The Use of Ferrate(VI) Technology in Sludge Treatment - ACS

Chapter 18

The Use of Ferrate(VI) Technology in Sludge Treatment 1

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Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 17, 2013 | http://pubs.acs.org Publication Date: July 25, 2008 | doi: 10.1021/bk-2008-0985.ch018

Jia-Qian Jiang and Virender K. Sharma

C e n t r e for Environmental Health Engineering, School of Engineering, C5, University of Surrey, Guildford, Surrey GU2 7XH, United Kingdom ([email protected]; fax: +44 1483 450984 ) Department of Chemistry, Florida Institute of Technology, 150 West University Boulevard, Melbourne, FL 32901 *Corresponding author : [email protected]; Fax: +44 1483 450984 2

Sludge in large quantity is generated as byproducts of wastewater treatment processes. Various approaches have been taken to treat sludge, such as land-filling, ocean dumping, or recycling for beneficial purposes. In the USA, about 60% of sludge generated is land applied as a soil conditioner or fertilizer. Due to increasing public concern on the safety of land-applied sludge, various sludge treatment technologies are being developed or under evaluation in order to improve the quality of sludge in terms of pathogen content, odor characteristics, accumulated organic micro-pollutants. This paper summarizes the results of various reported or on-going researches on the potential use of ferrate [Fe(VI)O ] as a conditioning agent for sludge. Ferrate(VI) has high oxidizing potential and selectivity, and upon decomposition produces a non-toxic by-product, Fe(III), which is a conventional coagulant; the ferrate(VI) is thus considered to be an environmentally-friendly oxidant. Rates of oxidation reactions increase with decrease in pH. Oxidation of sulfur- and aminecontaining contaminants in sludge by Fe(VI) can be accomplished in seconds to minutes with formation of nonhazardous products. Ferrate(VI) can also coagulate toxic metals and disinfect wide ranges of microorganisms including human pathogens. With its multifunctional properties, ferrate(VI) has the potential for sludge treatment. 2-

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© 2008 American Chemical Society In Ferrates; Sharma, V.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.

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Introduction In the development of municipal wastewater treatment strategies, except for the efforts made to improve the quality of the effluent by using more effective treatment technologies, the problems associated with the sewage sludge produced in wastewater treatment processes have been taken into account. Toxic pollutants (e.g., heavy metals and endocrine disruptors) together with a large number of the pathogens are concentrated in the sludge, and this increases in the risks to the health and environment. Moreover, a number of organic sulfides and amines are produced in wastewater treatment which results in unpleasant odors (7). In the United States, 5.6 million tons of sludge is generated annually, of which 60% is applied as a fertilizer (2). Complaints of illness related to the land application of biosolids have been increasing (5), and the original application of the sludge as a fertilizer in agricultural systems has thus become increasingly under pressure. The legislation and regulations regarding the application of sludge in agriculture have changed considerably. According to Environmental Protection Agency (EPA) under 40 CFR Part 503, biosolids designated for land application are classified as Class A or Class B, depending on the presumed pathogen content. Class A biosolids are intended to have undetectable bacteria, enteric viruses, and helminthes. The revised European Union Directive on sewage sludge revises the previous one (EU Sludge Directive (86/278/EEC)) significantly. The most important new aspects are the requirement of sludge hygienization and odor reduction using advanced treatments, and the treated sludge shall not contain Salmonella spp. in 50 gram wet weighted sludge, and achieve at least a 6 Log 10 reduction in Escherichia coli to less than 500 colony forming units (CFU) per gram dry solids. Also, the use of conventional treated sludge on parks, green areas, and city gardens as well as any use on forests is to be forbidden. The revised EU Directive on sewage sludge has also proposed considerably stricter regulations for heavy metals. It has also added new limit values on organic micropollutants, such as sum of halogenated organic compounds (AOX); linear alkylbenzene sulfonates (LAS), di (2-ethylhexyl) phthalate (DEHP), nonylphenol and nonylphenolethoxylates (NPE), sum of polycyclic aromatic hydrocarbons (PAH), polychlorinated biphenyls (PCB) and dioxins. This should require water industries to apply more advanced technologies to meet the proposed treated sludge standards. Due to all these developments the current practice of sludge management has considerably changed during the past twenty years. Improved biogas production, advanced sludge dewatering processes, controlled land filling and thermal processes are increasingly applied in practice. There is a need of innovative sludge management practices, which could not only effectively treat a wide range of contaminants and health hazardous pathogenic organisms, but could also remove unconventional contaminants (e.g., personal care products and endocrine disruptors) from sewage sludge. Due to the urgency to develop

In Ferrates; Sharma, V.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.

308 more sustainable and efficient sludge treatment technologies, an increasing growth in research is observed (4). Ferrate(VI) ion has the molecular formula, Fe0 ", and is a very strong oxidant. Under acidic conditions, the redox potential of ferrate (VI) ions is greater than ozone and is the strongest of all the oxidants/disinfectants practically used for water and wastewater treatment (5,6). Moreover, during the oxidation/disinfection process, ferrate(VI) ions will be reduced to Fe(III) ions or ferric hydroxide, and this simultaneously generates a coagulant in a single dosing and mixing unit process. The use of ferrate(VI) in treating sludge can accomplish the important objectives of sludge management. This review aims to address currently emerging issues in sludge management and discuss the potential role of ferrate(VI) in sewage sludge treatment and management. 2


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Sludge Management In sludge all contaminants are present as one mixture, which includes pathogens and other harmful microbiological pollutants, odor-causing compounds, toxic heavy metals, and toxic organic micropollutants such as pesticides, endocrine disrupters. These contaminants are mixed with non-toxic organic carbon compounds (approximately 60% on dry basis), and total nitrogen and phosphorus containing components, which makes the management of sludge to be complicated. Organic carbon, phosphorus, and nitrogen containing compounds can be considered as valuable compounds and are often recovered and reused after treatment. Sludge treatment also involves the minimization of the possible adverse impact of sewage sludge on the environment and on human beings.

Sludge hygienization Pathogens, such as bacteria, viruses, and human parasites, which may cause human diseases, exist in raw wastewaters. Common bacterial pathogens include Escherichia coli, Helicobacter pylori and Listeria montocytogenes. Of the emerging pathogens enteric viruses present the greatest risk because of their resistance to inactivation and longer survival. The literature (7) suggested that the concentration of enteric viruses (enteroviruses) ranged from 102 to 104 per gram dry weight of solids in raw biosolids and an average of 300 per gram in secondary biosolids. E. coli and L. montocytogenes are known capable of surviving anaerobic digestion and may regrow after land application in some cases. This is the reason that sewage sludge must be subjected to additional pathogen reduction treatment prior to land application. Aerobic and anaerobic digestion and lime treatment are the common processes for the pathogen control. The inactivation of enteric viruses by aerobic digestion is usually pH and temperature dependent. In general, a plant using mesophilic digestion (37°C)



309 with a mean retention time of 10-20 days can reduce enterovirus concentration by 90%. Anaerobic digestion can result in a similar reduction. More recently, inactivation of vaccine-strain poliovirus and eggs from the helminth Ascaris suum in biosolids under thermophilic anaerobic digestion conditions have been reported (8). Interestingly, Ascaris ovum could be inactivated with the use of electron beam irradiation process (9). Moreover, this process may result in morphological changes in ovum as has been found in Ascaris lumbricoides ova sewage sludge water treated by gamma irradiation (70). Lime treatment has been one of the most frequently applied pathogen control processes in a domestic wastewater treatment plant. The lime addition to sludge causes pH to increase to over 12 for over 2 hrs and leads to the inactivation of pathogen. However, the liming process promotes to generate ammonia and amines (e.g., trimethylamine) and odor, resulting in complaints from the public (7). Alternative treatment chemicals, e.g., chlorine or ozone, have been used to disinfect sludge (77). Ozonation has also been suggested a suitable process for minimizing the sludge production (72). However, due to high cost and the generation of harmful by-products, they are not commonly employed. Therefore, other chemical products which can economically favorably stabilize sludge and do not produce any harmful by-products and odors should be explored.

Odors Odor is a problem at many urban and industrial wastewater treatment plants (13). Therefore, most unit processes, e.g., preliminary treatment, primary clarifiers, activated sludge basins, secondary clarifiers, sludge thickening, and conditioning, and dewatering, in a wastewater treatment plant are potential resource of odor (14). Especially, sludge conditioning systems, such as thickening, drying, and lime stabilization, are normally the most significant source of odors, since these unit processes are usually open to the air, and they emit odors with intensity raging from mild to nauseating. Moreover, cationic organic polymers used in enhancing thickening and dewatering processes become potential sources of strong odors (75). Chemical structures of these polymers are such that they are susceptible to biotic or abiotic degradation, which produce volatile odorous organic amines. Fresher sludge emits less offensive odor and septic sludge is more offensive. The odors from the sludge during solid handling processes are often carried over to the final biosolids, which is to be land applied (14). Since odors are considered as the most important factor affecting the operation of a wastewater treatment plant and the public acceptance of the final biosolids for land application, appropriate odor control should be designed and applied to the processes. Mostfrequentlyused odor control techniques include pH adjustment, addition of metal salts and nitrate, and addition of chemical oxidants such as hydrogen peroxide, chlorine, potassium permanganate, and ozone (16,17). Especially, through the addition of oxidants, the oxidation environment of the system is promoted and septic or


310 anaerobic condition in which odorous sulfur compounds are generated can be avoided. Furthermore, some chemicals with odor potential can be destroyed through oxidation (//). Oxidation technologies have been applied to control odorsfromwastewater and sludge. Hydrogen peroxide has been applied to oxidize H S, mercaptanes, thiosulfate, and sulfur dioxide in wastewater and waste activate sludge (18). However, hydrogen peroxide can be dangerous to handle and takes time to react, so several other oxidants also have been tested and applied. For example, potassium permanganate was added to raw dewatered sludge to reduce hydrogen sulfide production (19). Farooq and Akhlague (20) tested ozone to condition and oxidize heavy metals and organics in raw and thickened waste activate sludge. They reported a considerable reduction in odor released from the sludge, although they did not quantify their results. Gao et al. (21) used sodium hypochlorite to remove 95 % of hydrogen sulfide from gas emissions released from gravity thickened sludge. In the use of various chemicals for hydrogen sulfide treatment, it has been a trend since 1970s that an increase in the use of metal salts (e.g., ferric and ferrous iron) and nitrate, with a decrease in the use of lime, chlorine and oxygen.


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Heavy metals Heavy metals present in the wastewater will be removed in the treatment processes. The concentration ratio of heavy metals in the sludge to wastewater is of the magnitude of 10000:1. This means that even very small concentrations of heavy metals in wastewater will result in greater concentrations in sludge. Such high concentrations of metals in sludge may risk crop yield, long-terms soil quality, wildlife and cattle heath, and eventually human health. As stated previously that the revised EU Directive on sewage sludge has proposed considerably stricter regulations for heavy metals (see Table 1) and this should require water industries to apply more advanced technologies to meet the proposed treated sludge standards.

Table 1. Limit values of heavy metals in sludge for use on land Limit values Revised Directive Revised Directive (mg/kg P) (mg/kg-dry sludge) 250 Cd 10 Cr 25,000 1,000 Cu 25,000 1,000-1,750 1,000 250 16-25 10 Hg Ni 7,500 300 300^100 18,750 Pb 750-1,200 750 Zn 62,500 2,500-4,000 2,500 N O T E : The sludge producer may choose to observe either the dry matter related or the phosphorus related limit values. Metals

Directive 86/278 (mg/kg-dry sludge) 20-40

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Micro pollutants and endocrine disruptors Sludge contains the components both in wastewater (domestic and industrial effluents) and thatfromthe formation of by-products of biological and chemical treatment. The wastewater includes the expected urine and faecal matter but also synthetic organic chemicals, contributed not only by the commercial and industrial sectors, but also by residents via sinks and floor drains, and even as the pharmaceutical chemicals contained in their own wastes. With the advance of chemical/biochemical synthetic technologies, millions chemicals have been developed and manufactured but might not be considered for their complete recycle and reuse, and a significant fraction of these chemicals is sent into waste streams such as wastewater and its residual sludge. The revised EU Directive on sewage sludge has added new limit values on organic micropollutants (see Table 2), such as sum of halogenated organic compounds (AOX), linear alkylbenzene sulfonates (LAS), di (2-ethylhexyl) phthalate (DEHP), nonylphenol (NP) and nonylphenolethoxylates (NPE), sum of polycyclic aromatic hydrocarbons (PAH), polychlorinated biphenyls (PCB) and dioxins. This should require water industries to apply more advanced technologies to meet the proposed treated sludge standards.

Table 2. Limit values of organic compounds and dioxins Compound AOX (sum of halogenated organic compounds) LAS (linear alkylbenzene sulfonates) DEHP (di (2-ethylhexyl) phthalate) NPE (nonylphenol and nonylphenolethoxylates with 1 or 2 ethoxy groups) PAH (sum of various polycyclic aromatic hydrocarbons) PCB (sum ofpolychlorinated biphenyls) PCDD (polychlorinated dibenzodioxJdibenzofur) /(ng-TE/kg-dry matter)

Limit values (mg/kg-dry matter) 500 2,600 100 50 6 0.8 100

Endocrine disruptor chemicals (EDCs), e.g., nonylphenols (NPs) and polybrominated diphenyl ethers (PBDEs) in wastewater and sewage sludge have been detected at trace levels (22). The adverse effects of EDCs have been well addressed. EDCs can reach water resources indirectly through the farm runoff where biosolids has been applied. Entering of EDCs into the environment can possibly threaten the long-term viability of eco-system (23). NPs result from the incomplete biotransformation of domestic or industrial detergents (alkylphenolpolyethoxylate surfactants). Therefore, in the effluent of a swage treatment plant



312 relatively high concentration of NPs can be observed (24). Due to their high lipophilicity, NPs can accumulate to higher concentrations in thickened sludge and the final biosolids, which can be land applied (22). Ahel et al. (25) found about 60% of total nonylphenols to a wastewater treatment plant accumulated in sludge. PBDEs have been used as a flame retardant for furniture, televisions cast and other plastics (26). They can be introduced into wastewater and also accumulate in biosolids as the case with NPs (27). Recently, the public concerns on the land-applied biosolids have been elevated since it is thought as a potential source which delivers the endocrine disruptors to the environment, such as soil and especially aquatic ecosystem (28). If these endocrine disruptors are introduced into surface water and enter the body of a fish, it is known the action of the endocrine system through various mechanisms is inhibited (29). Researches have shown estrogen activity of NPs in rainbow trout (30) and retardation in the development of secondary sex characteristics in Fathead Minnow induced by NPs. Zennegg et al. (57) also found PBDEs in the tissues of whitefish and rainbow trout. Although it is still under investigation, possible carcinogenic effect of the endocrine disruptors on humans is being discussed. Wren (32) reported the presence of endocrine disruptors at the concentrations enough to warrant concerns on the health of operators at the plant. Therefore, it is desirable to destroy the endocrine disruptors within the treatment processes before they are released into the environment. Since endocrine disruptors in wastewater and biosolids have been recently addressed, only a few researches have been performed on their destruction (33). Biological treatment alone cannot eliminate endocrine disruptors from wastewater. Lee and Paert (34) found that only 50% of NP could be eliminated through the secondary process in wastewater treatment. On the other hand, sonication, or peroxidase-mediated or Fenton's reagent-mediated oxidation showed more than 95 % NP removal efficiency (33). However, these processes require considerable capital demand. Therefore, more economically favorable alternative oxidation process should be pursued.

Ferrate as a Potential Sludge Conditioner Properties of ferrate(VI) Iron commonly exists in the +2 and +3 oxidation states; however, in a strong oxidizing environment, higher oxidation states of iron, +6 can be obtained. In the laboratory, ferrate(VI) can be produced from the reaction of ferric chloride with sodium hypochlorite in the presence of sodium hydroxide (35). This method produces sodium ferrate(VI) (Na Fe0 ) and potassium 2

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313 hydroxide is added into to precipitate potassium ferrate(VI) (K Fe0 ). The basic reactions are as follows: 2

4

2 FeCl + 3 NaOCl + 10 NaOH -> 2 Na Fe0 + 9 NaCl + 5 H 0 3

2

Na Fe0 + 2 KOH


2

4

4

K Fe0 2

2

(1) (2)

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Ferrate(VI) exhibits a multitude of advantageous properties; including higher reactivity and selectivity than traditional oxidant alternatives, disinfectant, and coagulant properties (5). Ferrate(VI) is one of the most powerful multi-purposes oxidizers known in treatment processes. Under acidic conditions, the redox potential of ferrate ion is the highest of any oxidant such as chlorine, ozone, hydrogen peroxide, and potassium permanganate used in treatment processes (eqs 3, 4). 2

+

_

3+

Fe0 " + 8H + 3e Fe + 4 H 0 4

E° = 2.20 V

2

2

Fe0 * + 4 H 0 + 3e o Fe(OH) + 5 OH" 4

2

E° = 0.70 V

3

(3) (4)

The spontaneous oxidation of Fe(VI) in water forms molecular oxygen (eq 5), which can minimize anaerobic conditions in sludge. Fe0

2_ 4

3+

+ 5H 0 -> Fe + 3/20 + 10OH" 2

2

(5)

A by-product of Fe(VI) is non-toxic, Fe(III), making Fe(VI) an environmentally friendly oxidant for sludge treatment (36,37). Moreover, ferric hydroxide, produced from Fe(VI), acts as a coagulant which is an excellent dewatering agent and is also suitable for removal of toxic substances such as metals in sludge. Tests show that Fe(VI) has a greater efficiency than commonly used inorganic coagulants (38).

Ferrate(VI)'s potential as a disinfectant The disinfecting properties of ferrate(VI) were first observed by Murmann and Robinson (39) when they investigated the effectiveness of ferrate as a disinfectant to kill two pure laboratory cultures of bacteria (Non-recombinant Pseudomonas and Recombinant Pseudomonas). At a dose range of 0 - 50 ppm as Fe0 ", the bacteria could be completely destroyed. Later, another study (40) showed that ferrate (VI) has sufficient disinfection capability to kill Escherichia coli (E. coli). At pH 8.2 and a dose of 6 mg/1 as Fe, the E. coli percentage kill was 99.9% when the contact time was 7 min. The results also demonstrated that the disinfecting ability of Fe0 " increased markedly if water pH was below 8.0. 2

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2

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314 In another secondary effluent disinfection study with ferrate(VI) (36), 99.9% of total coliforms and 97% of the total viable bacteria were removed at a dose of 8 mg/1 as Fe0 (Figure 1). Using ferrate(VI) to treat real drinking water and wastewater were conducted by Jiang et al. (41-43). Study results demonstrated that ferrate(VI) is effective in killing Escherichia coli (E. coli) and total coliforms. For example, in treating sewages, ferrate can achieve >99.99% inactivation of total coliforms at relative low dose comparing with other reagents (Table 3). Kazama (44) demonstrated that Fe(VI) rapidly inactivated f2 Coliphage at low concentrations and a survival ratio of the virus decreased rapidly within 10 minutes after the addition of ferrate(VI). The treatment of DNA solution with micromolar concentrations of ferrate(VI) inhibits irreversibly further DNA polymerization and polymerase-chain-reaction (PCR) synthesis. Ferrate(VI) also inhibited the respiration of the bacterium Sphaerotilus; suggesting potential role of ferrate in treating sludge for disinfection. In a recent study (45) on sludge, more than 99% of indicator organisms could be inactivated at a dose of 0.2 g Fe0 " / L sludge. However, a spore formers such as Clostridium perfringens, which is resistant to disinfectants, required a dose of 0.8 g/L sludge. 2


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2

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Figure 1. Percentage inactivation of bacteria with ferrate (VI)

Table 3. Comparative performance of bacteria inactivation (41) 1

Coagulation pH Total Coliform Faecal Coliform

AS 6.75-7.48 87-91% 89-90%

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FS 6.75-7.48 87-90% 90-91%

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Ferrate(VI) 5 7 >99.99% >99.99% >99.99% >99.99%

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Aluminium sulfate (AS) and ferric sulfate (FS) dose required was >0.50 mmol/L as either Al or Fe(III). Ferrate(VI) achieved >99.99% bacteria inactivation at doses

The Use of Ferrate(VI) Technology in Sludge Treatment - ACS

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