

techniques include conditioning using various salts along with acid

132

composite polymerization

101

. Other modification

, co-polymerization or

and immobilization 95. Although promising, further investigations

have to be conducted to confirm its economic viability and performance especially in the treatment of real industrial wastewater.

7.

Acknowledgement

The authors would like to thank Monash University Malaysia for providing Chee Yang Teh, Pretty Mori Budiman and Katrina Pui Yee Shak with PhD scholarships.



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673. (206) Sklyar, V.; Epov, A.; Gladchenko, M.; Danilovich, D.; Kalyuzhnyi, S. Combined Biologic (Anaerobic-Aerobic) and Chemical Treatment of Starch Industry Wastewater. Appl. Biochem. Biotechnol. 2003, 109, 253. (207) El-Gohary, F.; Tawfik, A. Decolorization and COD Reduction of Disperse and Reactive Dyes Wastewater Using Chemical-Coagulation Followed by Sequential Batch Reactor (SBR) Process. Desalination 2009, 249, 1159. (208) Diamadopoulos, E.; Megalou, K.; Georgiou, M.; Gizgis, N. Coagulation and Precipitation as Post-Treatment of Anaerobically Treated Primary Municipal Wastewater. Water Environ. Res. 2007, 79, 131. (209) Fan, L.; Nguyen, T.; Roddick, F. A. Characterisation of the Impact of Coagulation and Anaerobic Bio-Treatment on the Removal of Chromophores from Molasses Wastewater. Water Res. 2011, 45, 3933. (210) Chuang, S. H.; Chang, W. C.; Chang, T. C.; You, S. J. Improving the Removal of Anions by Coagulation and Dissolved Air Flotation in Wastewater Reclamation. Environ. Technol. 2006, 27, 493. (211) Aryal, A.; Sathasivan, A.; Vigneswaran, S. Synergistic Effect of Biological Activated Carbon and Enhanced Coagulation in Secondary Wastewater Effluent Treatment. Water Sci. Technol. 2012, 65, 332. (212) García-Morales, M. A.; Roa-Morales, G.; Barrera-Díaz, C.; Balderas-Hernández, P. Treatment of Soft Drink Process Wastewater by Ozonation, Ozonation-H2O2 and Ozonation-Coagulation Processes. J. Environ. Sci. Heal. Part A 2012, 47, 22. (213) Lee, K. E.; Morad, N.; Teng, T. T.; Poh, B. T. Development, Characterization and the Application of Hybrid Materials in Coagulation/flocculation of Wastewater: A Review. Chem. Eng. J. 2012, 203, 370.



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List of tables Table 1: Typical pH dependence coagulation process for various effluents Type of Wastewater

Initial pH of Effluent -

Coagulant (Optimum pH)

Finding (s)

Reference

Alum (3) Ferrous sulfate (9)

Asilian et al. 174

7

M. oleifera (7)

Domestic wastewater containing phosphorous

6.6-6.7

Alum (5.7-5.9)

Consumer products wastewater

11

Alum (10)

Aquaculture effluent

7.14

Alum (6.5-7.5) Ferric chloride (4-11)

Paper and pulp mill effluent

10.45

Aluminum chloride (4) PAC (5) Copper sulfate (6)

Palm oil mill effluent

4-5

Alkaline solution was added to adjust the pH of the raw effluent. Coagulation was enhanced under acidic environment due to the cationic character of M. oleifera protein. For low alkalinity wastewater, the optimum pH influenced the coagulant dosage and the removal of phosphorous content. pH 10 was chosen due to the closest similarity to the pH of raw effluent. Bench scale test would be required to find optimum pH for coagulation. pH value influences the coagulation efficiency due to the amount of functional groups that participated in the complexation and coordination of metal cations. In order to achieve 99% residual oil removal, the optimum pH value of effluent for each coagulant was found to be specific.

Dye wastewater (acrylic water base paint) Dye wastewater (Chicago Sky Blue)

Chitosan (4) Alum (4.5) PAC (4.5)


Beltrán-Heredia and SánchezMartín 37 Banu et al. 175

Jangkorn et al. 125

Ebeling et al. 156

Chaudhari et al. 76

Ahmad et al. 56

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Petrochemical wastewater containing purified terephthalic acid (PTA)

5.6

Ferric chloride (5.6) Ferrous sulfate (8.5) Alum (8.5) PAC (5.6)

The pH of raw Verma et al.124 effluent was measured at 5.6. Thus, the use of ferric chloride and PAC as coagulant was preferred due to the absence of pH adjustment.



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1

Table 2: Integration of coagulation-flocculation process with other treatments in wastewater treatment Integrated Type of Initial Coagulant Efficiency (%) Remarks Treatment Wastewater Parameter Coagulatio Integrated n Alone Treatment Coagulationflocculation (CF) with membrane filtration CF + filtration Secondary -Initial pH: 70 mg/L alum 95 (TP) 94.2 (TP) Slightly lower effluent 7.78 efficiency in the 94.2 (TP) -Initial COD: 40 mg/L ferric 95 (TP) integrated scheme, from chloride municipal 28.9 mg/L however, reduction in wastewater -Initial TSS: optimum dosage of 10.1 mg/L coagulant and treatment -Initial TP: flocculation time was plant 5.21 mg/L obtained as compared to coagulation alone. CF + Textile dye -Initial pH: 100 mg/L alum 20 37 (COD); microfiltration effluent 8.6 – 9.9 + 4 mg/L (Turbidity); 65 (Color) (MF) -Initial COD: flocculant 65 (COD) 1000-1200 mg/L -Initial turbidity: 45117 NTU Synthetic Ferric sulfate + 91-98 98 (Copper) Application of MF wastewater Magnafloc (Copper) alone achieved 90% of with humic LT25 (anionic copper removal. acids, flocculant) or kaolin PFS-PAM


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Reference

Citulski et al.

203

Harrelkas et al. 204

Zouboulis and Tzoupanos 101

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CF + ultrafiltration (UF)

CF + microfiltration (MF) using polyvinyliden e difluoride membranes with 0.22 µm pore size CF + nanofiltration (NF)


(clay), and CU ions effluent Textile dye effluent

Secondary effluent from biological wastewater treatment

Textile effluents

Textile wastewater

-Initial pH: 8.6 – 9.9 -Initial COD: 1000-1200 mg/L -Initial turbidity: 45117 NTU -Initial pH: 7.2 -Initial COD: 35 mg/L -Initial TP: 2.98 mg/L -Initial DOC: 6.29 mg/L -Initial pH = 6 -Initial dye content: 0.1 g/L

-Initial pH: 6.45-7.5 -Initial turbidity: 31-

100 mg/L alum + 4 mg/L flocculant

20 (Turbidity); 65 (COD)

42 (COD); 74 (Color)

-

Harrelkas et al. 204

30 mg/L PAC

84.3 (Turbidity); 35 (DOC); 100 (T-P)

84.3 (Turbidity); 100 (TP)

Coagulation process using PAC enhanced the extent of flux decline rate up to 88%

Park et al. 205

400 ppm of ferric chloride + 0.5 ppm anionic polyelectrolyte (HIMOLOC SS120) + 600 ppm resin 800 to 900 mg/L of alum + 3.4 mg/L of anionic

85 – 95 (Color)

98 (Color)

Application of NF Riera-Torres alone achieved 40-80% et al. 42 color removal

85% (Color); 2 NTU (Final turbidity)

100% CF + NF was operated (Color); at 10 bar trans60% (COD) membrane pressure and 40oC.


Ellouze et al. 44


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85 NTU polyelectrolyte -Initial COD: Magnafloc 919 227-627 mg/L Coagulationflocculation (CF) with biological treatment CF + aerobic treatment

Starch industry wastewater

-Initial 7.2 – 7.5 mg/l phosphate: of ferric 0.4 g/L chloride -Initial COD: 20 g/L mg/L

Purified terephthalic acid wastewater

-Initial pH: 5.3 -Initial COD: 9222 mg/L -Initial TS: 6123 mg/L -Initial BOD5: 4150 mg/L -Initial pH: 8.8-9.4 -Initial COD: 595 mg/L -Initial TSS:

Reactive dyes wastewater

500 mg/L PAC + 450 mg/L lime + 1.5 mg/L polyelectrolyte

-

1.4 mg of COD and 0.5 mg of phosphate could be removed by each mg of iron added. 63.1 (COD) 97.4 (COD)

200 mg/L alum 94 (Color); + 1 mg/L 44 (COD) polyacrylamide

68.2 (COD); 76.3 (BOD5); 61.4 (TSS)


Since COD and Skylar et al. 206 phosphate content remained quite high after aerobic treatment, coagulation was done to further treat the effluent. An increase of biodegradability from 0.45 to 0.67 was achieved in the integrated treatment.

Krishnan et al. 60

Treatment includes combination of alum and SBR process with 5 h HRT.

El-Gohary and Tawfik

207

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276 mg/L

CF + anaerobic treatment

Winery wastewater

-Initial COD: 34.88 g/L -Initial TSS: 5575 g/L

15 ml of calcium hydroxide (CaOH)

Cottontextile wastewater

-Initial pH: 8.5 -Initial TSS: 320 mg/L -Initial COD: 474 mg/L -Initial COD: 150-950 mg/L

800 mg/L ferrous sulfate + 1000 mg/L lime + 8 mg/L cationic polyelectrolyte 7 mg/L of alum -

Municipal wastewater

90.1 (Turbidity); 37.9 (COD); 84.1 (TSS) 70 (COD)

96.6 (Turbidity); 84.5 (COD); 99.1 (TSS) 80 (COD)

Additional removal of 35-65% TSS, 70% COD, and 80% turbidity.

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