Review on Greywater Treatment and Dye Removal from Aqueous

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Chapter 14

Review on Greywater Treatment and Dye Removal from Aqueous Solution by Ferrate (VI) S. Barışçı,* O. Turkay, and A. Dimoglo Gebze Technical University, Environmental Engineering Department, 41400, Gebze, Kocaeli, Turkey *E-mail: [email protected]

Greywater reuse is an attractive option for the conservation of water resources and sustainable environmental management. The gap between available water resources and water demand is growing significantly. Safe and adequate quantity of water is essential for human beings and their health. Appropriate hygiene and water resources management can be done by greywater reuse. Usually, greywater flow from a household is around 65% of the total wastewater flow. Therefore, greywater has a big potential for recycle and reuse. Dyes are an important class of contaminants, and can even be recognized by the human eye. Release of dye containing wastewaters to valuable water resources must be avoided. However, specific treatment technologies are not available for the removal of dyes. Fe (VI) has several advantages as its high redox potential and coagulant properties. Fe (VI) are able to oxidize organic and inorganic contaminants successfully with its unique properties. This chapter highlights and provides an overview of greywater treatment applications by Fe (VI) which is lack in the literature. Disinfection efficiencies of Fe (VI) on greywater treatment and the removal of other pollution parameters such as COD, BOD5, surfactant and TOC can be found in this chapter. Also, removal of dye contents such as indigo, methylene blue, orange II, azo dye X-3B, acid yellow-36 by Fe (VI) and the applications of Fe (VI) for the treatment of real dye wastewater were evaluated in

© 2016 American Chemical Society Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

this chapter. Kinetic studies and treatment efficiencies for dye removal are also complied. In addition, many other methods used for the removal of dyes are also complied in brief. From a comprehensive literature review, it was found that Fe (VI) has fast kinetics and significant oxidation and adsorption capacities on both greywater treatment and dye removal.


Introduction Rising urbanization and increasing stress on water resources via domestic, industrial, commercial, and agricultural consumption cause water insufficiency in all over the world. Imbalance between rapid growth and fresh water amount has focused attention from both scientific researchers and the public to alternative water sources (1); this includes water recycling. The emphasis is, however, is mostly on domestic wastewater (2) due to causing water pollution significantly (3). For this reason, reclaimed water should be considered as a new source for potential use in many areas such as industry, domestic, and agriculture. Due to decline in availability of freshwater sources, it is important to search for affordable, implementable, and safe solutions to alleviate water problems. “Guidelines for Safe Use of Wastewater, Excreta and Greywater,” published by the World Health Organization (WHO), highlights the significance of greywater as an alternative water resource. According to the WHO, greywater can contribute to decreasing water stress as: (a) it is still water, (b) it has the largest volume of the waste flow from households, (c) it has nutrient content and it can be beneficially used for crop irrigation, (d) it has low pathogen content compared to black water, and (e) it can reduce the demand for the first use water (4). Dye containing wastewaters, especially from textile industry, are among the most contaminated ones because they contain high concentration of non-biodegradable compounds, high temperature and pH values and persistent color. Due to their persistent color, dye effluents may prevent light penetration in water bodies. Additionally, this type of wastewaters may contain toxic, carcinogenic and mutagenic chemicals which affect aquatic organisms adversely. Therefore, dyeing process has one of the biggest risk to the environment. Dyes are used in many industries such as textile, food, plastic, furniture, film, and printing house extensively. Dyes are retained on the substrates through adsorption, making covalent bonds with other compounds and forming compounds with salts and metals. The consumption of dyes for cellulosic fibers are around 60 000 tonyear1- in the World (5). Dye fragmentation consist of the chromophores and the auxochromes as two fundamental components. The first one is responsible for making the colour and the second is for supplement the chromophore together with rendering the molecule soluble in water and giving improved affinity to the fibers. The classification of dyes can be in several ways due to their structures which show diversity. 350 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Due to their toxicity and persistent color, special technologies should be applied for the removal of dyes. Adsorption, biological, electrochemical and photochemical processes are amongst the applied technologies so far (6, 7). Fe (VI), due to its unique properties has been used for many environmental applications. Application of Fe (VI) is a promising technology for the removal of dyes by its oxidant and coagulant functions. Whereas Fe (VI) oxidizes pollutants as it has high redox potential, improves coagulation efficacy. Additionally, the formation of toxic by-products does not seem in Fe (VI) process. Several reactions may occur during its use for the removal of organic or inorganic matters. These reactions can be Fe (V) and Fe (IV) generation by 1 and 2 electron transfer, production of radicals, self-decay of Fe (VI), reactions between formed Fe (VI) species and contaminants, reducing reactions resulted in the formation of Fe (III) and Fe (II) and reactions between Fe (VI) species and oxygen. The reaction mechanism between Fe (VI) species and contaminants depends on the formed Fe (VI)s and the structure of contaminants. pH of the medium is very key parameter for describing the reaction mechanism as Fe (VI) species strongly depend on pH, so as the structure of contaminant can also be differentiated according to pH. This chapter aims to understand the efficiency and the mechanism of Fe (VI) for the treatment of two different wastewater types: greywater and dye wastewater. Although both types of wastewater contain organic substances, they differ from each other in terms of ingredient, color, and pollution load. Furthermore, these two types of wastewater are different in terms of treatability, as greywater is easy to treat, while dye wastewater is more challenging to treat. Therefore, the treatment of these kinds of wastewater with Fe (VI) is combined and investigated in this chapter.

Greywater Sources, Characteristics and Reuse Potential Greywater is produced as a consequence of the lifestyles of habitants involved, the products used, and the nature of the installation; therefore, its characteristics are highly variable (8). The quantity of produced greywater also depends on living standards, culture, and so on. Depending on its source, greywater can be divided into different categories such as bathroom, laundry, kitchen, washbasin, and mixed origin. The characteristics of greywater sources can be seen in Table 1. The greywater sources are highly important for assessment for its possible reuse. Greywaters are defined as high-load and low-load according to their contaminant concentrations (see Table 2). High-load greywater contains wastewater coming from kitchen, washing machine, and dish washer sources and presents complex chemical composition including contaminants such as detergents, soaps, personal-care products, and other chemicals (9, 10).

351 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

Table 1. General characteristic of greywater sources.


Parameter

Bathroom

Laundry

Kitchen

Mixed

pH

6.4-8.1

7.1-10

5.9-7.4

6.3-8.1

TSS (mg L1-)

7-505

68-465

134-1300

25-183

Turbidity (NTU)

44-375

50-444

298

29-375

COD (mg L1-)

100-633

231-2950

26-2050

100-700

BOD (mg L1-)

50-300

48-472

536-1460

47-466

TN (mg L1-)

3.6-19.4

1.1-40.3

11.4-74

1.7-34.3

L1-)

0.11->48.8

ND->171

2.9->74

0.11-22.8

Total Coliform (TC) (CFU 100 mL1-)

10-2.4x107

200.5-7x105

> 2.4x108

56-8x107

Fecal Coliform (CFU 100 mL1-)

0-3.4x105

50-1.4x103

-

0.1-1.5x108

TP (mg

Table 2. Greywater sources and their constituents. Greywater High-load Kitchen sinks: Contains food residues, high amounts of oil and fat, dish washing detergents.

Low-load

Laundry: Contains soap, bleaches, oils, paints, solvents, and non-biodegradable fibres from clothing.

Bathroom: Contains soaps, shampoos, body care products, hair, body fats, lint, and traces of urine.

Washbasin: Contains soaps, toothpaste, body care products, shaving waste, and hair.

Average COD values of high-load greywater vary from 483 to 1164 mg L1-. While COD concentration of greywater coming from kitchen or mixed origin (bathroom and kitchen) varies between 483 and 644 mg L1, greywater from washing machines has a COD value of 1164 mg L1- demonstrating high concentrations of detergents and chemicals in washing waters. Phosphorus and nitrogen concentrations also represent differences from source to source. The majority of nutrients (N and P) comes from kitchen sinks in greywater flow (11). NH4+-N concentration is 5.7 mg L1- for greywater coming from kitchen and bathroom, while it is 2 mg L1- for washing machine based greywater. Phosphorus concentrations are also high in almost all sources, at 7.4 mg L1- for bathroom and kitchen water, 26 mg L1- for kitchen greywater, 8.4 mg L1- for mixed origin, and 21 mg L1- for washing machinewater (1, 11–13). Low-load greywater is sourced from bath, shower, and wash basin wastewater and it includes naturally low concentrations of pollutants compared to high-load greywater, domestic wastewater, and blackwater. Average COD concentrations are given in a range between 244 and 371 mg L1-. Ammonia concentration is 0.3 352 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

mg L1- in handbasin greywater, while it is higher in bathroom- and shower-based greywater because of urine. Phosphorus concentrations range between 2.58 mg L1and 19.2 mg L1-. Fecal contamination can also be found in low-load greywater, though in lower amounts than in blackwater (14–16).


Greywater Treatment Technologies There are numerous technologies applied for greywater treatment; these include a wide range of alternatives, such as physicochemical, biological, and advanced oxidation technologies. Most of these technologies are followed by a solid–liquid separation stage as pre-treatment, followed by a disinfection stage as a post treatment. To elude the clogging of the following treatment, pre-treatments such as septic tank, filter bags, screens, and filters are applied to diminish the amount of particles, oil, and grease. The disinfection stage is used to meet the microbiological necessities. Characteristics of grey water and reuse purposes are the main factors taken into consideration in determining suitable greywater treatment technology. Consequently, implementing combinations of these different technologies is also another way favored in greywater treatment in order to get positive results (17, 18). Physicochemical Treatment Technologies Coarse filters, membrane filters, and other natural environments such as sand and mulch are known to be different physical treatment technologies followed by a disinfection step for removing pathogens. Application of filtration and sedimentation followed by disinfection has been applied for the treatment of greywater from bathtubs and hand-washing basins and process efficiency was assessed by turbidity, suspended solids, total nitrogen, TOC and COD (15). It was also reported that the treated greywater could be used for toilet flushing. Low-load greywater was treated by coagulation and chlorination, and the treatment efficiency was reported by similar parameters representing organic content (e.g. COD) as well as by hardness causing divalent cations (19). In a recent study reported by Pidou and colleagues, greywater obtained from showers was treated by coagulation and magnetic ion exchange resin. Treatment efficiency has also been expressed by the removal of coliform bacteria (18). However, physical treatment options alone are not effective for removing pollutants such as organics, nutrients, surfactants, and other micropollutants (e.g., xenobiotic organic compounds and metals). Biological Treatment Processes Biological treatment processes are known to be one of the most common greywater treatment technologies studied and applied in the literature. Common biological treatment processes used for greywater treatment are membrane 353 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


bioreactors (MBRs), rotating biological contactors (RBCs), and constructed wetlands. This section presents a brief literature review on biological treatment processes and their application in greywater treatment. MBR systems are commonly used for greywater treatment and reuse purposes. Several studies showed that MBR technologies used for greywater treatment resulted in desirable reuse standards due to its stability and pathogen removal. Hence, different MBR technologies for greywater treatment have been studied with small modifications such as different solid retention times (SRT) and hydraulic retention times (HRTs) (13), submerged membrane-sequencing batch reactor (SM-SBR) (20), and HUBER-MBR process (21). RBCs also have been applied for grey water treatment in several studies (22, 23). Although biological treatment processes are commonly studied and applied for greywater treatment, its operation would have some limitations in terms of removal of non-biodegradable, xenobiotic and toxic compounds that are quite resistant and have inhibitory impact on the bacterial activity of biological treatment systems (24). Up-flow anaerobic sludge blanket (UASB) reactors (25) and recycled vertical flow bioreactors (RVFBs) (26) are some other different biological treatment technologies investigated for greywater treatment.

Advanced Oxidation Processes Although physicochemical and biological treatment processes have displayed appropriate results in greywater treatment, they are not sufficient for removing several micropollutants that are substantially resistant and toxic. Accordingly, treatment methods other than conventional ones should be investigated in order to remove those micropollutants. At this point, advanced oxidation processes (AOPs) may be taken into consideration in order to overcome such a problem. AOPs are known to be some of the most innovative water and wastewater treatment technologies in which the main mechanism is production of highly reactive transitory oxygen species (ROS) (H2O2, OH•, O2•-, O3) that are able to degrade even the recalcitrant organic molecules into biodegradable compounds, and eventually mineralize them to water, yielding CO2 and inorganic anions. Among these reactive oxygen species, hydroxyl radical is the most powerful oxidizing agent. Homogeneous photolysis, heterogeneous photolysis or photocatalysis, Fenton process, photo-Fenton process, dark oxidation processes, hydrothermal oxidation, wet oxidation, radiolysis, and sonolysis are some of the AOPs classified and studied throughout the literature (27–29).

Greywater Treatment by Fe (VI) Fe (VI) has a great potential with high oxidizing capability and coagulant properties. While Fe (VI) oxidizes contaminants, the by-product produced by its self-decay can remove the contaminants simultaneously. The decomposition product of Fe (VI), Fe (III), is non-toxic and is a common coagulant used in water and wastewater treatment. 354 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Although, there are limited studies on greywater treatment by Fe (VI), the results of the current studies show that the usage of Fe (VI) for greywater treatment is efficient and applicable. Table 3 presents the comparison of Fe (VI) and some other technologies used in greywater treatment. As seen in Table 3, the sand filter and granular-activated carbon system was not effective for greywater treatment and the treated greywater did not meet the reuse standards. However, the UV/H2O2 process provided 86.7% COD removal efficiency, and the UV/TiO2 process resulted in a 54% reduction in COD value, which is not suitable for non-potable reuse. On the other hand, both COD removal and MBAS degradation efficiencies were considerably improved by the synergistic effect of Fe2+ through the Fe2+/Fe0/H2O2 system. Electrocoagulation was also found to be effective for COD, MBAS, and turbidity removal from greywater. The highest turbidity removals were achieved with the Fe (VI), Fe (VI)/Al(III), electrocoagulation, and recycled vertical flow bioreactor (RVFB) systems. It seems that Fe (VI) and electrocoagulation met reuse standards, as the COD, turbidity, and MBAS values were below 160 mg L1-, 2 NTU, and 1 mg L1-, respectively. Additionally, the Fe (VI)/Al(III) system offers reuse applications with 3 mg L1- COD and 0.5 NTU turbidity in the treated greywater. The Fe2+/Fe0/H2O2 system also meets the reuse standards in terms of COD and turbidity; however, MBAS was still very high after the treatment of greywater due to a high initial MBAS value. In the following sections, the treatment of greywater from two different sources (domestic and restaurant) is described in detail.

Restaurant Greywater This section assesses the treatment of greywater from a restaurant by electrosynthesized Fe (VI) considering removal of chemical oxygen demand (COD), total organic carbon (TOC), turbidity, anions (SO42-, NO3-, NO2-, PO43-, F-, and Cl-) and anionic surfactant (MBAS) for its possible real-scale applications.

Degradation of Organics and Removal of Physical Contaminations The performance of Fe (VI) in greywater treatment considering water quality parameters such as COD, TOC, and turbidity have been evaluated (35). The results indicate that the removal rates increased significantly after treatment by Fe (VI) along with the applied Fe (VI) dose ranging from 2–100 mg L1- (see Figure 1a). Low COD removal (37%) occurred with a Fe (VI) dose of 2 mg L1-, demonstrating low degradation of the pollutants in the greywater. Substantial COD removal (70–89%) was observed with a 20 mg L1- dose of Fe (VI) and its increasing concentrations. The COD value reduced to 105 mg L1- at Fe (VI) dose of 100 mg L1-, corresponding to 89.3% COD removal. In addition, the decrease in applied Fe (VI) dose to 75 mg L1- did not induce significant change. It can be said that most of the pollutants were oxidized by Fe (VI). TOC was removed in the range of 21.6-61.7% according to the applied Fe (VI) dose. 355 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

356


Table 3. Literature review of greywater treatment. Untreated GW

Treated GW

Process

Conditions

COD (mg L1-)

Turbidity (NTU)

MBAS (mg L1-)

TSS (mg L1-)

COD (mg L1-)

Turbidity (NTU)

MBAS (mg L1-)

TSS (mg L1-)

References

UV/H2O2

3 h UVC/10 mM H2O2, pH 10

225

18.10

-

19

30

-

-

-

(27)

Anaerobic filter followed by UV disinfection

_

170

40.40

-

76

48

4.80

-

17

(30)

Sand filter and granular activated carbon (GAC)

Filtration rate of 6 m3/m2/day, pH 7, permeability of sand 2.7x1010- m2-, permeability of GAC 1.8x109- m2-

350

263

14.84

-

221

108

6.64

-

(31)

339

-

12.30

46

46.60

-

0.20

3

(1)

Recycled vertical flow bioreactor (RVFB) UV/TiO2

330 min of process time, TiO2 loading of 0.1 g/L, pH 5

620

-

-

-

285

-

-

-

(32)

Electrocoagulation (EC)

30 min operating time, Al-Fe-Fe-Al electrode combination, current density of 1 mA/cm2, pH 7.6

229

53.40

72

-

4.40

0.45

0.78

-

(33)

Zero valent iron (ZVI)-mediated Fenton-like system

120 min process time, 0.5/18/15 mM Fe2+/Fe0/H2O2, pH 3-6.5

301

8.80

174

-

66.20

1.94

13.92

-

(34)

ZVI/H2O2

120 min process time, pH 3

301

8.80

174

-

135.45

0.44

83.52

-

(34)

Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

Treated GW

Conditions

COD (mg L1-)

Turbidity (NTU)

MBAS (mg L1-)

TSS (mg L1-)

COD (mg L1-)

Turbidity (NTU)

MBAS (mg L1-)

TSS (mg L1-)

References

Electrosynthesized Fe (VI)

75 mg/L Fe (VI), pH 7

984.60

303.07

14.55

-

105

0.30

0.23

-

(35)

Fe (VI) and Al(III) salt

25/25 mg/L Fe (VI)/Al(III) salt, pH 6.5

151.50

36.50

-

-

3

0.50

-

-

(36)

Electrosynthesized Fe (VI)

75 mg/L Fe (VI), pH 7

270

50.10

29.80

-

30.60

0.23

8.05

-

Unpublished data

357


Untreated GW Process



Figure 1. COD, TOC and Turbidity removal efficiencies at corresponding (a) Fe (VI) dose at pH 7 and (b) pH with 75 mg L1- Fe (VI) dose. (Adapted with permission from reference (35). Copyright 2016 Taylor and Francis).



Figure 1a also shows the removal efficiencies for turbidity. Turbidity removal rates were between 97–99.9% along with the applied Fe (VI) dose. A relatively low Fe (VI) dose (2 mg L1-) appeared to be very effective for the removal of turbidity. Thus, it can be concluded that the coagulation effect of Fe (VI) is very high, which provides the efficient removal of particular matter. However, the COD and TOC removals were only 37% and 21.6%, respectively, under the same conditions, as mentioned, because the oxidation effect of Fe (VI) remained low at the 2 mg L1dose compared with the coagulation effect. Additionally, the removal rates were almost unaffected for applied Fe (VI) doses. The effect of pH on greywater treatment by Fe (VI) can be seen in Figure 1b. The decrease in COD value was higher in the pH range of 6–9. The highest COD removal efficiencies were achieved at pH 7 and 8. Turbidity removal efficiency ranged between 87.93–99.56%. It was clear that all pH values were effective for turbidity removal. These results for greywater treatment are different from those of other AOPs as they are more effective under acidic conditions (37–39). The decay of organics at different pH values may be clarified by the higher redox potential and higher stability of Fe (VI). Furthermore, the Fe (VI) process reaches the maximum sorption capacity between pH 7 and 8, as the by-product of Fe (VI) decay, Fe(OH)3, is produced, and this mechanism provides coagulation and precipitation of organics. Thus, degradation is not the only mechanism in this case; the sorption mechanism is also involved in the process.

Degradation of Anionic Surfactant (MBAS) Surfactants may have adverse effects when they are present in irrigation water. It is suggested that the concentration of MBAS should not exceed 1 mg L (1–17). Fe (VI) was very effective for the degradation of MBAS, with more than 98% degradation efficiency obtained (35). When a Fe (VI) dose increased from 20 to 100 mg L1-, MBAS concentration decreased from 0.63 to 0.23 mg L1-. The degradation of MBAS increased with the increase in Fe (VI) concentration (see Figure 2). The degradation of MBAS was affected by the change of pH as Fe (VI) process is pH dependent (see Figure 2). The removal rate reached its maximum value (96.08%) at pH 6. The removal rate was lower in both acidic (pH 4 and 5) and basic conditions (pH 9 and 10). It can be concluded from this that the reactivity of Fe (VI) species at mid-range of pH (6–8). HFeO4- is the dominant species at mildly acidic and neutral conditions, and it is known that the protonated form of Fe (VI) (HFeO4-) reacts faster than the its unprotonated form (FeO42-) (40). Therefore, HFeO4- can oxidize MBAS easily. In the case of pH 8, FeO42- is the more dominant Fe (VI) form, and the sorption mechanism also takes place, providing efficient removal. Additionally, surfactants are typically amphiphilic organic compounds meaning that they contain both hydrophobic and hydrophilic groups (their tails and heads, correspondingly). Hence, with the produced Fe(OH)3 particles in the Fe (VI) process, hydrophobic surfactant adsorbs onto the solid particles. The particles 359 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


collide and aggregate in hydrophobic–hydrophobic association by flocculation/ coagulation and can be removed from greywater.

Figure 2. Degradation of surfactant (□ shows the effect of Fe (VI) dose and ▵ shows the effect of pH). (Adapted with permission from reference (35). Copyright 2016 Taylor and Francis).

Removal of Anions The Fe (VI) process was found to be efficient in the removal of anions, providing a >65% removal rate (35). A significant effect of pH on the removal of anions was observed. The efficiency was lower at acidic pH values than neutral and basic values. This was due to the formation of Fe(OH)3 during the process. Iron hydroxide provides sweeping floccules with a large surface area, and this is advantageous for the rapid adsorption of anions. However, PO43- and F- removals were greater than the other anions due to the solubility of NO3-, NO2-, and SO42-, which are more than PO43and F-. Sweeping floccules, which are produced by Fe (VI) decay, can easily remove those relatively insoluble anions. From this behavior, it can be concluded that anions (An) adsorb into formed colloidal particles and make granules that aggregate easily due to charge neutralization according to the following equation:



The particle Fe(OH)3/FeO+ is formed at the first stage of coagulation. It adsorbs (n-x)An- and produces a granule. After that, diffusive layer with xAnis formed, which suggests the formation of the particles’ isoelectric points. All this causes gluing of the colloidal particles and the growth of their size, which improves anion removal efficiency.

Particle Size Distributions and Zeta Potentials The coagulation effect due to Fe (VI) decay alters the particle size distribution. According to the Figure 3, much larger particles were observed, with an average size of over 500 nm, after Fe (VI) process compared to raw greywater solution with 59-nm particle size (35). This was due to higher Fe(OH)3 precursor mass production leading to a higher collision frequency. In addition, due to increasing Fe (VI) concentration, the closer proximity of Fe(OH)3 particles results in higher collision frequency, which enhances the flocculation process. The dependency of floccules’ size on Fe (VI) dose shows a similar parabolic trend for 75 and 100 mg L1-.

Figure 3. Effect of Fe (VI) dose on particle size number distributions. (Adapted with permission from reference (35). Copyright 2016 Taylor and Francis). 361 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Additionally, zeta potential (ZP) values varied between -3.65 and -0.99 mV. This specifies that the solution was at rapid coagulation or flocculation. The applied Fe (VI) dose affected ZP values as reaching values of isoelectric point at a Fe (VI) dose of 75 mg L1-. Charge reversal was not observed with further increase in Fe (VI) dose. ZP points increase to charge neutralization, which takes place together with sweep coagulation and adsorption. The effect of pH was also observed, and the ZP of floccules increases progressively during the process for pH values in the range of 4–10. It was observed that pH 7 and 8 provided larger floccules and ZP values closer to zero. This is because HFeO4- reacts with contaminants more rapidly than FeO42- does. Density functional theory calculations of Fe (VI) reactivity with pollutants have shown that the protonated Fe (VI) form has a larger spin density on the organic pollutants than the unprotonated Fe (VI) form, and this increases the oxidation ability of protonated Fe (VI) (41, 42).

Domestic Greywater Degradation of Organics and Removal of Physical Contaminants Removal efficiencies for key wastewater parameters such as COD, TOC, and turbidity by electrosynthesized Fe (VI) ion are shown in Figure 4. In terms of physical impurities, Fe (VI) process showed excellent removal efficiencies for turbidity with all applied Fe (VI) doses, as shown in Figure 4a. Corresponding removal efficiencies varied between 97.8–99.54%. When pH effect considered for turbidity removal, high removal efficiencies were gained, too. However, the efficiency was relatively lower for pH 10, as shown in Figure 4b, with 95.4%. In terms of organic matter, the treatment by electrosynthesized Fe (VI) ion process reduced COD up to 88.67% with a 100 mg L1- Fe (VI) dose (see Figure 4a). COD removal efficiency was affected by the increase of Fe (VI) dose. Only 47.3% removal was provided with 2 mg L1- Fe (VI) dose. TOC removal efficiency showed a similar trend to that of COD removal. TOC removal efficiency varied between 40.2–73.8% according to the applied Fe (VI) dose. The highest TOC removal was gained at pH 7 and 8. This can be attributed to the coagulation effect of Fe (VI) together with the oxidizing effect. When two types of greywater (restaurant and domestic) treatments are compared in terms of COD and TOC removals, the removal efficiencies showed similar trends in the same conditions. Furthermore, the initial values of the target parameters. such as COD and TOC, were different for each type of greywater. For example, the initial COD values were 984.60 mg L1- and 270 mg L1- for the restaurant and domestic greywater samples, respectively. Thus, the COD value decreased to 105 mg L1- for the restaurant case, with an almost 89% removal ratio, and it decreased to 30.6 mg L1- with an 88% removal ratio for the domestic case. These two values met the greywater reuse guidelines (≤160 mg L1-). In conclusion, Fe (VI) was effective for varied initial COD values (i.e. low and higher values). 362 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 4. Turbidity, COD and TOC removal efficiencies according to (a) Fe (VI) dosage at pH 7 and (b) pH with 75 mg L1- Fe (VI) dosage.

Degradation of Biochemical Oxygen Demand (BOD) Treatment of domestic greywater source with Fe (VI) showed good performance for BOD removal (see Figure 5) as the removal efficiency reached 92%. The treatment appeared to be dependent on Fe (VI) dose and pH. 363 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Corresponding removal efficiencies varied between 70–92% for applied Fe (VI) doses, which reflects the low level of BOD in the influent. As mentioned, pH affected BOD removal efficiency. Residual BOD concentrations varied between 9.6–37.2 mg L1- under all conditions tested in terms of pH. The best performance for BOD removal was obtained via the pH range of 6–8, which was similar to COD and TOC removal. On the basis of BOD removal data, it can be stated that the reduction of most of the organic suspended solids (70%) was obtained in domestic greywater using only 2 mg L1-Fe (VI) dosage. When Fe (VI) dose increased to 75 mg L1-, the efficiency reached to 92%. Applying higher Fe (VI) dose, higher BOD removal was achieved.

Figure 5. BOD removal efficiencies (□ shows the effect of Fe (VI) dose and ▵ shows the effect of pH).

Degradation of Anionic Surfactants (MBAS) Figure 6 specifies surfactant degradation according to Fe (VI) dose and pH. Increasing Fe (VI) dose showed increasing surfactant removal efficiency. A minimum applied Fe (VI) dose (2 mg L1-) provided about 48% surfactant removal. After 75 mg L1- Fe (VI) dose, a stable stage for the removal efficiency was observed. Almost 73% surfactant removal was obtained with Fe (VI) treatment process. pH had significant effect on MBAS removal. Just like TKN and TP removal, pH 7 showed the best performance for surfactant removal. All applied pH values provided more than 50% MBAS removal efficiency.



Figure 6. Surfactant removal efficiencies (□ shows the effect of Fe (VI) dose and ▵ shows the effect of pH).

In comparison with the two types of greywater, Figures 3 and 7 show MBAS removal, and it can be seen that the removal efficiencies were different from each other (96.08% and 73.0% for restaurant and domestic greywater, respectively) because the initial concentration of the surfactant was different for each greywater sample. The values were 14.55 mg L1- for the restaurant and 29.80 mg L1- for the domestic greywater. With increasing MBAS concentrations, the effectiveness of Fe (VI) decreased. There has been no study about greywater treatment with Fe (VI) to compare MBAS removal; however, in comparison with other oxidants, H2O2 has been used for MBAS removal (initial concentration of 174.24 mg L1-). The study shows that MBAS removal achieved only about 20% with H2O2 (34).

Removal of Total Kjeldahl Nitrogen (TKN) and Total Phosphorus (TP) Figure 7a shows the effect of Fe (VI) dose on TKN and TP removal efficiencies for greywater treatment. According to Figure 7a, about 55% TKN removal and more than 90% TP removal efficiencies were gained with 100 mg L1Fe (VI) dose. Increasing Fe (VI) dose provided better efficiencies for both TKN and TP parameters. It can be said that after 50 mg L1- Fe (VI) dose, the removal of TP reached to stable stage. However, TKN removal showed increasing trend with higher doses of Fe (VI).



Figure 7. TKN and TP removal efficiencies according to (a) applied Fe (VI) dose at pH 7 and (b) the change of pH at 75 mg L1- Fe (VI) dose.



According to Figure 7b, pH was very effective for TKN and TP removal. A neutral pH value showed the best removal efficiency for both parameters. The removal trends were almost identical for TKN and TP removal. The efficiencies increased up to pH 7 and then decreased with increasing pH values. In the literature, the studies on greywater treatment by using alum and iron chloride indicate that 420 mg L1- alum concentration provided 14.93% TKN removal and 94.58 TP removal and 150 mg L1- FeCl3 provided 8.96% and 96.39% TKN and TP removal, respectively (18). In the case of Fe (VI), as mentioned above, higher removal efficiencies were gained with relatively low Fe (VI) doses, particularly for TKN. Only 2 mg L1- Fe (VI) dosage delivered more than 10% TKN removal from greywater samples, while almost 20% TKN removal was gained with 5 mg L1- Fe (VI) dosage. This proves that Fe (VI) with coagulation properties together with high oxidizing capacity can be used instead of well-known coagulants in water and wastewater treatment. The effect of pH on removal efficiencies was significant for all measured parameters. According to the results, neutral pH values are the most suitable values for greywater treatment. It can be concluded that Fe (VI) has a higher oxidation potential at low pH values than in alkaline media. When pH < 5, the self-decay of Fe (VI) occurs, and this phenomenon causes the incomplete degradation of contaminants by Fe (VI). Furthermore, the precipitates of Fe (III) hydroxides could not form in an acidic condition. When the pH increased to 7, Fe (VI) possessed strong oxidation capability, since Fe (VI) presented a high protonation grade with strong oxidation ability. However, at higher pH values, Fe (VI) was quite stable and the oxidizing ability of Fe (VI) was weak. Additionally, the coagulation of Fe (III) hydroxides played a key role. A study investigating greywater treatment with the Fe (VI) and Al (III) system also indicates that pH 6.5 is the most effective value for providing the efficient removal of contaminants in greywater (36).

Removal of Pathogens Attempts to identify pathogens in domestic greywater revealed that total coliform (TC) was present in all the greywater samples tested, at a mean concentration of 5.2±0.34 log10CFU 100 mL1-. TC survival in Fe (VI) treated greywater was greater with increasing contact time. As seen in Figure 8, TC reduced to 4.1 from 5.2 log10CFU 100 mL1- at 1 min, which means that only 21.2% removal was provided. However, TC reduced to 0.052 and 0.0052 log10CFU 100 mL1- with 99 and 99.9% removal at 9 and 10 min, respectively. Longer Fe (VI) contact times resulted in less TC survival in domestic greywater. Fe (VI) was very successful for inactivation of TC. The process of Fe (VI) can be used for wastewater disinfection according to the results.



Figure 8. Effect of contact time for inactivation of TC by Fe (VI) (experimental conditions: 75 mg L1- Fe (VI) dose and pH 4). In Figure 9, the effect of Fe (VI) dose at pH 4 and 10 min contact time can be seen.

Figure 9. Effect of Fe (VI) dose for inactivation of TC (experimental conditions: 10 min contact time and pH 4). The lowest Fe (VI) dose (2 mg L1-) gave the poorest removal of indicator bacteria. To illustrate, TC reduced to 4.5 log10CFU 100 mL1- from the mean value of 5.2 log10CFU 100 mL1- in 10 minutes’ contact time. However, Fe (VI) dose increased to only 5 mg L1- and TC reduced to 2.4 log10CFU 100 mL1-. TC 368 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


removal by higher doses of Fe (VI) such as 10 and 20 mg L1- seemed to be efficient for providing some type of reuse standards. To illustrate, TC reduced to 1.1 and 0.84 log10CFU 100 mL1-with 10 and 20 mg L1- Fe (VI) doses, respectively. This means that treated greywater can be used for construction facilities such as soil compaction, dust control, washing aggregate, and making concrete. The effect of pH can be seen in Figure 10. Acidic and mildly acidic pH conditions were more effective than basic conditions for the inactivation of TC. To illustrate, in greywater samples, TC reduced to 0.0052 log10CFU 100 mL1- at the minimum studied pH value (4), and at pH 5, to 0.891 log10CFU 100 mL1- TC. Determined TC values were 0.98, 1.12, 1.76, 2.89, and 3.12 log10CFU 100 mL1at pH 6, 7, 8, 9, and 10, respectively.

Figure 10. Effect of pH for inactivation of TC (experimental conditions: 75 mg L1- Fe (VI) dose and 10 min contact time). The above studies show that Fe (VI) technology is closely matched to some types of reuse standards when TC removal considered. In other words, Fe (VI) was very efficient for the disinfection of greywater considering the established quality standards for reuse. If treated greywater contains ≤100 CFU 100 mL1TC, it can be used for ornamental fountains, recreational impoundments, lakes and ponds for swimming, ponds for recreational uses without body contact, toilet flushing, laundry; air conditioning, process water, landscape irrigation, fire protection, construction, surface irrigation of food crops and vegetables (consumed uncooked), and street washing; landscape irrigation where public access is infrequent and controlled, and subsurface irrigation of non-food crops and food crops and vegetables (consumed after processing). However, the other parameters such as BOD, TN, TP, turbidity, pH, TSS, and surfactants should also provide the quality standards for these intended purposes. The efficiency of greywater treatment and/or disinfection in practice should be evaluated by risk assessment in terms of pathogen transmission from greywater 369 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


reuse. The different risks associated with urban water reuse applications are revealed in many guidelines. Disinfection of all treated effluents is expected to ensure the strictest microbiological standards for reuse. The occurrence of pathogenic protozoa and/or viruses in treated greywater effluent intended for reuse will impact the health risks associated with reuse. In addition, the quality of the treated greywater effluents will influence subsequent disinfection and the potential for regrowth of bacteria. The superior removal of organics by Fe (VI) diminishes the potential for regrowth and reduces the demand for chemical disinfectant. Regrowth of TC bacteria was investigated for 0.5, 3, 6, and 12 h after Fe (VI) treatment and for untreated greywater samples. As seen in Figure 11, TC did not exhibit regrowth ability after treatment at all applied Fe (VI) doses. When only 2 mg L1- Fe (VI) dose was used for disinfection of domestic greywater, after 0.5 h, TC remained at the same value and after 12 h, TC increased to 4.93 log10CFU 100 mL1- from 4.5 log10CFU 100 mL1-.

Figure 11. TC concentrations as a function of Fe (VI) dose and storage time (before and after treatment).

TC increased to 2.78, 1.45, 0.95, 0.523, 0.005255, and 0.00133 log10CFU 100 mL1- from 2.4, 1.1, 0.84, 0.5, 0.0052, and 0.0013 log10CFU 100 mL1-, respectively, with 10, 20, 50, 75, and 100 mg L1- applied Fe (VI) dose, respectively. However, TC regrowth showed a significantly increasing trend for untreated greywater. TC increased to 5.4, 5.8, 6.1, and 6.45 log10CFU 100 mL1- from 5.2 log10CFU 100 mL1- in untreated stored greywater after 0.5, 3, 6, and 12 h, respectively. Available carbon sources and nutrients in untreated greywater samples may cause TC regrowth. 370 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Reactivity of Fe (VI) in Greywater It is known that the stability of Fe (VI) in aqueous solution is low and that Fe (VI) will rapidly reduce to Fe (III) or insoluble Fe(OH)3. Mössbauer spectroscopy was used to define the oxidation states of iron in greywater samples after treatment by Fe (VI). The Mössbauer spectrum concludes the parameters including isomer shift (δ), which varies with the valence of iron in the sample (43). The spectrum of iron species displays that the Fe (VI) partly reduced to doublets of Fe (III). The values of the isomer shift (δ) were extremely sensitive to the oxidation state (OS) of iron and δ values reduced with increasing OS. There was no Fe (II) as a final product; in other words, the only final product of the reaction was Fe (III). Most of the Fe (VI) was consumed while oxidizing pollutants in greywater solution. When Fe (VI) reacted with contaminants in greywater, numerous reactions occur between Fe (VI) and contaminants. These reactions contain the formation of Fe (V) and Fe (IV) via 1-e− and 2-e− transfer processes, radical species that can also produce Fe (V) and Fe (IV), additional reactions between Fe (V) and Fe (IV) with contaminants, as well as Fe (III) formation, self-decay of Fe (VI), Fe (V), and Fe (IV) species (Fe (II) and Fe (III) formation), and reactions between Fe (VI)s and reactive oxygen species (44).

Chemical Constituents of Dyes Dyes are colored, ionizing, aromatic organic compounds. Dyes react with the cottons and color them by coating their surface (45). Dye molecules include three key constituents; the first group includes benzene, fused benzene, and rings, and the others are chromophores and auxochromes (46). A chromophore is the part of a molecule responsible for its color. Chromophores and auxochromes participate in dyeing the cottons, bonding with fiber and other materials (45). The chromophores are simple unsaturated groups attaching the benzene and rings, providing colorization. The auxochromes are basic groups and provide higher affinity toward the fibers and make the dye insoluble in water (46, 47). Some of the typical chromophoric groups are shown in Figure 12.

Figure 12. The typical chromophoric groups. 371 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


The explanation of the relationship between structure and color depends on the basic atomic structure of the aryl ring. This atomic arrangement appears in the shared or delocalized electrons. The color changes of substituted naphthalene can be given as an exapmle in Figure 13.

Figure 13. Color changes of naphthalene molecule. Adding groups of increasing electron-donating ability to the naphthalene, azobenzene and other structures have a bathochromic effect. This is illustrated in Figure 14, where the following effects of substituents are shown.

Figure 14. Structure change of acetamidine molecule. The effect of other atomic configurations is to modify the energy contained in the delocalized electron cloud so that the compound absorbs electromagnetic radiation at a wavelength in the visible range.

Classification of Dyes Dyes can be classified as natural and synthetic dyes, basically. Natural dyes are separated into three groups according to their origins as plants, animals, and minerals. Synthetic dyes are produced as a consequence of two or most reactions and can be classified based on their chemical structure as azo, indigo, phthalocyanine, anthraquinone, arylmethane and heterocyclic dyes (48). However, most of the synthetic dyes (60–70%) are bio-recalcitrant azo dyes, which consist of one or more N=N (azo) bridges linking substituted aromatic structures (49). An integrated classification according to their chemical structure, color properties, and application is given in this report. The classification of dyes is shown in Table 4. 372 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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Table 4. Classification of dyes. Dye

Water solubility and other specification

Acid Dyes

Soluble, anionic,

Hydroxyl and carbonyl groups (45)

Azo, anthraquinone, triphenylmethane, azine, xanthene, nitro, and nitroso (47)

Complete color range with very good bright shades

Protein (nylon, wool, paper, silk, ink and leather) (47) and polyamide fibers (45)

Reactive Dyes

Soluble, anionic, polar, smaller molecule size

Chlorotriazine, epoxy, ethyleneimide groups (45) and dye forms covalent bond with fiber polymer

Azo, anthraquinone, triarylmethane, phthalocyanine, formazan, and oxazine (47)

Large range of colors and brighter shades, require low temperature

Cotton, yarn and other cellulosic, silk painting, and polychromatic printing (47)

Direct Dyes

Soluble, anionic, polar, high molecular weight

Hydrogen bonds and Van der Waals forces between dye and fiber surface (45) and azo linkage –N=N–

Polyazo compounds, stilbenes, phthalocyanines, and oxazines (47)

Large range of colors and darker shades

Cotton and rayon, paper, leather, nylon (47), cellulose fibers, linen, wool, and silk (45)

Basic Dyes

Soluble, cationic

Amino groups to form hydrochlorides and oxalates (45)

Diazahemicyanine, triarylmethane, cyanine, hemicyanine, thiazine, oxazine, acridine (47), azine, and xanthene (45)

Complete color range with very good brilliant shades

Silk, wool, and tannin-mordant cotton, paper, modified polyesters (47), leather, wood, and straw

Azoic Dyes

A type of direct dye

Azo groups –N=N–

Stilbene, pyrazoles, coumarin, and naphthalimides (47)

Limited color range and bright shade, required low temperature

Cotton, other cellulosic materials, soaps, detergents, paints, and plastics (47)

Chemical and bond structure

Chromophoric groups

Color properties

Application

Continued on next page.


Dye

Water solubility and other specification

Chemical and bond structure

Chromophoric groups

Color properties

Application

Disperse Dyes

Insoluble, nonpolar, nonionic

Van der Waals forces

Azo, anthraquinone, styryl, nitro, and benzodifuranone groups (47)

Large range of colors and bright and lighter shades

Polyester, nylon, cellulose, cellulose acetate, and acrylic fibers (47)

Solvent Dyes

insoluble (solvent soluble), nonpolar or little polar

Sulfonic acid, carboxylic acid, or quaternary ammonium (47)

Azo, anthraquinone, phthalocyanine and triarylmethane (47)

Poor to good light fastness, require high temperature

Plastics, gasoline, lubricants, oils, and waxes (47)

Sulfur Dyes

Insoluble

Sulfur linkages within their molecules

Nitro and amino groups

Incomplete color range, dark shades, requires high temperature with large quantities of salt

Cotton, rayon, and to a small extent polyamide fibers, silk, leather, paper, and wood (47)

Vat Dyes

Insoluble but soluble with alkali

Redox reactions

Anthraquinone (including polycyclic quinones), indigoids (47), and carbazole

Large range of colors and dark shades ability

Cotton (cellulosic fibers), rayon, wool (47), linen, silk, and nylon

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Table 4. (Continued). Classification of dyes.



Dye Wastewater Dyes have been commonly used in textile, paper and pulp, dyeing, tannery, printing, photographic, and coating industries (47, 50). Varying volume and characteristics of wastewater are produced from these industries (51). It is estimated that nearly 106 tons of dye per year are produced and that 10–20% of the used dyes are discharged to the environment as waste (47, 48). Besides dye components, some dyeing auxiliaries, such as buffer solutions, salts, dispersing agents, and metal ions, are released into dye wastewater (48). Commonly, the dye wastewaters contain dye pigments, slowly or non-biodegradable organic and inorganic substances (52). When dye wastewaters are discharged to the surface water, they inhibit the biological processes such as photosynthesis, blocking light penetration in the water of lakes, rivers, or lagunas (51, 53). Furthermore, these chemicals in wastewater can have toxic, carcinogenic, mutagenic, or teratogenic effects on various microbiological or animal species (54). Dyes in water even at low concentrations (

Review on Greywater Treatment and Dye Removal from Aqueous

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