Evaluation of Ferrate(VI) as a Conditioner for ... - ACS Publications

Chapter 19 Evaluation of Ferrate(VI) as a Conditioner for Dewatering Wastewater Biosolids 1,*

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Hyunook Kim , Yuhun Kim , Virender K. Sharma , Laura L. McConnell , Alba Torrents , Clifford P. Rice , Patricia Millner , and Mark Ramirez 3

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Downloaded by MONASH UNIV on March 11, 2013 | http://pubs.acs.org Publication Date: July 25, 2008 | doi: 10.1021/bk-2008-0985.ch019

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Department of Environmental Engineering, The University of Seoul, 90 Jeonnong-dong, Dongdaemun-gu, Seoul 130-743, Korea Department of Chemistry, Florida Institute of Technology, 150 West University Boulevard, Melbourne, F L 32901 A g r i c u l t u r a l Research Service, U.S. Department of Agriculture, 10300 Baltimore Avenue, Beltsville, M D 20705 Department of Civil and Environmental Engineering, The University of Maryland, College Park, M D 20742 district of Columbia Water and Sewer Authority, 5000 Overlook Avenue SW, Washington, DC 20032 *Corresponding author: email: [email protected], fax: +82-2-2244-2245 2

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Land application of sludge/biosolids is a commonly used practice for final utilization. Therefore, adequate conditioning and stabilization of wastewater solids is very critical for safe land application. The addition of ferrate (FeO ) has the potential to improve the dewaterablity of solids, destroy pathogenic organisms, and reduce certain endocrine disrupters. In this study, the dewaterbility and stabilization of thickened sludge treated with ferrate was evaluated under controlled laboratory conditions. To evaluate dewaterbility, three different techniques; belt-press, centrifugation, and vacuum filtration, were applied to dewater a mixture of solids. In addition, once biosoilds are generated, their safety and quality especially in terms of reduced endocrine disrupting compounds (EDCs) as well as pathogens become an important issue to the public. Therefore, the effectiveness of ferrate (FeO ) in disinfecting microorganisms and in oxidizing EDCs, i.e., Nonylphenols and polybrominated diphenyl ethers, is briefly discussed. 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 2005, a total of 7.5 million dry tons of biosolids were generated by municipal wastewater treatment plants (WWTPs) in the United States (7), and about 60 % of the biosolids were land applied. Since the management of wastewater sludge is difficult and expensive, the utilization or disposal including beneficial reuse of biosolids has become a concern of high priority (2). In reality, final biosolids quality is influenced by the selection, design, and operation of upstream processes (3), including sludge conditioning, thickening, and dewatering. The overall cost for sludge treatment/handling is directly related to the bulk volume of sludge treated. Land application continues to be the preferred option for sludge/biosolids' final utilization, however, the production of less WWTPs solids is the most desirable and cost effective option. Minimization of water content will result in significant cost saving by reducing the overall volume of solids (4). Therefore, the solid-liquid separation processes, e.g., thickening and dewatering processes should be operated at the highest performance and reliability levels. The most commonly utilized dewatering technologies are beltfilterpressing and solid bowl centrifugation. Well managed operation of these technologies can increase solids content up to 25 ~ 30 % (5). However, most installations are not operated at optimum levels and fail to provide proper dewatering (4). To aid in the dewatering processes, thickened sludge is often conditioned by applying chemical additives (coagulants such as polymer, or ferric chloride, aluminum sulfate (alum), ferrous sulfate, and ferric chloro-sulfate (6)). Once added, the coagulant alters wastewater solids structure for better separation of solid and liquid. Although commercially available coagulants may show outstanding solidsliquid separation and chemical oxygen demand (COD) reductions in supernatant (7), they do not greatly affect the bio-chemical properties of the solids. Rather pathogenic microorganisms and organic pollutants are concentrated in dewatered solids. Therefore, further stabilization and disinfection procedures are required before biosolids are land-applied (5-70). Currently, alternative chemical additives to stabilize and disinfect as well as to condition and dewater thickened solids are being evaluated. Neyens et. al. (5) oxidized wastewater solids with H 0 (0.037 g H O /100 mL sludge) in the presence of Fe (1 mg Fe /100 mL sludge) and significantly improved the dewaterbility of the solids. In addition to improved dewaterbility, they observed higher degradable organic content and less residual heavy metals in the dissolved solids due to break-down of more recalcitrant compounds by H 0 . Iron with the +6 oxidation state, ferrate (Fe(VI)) is also a potential alternative to polymer for dewatering and improving the safety of biosolids, because it is a powerful oxidizing agent. Under acidic conditions, the redox potential of Fe(VI) ion (2.2 V) is higher than that of ozone (0.72 V). 2

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328 Upon decomposition, Fe(VI) in water forms molecular oxygen improving redox condition of the media, and Fe(III), a common coagulant used in water and wastewater treatment plants (Eq. 1). Ma and Liu (77) observed good coagulation of suspended solids in surface water preoxidized with Fe(VI). The floe size of the coagulated particles was larger in a Fe(VI) preoxidation process than those of an alum coagulated floe. They also found Fe(VI) was more effective in coagulation of organic-rich waters in which alum was less effective in reducing turbidity. 2

3+

Fe0 " + 5 H 0 -> Fe + 3/2 0 + 10 OH"


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(1)

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In this study, the dewaterbility of thickened sludge treated with ferrate was evaluated in the laboratory. Simulated belt-press, centrifugation, and vacuum filtration were applied to dewater a mixture of solids collected from a gravity thickener (GT) and from a dissolved air flotation (DAF) thickener where primary-settled sludge and waste activated sludges were fed, respectively. We also briefly discuss other benefits of using ferrate (Fe0 ') as a biosolids conditioner, such as oxidizing EDCs (nonylphenol (NP) and octylphenol (OP)) and inactivtivating microbes. 2

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Materials and Methods Ferrate Production In the laboratory, Fe(VI) has been produced using one of three procedures provided in the literature; wet synthesis (72), dry synthesis, and electrochemical synthesis. In wet synthesis, sodium ferrate(VI) (Na Fe0 ) is producedfromthe reaction of ferric chloride with sodium hypochlorite in the presence of sodium hydroxide. In dry synthesis, the fusion of Na 0 and Fe 0 at a molar ratio of 4:1 in the presence of dry oxygen (Temp.: 370 °C) is induced to produce Na Fe0 (75). Finally, Fe(VI) can be synthesized by applying electricity to anodic iron in NaOH solution whereby anodic iron is oxidized to Fe(VI) (14). Using any of the methods, Fe(VI) with relatively high purity can be produced. However, these technologies are labor and capital intensive, which has been a major obstacle to the wide application of ferrate in wastewater treatment processes. Ferrate used in this study was produced on site, using a pilot-scale ferrate producer, FERRATOR™, supplied by Ferrate Treatment Technology, Inc. (Orlando, FL). It consists of a 100 L reactor, where industry grade NaOCl with high concentration (in this study, 16 % weight basis), NaOH of 50 % and FeCl of 35 % are mixed to produce ferrate. A heat exchanger prevents quick decomposition of the generated ferrate. Using this reactor, which utilizes the chemical reactions described by Thompson et al (72), sodium ferrate could be produced at about 60 % conversion yield, and the pH of the final solution was 2

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329 >13. Once the ferrate solution was produced, it was utilized in the dewaterbility experiments within approximately one hour. The ferrate solution was stored at 4 °C prior to use to minimize decomposition.


Sludge Samples and Sample Treatments Thickened sludge samples were collected from a GT and a DAF thickener of the District of Columbia Advance WWTP at Blue Plains, District of Columbia, USA. The solids contents of sludges from a GT and a DAF were 3-5 %. GTs were fed with sludge from primary sedimentation tanks. DAFs were fed with waste activated sludge from secondary process and nitrification/denitrification processes. The collected GT and DAF sludges were blended at the ratio of 1:1 in a large plastic container to prepare 20 L. In the WWTP, the GT and DAF sludges are routinely mixed at a 1:1 ratio. Aliquots of the blended sludge (2 L) were placed in heavy duty mixers. Five treatments of ferrate solution were used in the experiment; 0, 20, 40, 100, 200 mL ferrate solution/2 L blended solids. As an additional treatment, one solids sample was treated with 25 % CaO (dry weight basis). The ferrate solution and CaO were added to the solids samples and mixed for a total of 5 min. One quarter ferrate solution or CaO was simultaneously added to each sample at 1, 2, 3, and 4 minutes, respectively. All the treatments were carried out in duplicate. In a separate set of experiments, effects of cationic polymer addition on the dewaterbility of ferrate treated solids were also evaluated by adding cationic polymer (Stock-Housen, Willmington, DE, USA) to the blended solids.

Procedures for Dewaterbility Tests Three conventional wastewater solids dewatering processes were tested; i.e., centrifiiging, belt-pressing, and vacuum filtering.

Dewatering of Sludge by Centrifugation Blended aliquots of treated solids (45 mL) were transferred to test tubes. Then the tubes were placed in holders of a lab-scale centrifuge (Cole-Palmer, USA) (Figure 1). The sludge-dewatering performance of the chemical treatments and controls were evaluated at three different centrifuge speeds: 1000, 2000, and 3000 rpm with a centrifuge time of 3 minutes. The treatment effectiveness was also evaluated by varying the centrifiiging duration at 1, 3, and 5 minutes at holding the speed constant 2000 rpm. The resulting volume and total suspended solid content of centrate were analyzed to calculating dewatering efficiency of the treatment.


330 Sludge Sample (50 mL)

Centrifuge: 1000-3000 rpm for 3 mins or 1 - 5 min at 2000 rpm


Figure 1. Procedure of sludge-dew atering using a centrifuge

Dewatering of Sludge by Belt-pressing Sludge dewatering effectiveness of the ferrate treatments and controls were further evaluated using a Crown Press, a bench-scale belt press simulator (Cat No.: 7800-100, Phipps & Birds, Richmond, VA) (Figure 2). Before applying belt pressing, 50 mL treated solids samples were drained for 5 minutes by gravity on the pre-filter in order to simulate real practices. Then, the solids remaining on the filter were transported on the press filter, where pressure of 100 psia was manually applied for 1 min. After the pressure was applied, the dewatered solids were scrapped out on to an aluminum dish for measuring the sample mass and solids content. Total suspended solids concentration of the filtrate were also measured. Lateral migration of solids under belt-press was visually assessed.

Figure 2. Procedure of sludge-dew atering using a lab-scale belt-press


331 Dewatering Sludge by Vacuum Filtering Vacuum filtration was also applied to assess the effectiveness of various ferrate treatments and controls. The study was performed by placing 50 mL treated sludge on a Buckner funnel with a paperfilterand by applying vacuum pressure of 49x1000 N/m for 10 min (Figure 3). As with the belt-press experiments, the volume and TSS concentration of thefiltratewere measured.


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Figure 3. Procedure of sludge-dew atering using a vacuum filter

Results and Discussion Characteristics of Solids after Ferrate Treatment Upon the addition of ferrate solution to blended solids, the solids exhibited the consistency of a gel, probably due to the coagulation effect of Fe(III) ions generated from ferrate decomposition. This hypothesis was supported by a color change of treated solids from dark grey (control) to light reddish (typical color of iron oxide). In Figure 4, the color of ferrate treated sludge turns to lighter reddish brown as the treatment levels increases. Since the pH of the ferrate solution was high (it contains concentrated OH"), the solids pH rose to > 12 at the lower level treatments (10 mL/L). Since the decomposition of ferrate produces OH' as depicted in Eq. 1, it should contribute to the pH increase of sludge, too.


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Ferrate 100 mL Ferrate 200 mL

Ferrate 20 mL

Ferrate 40 mL

Figure 4. Pictures of dewatered sludge with ferrate treatments.

Effects of Ferrate Treatment on Solids Dewaterbility by Centrifuge

Effects of ferrate treatment on solids dewatering using a centrifuge were evaluated by measuring the volume and TSS of extracted water (i.e., centrate) from each solids sample. First, centrifugation was operated at three different rotational speeds, i.e., 1000, 2000, and 3000 rpm and for constant operational duration, i.e., 3 minutes. In general, at lower doses of ferrate solution, the dewatering efficiency of centrifiiging was less than the solids without ferrate solution treatment; especially at lower rotating velocity (Figure 5(a)). At 1000 rpm and with 40 mL ferrate solution, separation between solid and liquid was not observed. It is hypothesized that organics with large molecular weights in biosolids were degraded into smaller molecules and suspended. The hypothesis was supported with the observed high TSS levels of centrates (refer to TSS of each treatment in Figure 5). Figure 6 shows the color of centratesfromdifferent ferrate solution treatments. The centratefromsolid sample with 200 mL ferrate treatment is dark brown, implying higher TSS level. Ayol et al. (15) reported a similar observation in their study. In their anaerobically digested solids, they found COD increased in the centrate of solids treated with potassium ferrate along with hydrogen peroxide. A similar result was obtained when the duration of centrifiiging was varied from 1 to 5 minutes and the rotating speed of the centrifuge wasfixedat 2000 rpm. It is noted that the TSS level of centratefromsamples treated with ferrate was significantly higher than the control when the centrifuge duration was short. One minute of centrifuge time was not enough to force suspended solids to separatefromthe liquid.



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Figure 5. Dewaterbility of centrifuge at different rotating velocities ((a)~(c); duration was set at 3 minutes) and at different durations ((d) ~ (f); rotating velocity was set at 2000 rpm).

OmL 40 mL 100 mL 200 mL Limed Figure 6. Centrate color of dewatered sludge with ferrate treatment


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Effects of Polymer Addition on Dewaterbility of Sludge Treated with Ferrate Polymer obtained from the Blue Plains WWTP was added to blended and thickened sludge to observe its effects on dewatering of the sludge. In practice, the polymer was added at the rate of 2.25 mL/2L thickened sludge at the plant. Therefore, in this study 1.125, 2.5, 3.5 mL polymer were applied to 2 L blended sludge treated with ferrate solution. Centrifugation was performed at 2000 rpm for 3 minute in this specific experiment. In general, ferrate solution applied with polymer showed to some degree improved dewaterbility; especially when higher doses of ferrate were applied (Figure 7). 12 mL more water was extracted from the solids treated with 200 mL ferrate solution and 1.125 mL polymer, comparing to the solids with only polymer addition. Under the same operational condition, only 7 mL more water could be extractedfromthe solids with 200 mL ferrate treatment than the control (Figure 5(b)).

Treatment

Figure 7. Dewaterbility of centrifuge at different polymer doses (Rotating velocity was set at 2000 rpm; duration: 3 minutes).


335 Regarding TSS concentration of centrates, polymer alone performed dramatically better than other treatments. Although it was still high, TSS levels of the centrate from ferrate treated solids were lowered with polymer addition. It is improved from 15,000 ppm to 9,000 ppm for 200 mL ferrate treatment as the polymer dose increases (Figure 7). It is hypothesized that small organic particles produced from reactions between ferrate and polymeric organic materials in sludge were coagulated by the polymer. The effects of polymer addition to ferrate treated sludge should be investigated more in detail in future study. On the other hand, limed solids with polymer addition did not show any improvement in sludge dewatering comparing to the control.


Effects of Ferrate Treatment on Solids Dewaterbility by Belt-pressing Dewatering of sludge treated with ferrate via belt-press was tested with a bench-scale belt-press equipped with a pre-filter and a filter-press (Figure 2). In general, belt-press following ferrate treatment did not enhance the dewaterbility of the sludge. When the ferrate solution was applied, the sludge texture changed to liquidized gel, so more sludge was drained through the pre-filter. In addition, the lateral migration of the sludge treated with ferrate was significant, so a considerable amount of sludge was not pressurized for dewatering; the volume shown in Figure 8 includes liquid from pre-filter, from lateral migration and through the belt-filter. For the sludge treated with highest dose of ferrate (200 mL/2L sludge), around 40 mL sludge out of 50 mL was drained through the prefilter and lateral migration during belt-pressing. Once dewatered, the sludge with ferrate treatment showed better solids contents levels, compared to the control and sludge with lime treatment; about 15 % for the sludge treated with 200 mL ferrate and about 9 % for the control and lime treated sludge. 16 14 12 10

E 20

CO

Control

Fe40

Fe 100

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Fe 200

Treatment

Figure 8. Solids contents and drainage volume of belt-pressed sludge with ferrate treatments


336 Polymer addition to ferrate treated solids before belt-pressing did not improve dewaterbility of the sludge (data not shown). Still a considerable amount of sludge was lost through prefilter and lateral migration. Since too much solids was lost through lateral migration, the use of a bench scale laboratory system to dewater ferrate conditioned solids did not appear to be the best technique to simulate full scale belt press dewatering. Additional testing on a larger scale should be evaluated.


Effect of Ferrate on the Performance of Vacuum Filtering in Sludge Dewatering Vacuumfilteringwas also applied to the ferrate treated sludge as illustrated in Figure 3. Sludge with ferrate treatment could not be filtered at all, probably due to the blockage of filter pores by small particles formed through coagulation by reaction products of ferrate degradation, i.e., Fe(III). Sludges with both ferrate treatment and polymer addition were not filterable with the vacuum system, either. In future studies, a robust filter assembly should be used and the surface of the filter also should be examined for blockage by ferrate-induced coagulated small particles.

Other Benefits of Using Ferrate(VI) as a Sludge Conditioner Since ferrate is a strong oxidizer, its application as a sludge conditioner can provide several potential benefits such as destruction of endocrine disrupters and pathogens. In fact, a significant amount of EDCs like NP or OP and its ethoxylates (NPEOs and OPEOs) and PBDEs are frequently detected in sludges. We also detected 2000-10000 ng/g NP and NP(l-5)EOs and 200-1700 ng/g OP and OP(l-3)EOs from sludge samples from the Blue Plains WWTP. However, their levels were significantly reduced with the addition of ferrate. With the ferrate treatment (dose of 10 ~ 40 mL/L), up to 70 % NP and NPEOs of the biosolids were oxidized and complete removal of OP and OPEOs were observed. Although 2-10 ng/g PBDE (i.e., BDE47, BDE99, BDE100) was also detected in the solids, their level did not change with the ferrate addition, probably due to bromide functional groups on the PBDEs which affect organic compound by making them less susceptible to oxidative reductive removal mechanisms. Regarding microbial inactivation, ferrate was very effective in inactivating the microorganisms (Table I); even Clostridium sp. could be completely inactivated at the dose of 40 mL ferrate/L thickened solids. This superior disinfection efficiency can be attributed both to the oxidation power of ferrate(VI) itself and to high solids pH. Although data was not provided, one thing should be mentioned. The ferrate treatment could not inactivate Ascaris ova, which should be achieved to comply with the Class A biosolids guideline (17). In an experiment performed with pure chemicals, the ferrate treatment could not inactivate the ova even at the dose of 0.01 g Fe0 7mL except for causing some color change of the ova. 2

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Table I. Microorganism removal rates (%) of treated thickened sludge(/6") Indicator microorganisms

No

Total Heterotrophs

1x10"

90%

>99 %

>99.9%

>99.9 %

5

70%

>99.9 %

>99.9 %

>99.9 %

6

b

Ferrate solution dose", mL/L thickened solids 80 10 20

Enterococci

5*10

Total coliform

8*10

>99.9 %

>99.9 %

>99.9 %

>99.9 %

Fecal coliforms

5xl0

6

>99.9 %

>99.9 %

>99.9 %

>99.9 %

E. coli

2*10

>99.9 %

>99.9 %

>99.9 %

>99.9 %

55%

>90 %

>99.9 %

>99.9 %

>99.9 %

>99.9 %

>99.9 %

>99.9 %

6

Clostridium perfringens

2xl0

Coli-phage

1*10

5

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Amount of Ferrate in the solution: 0.2 g of ferrate/10 mL solution. Solids content of the thickened solids: approximately 4 %.

b

Conclusion In this study, the effect of ferrate on the performance of conventional bench scale laboratory simulated sludge-dewatering processes, i.e., centrifugation, beltpress, and vacuum filtration were evaluated. In short, sludge treated with ferrate could not be dewatered with a vacuumfilter at all, possibly due to the clogging caused by small particles formed by ferrate. In the case of belt-press, the texture of sludge after ferrate treatment, i.e., liquidized gel, hindered the sludge from being dewatered. A significant amount of sludge were drained through pre-filter before belt-pressing. In addition, significant amount of sludge was lost through lateral migration during the pressing. The effects of ferrate treatment on dewaterbility using centrifugation of thickened sludge, showed a slightly more promising result. In this case, dewaterbility of sludge treated with ferrate increased in proportional to the Gforce or rotating speed (from 1,000 rpm to 3,000) and the time of centrifugation, duration (from 1 min to 5). However, no separation between liquid and solids could not be obtained at the lower ferrate treatment. At relatively higher ferrate level, better dewatering could be achieved. Up to 100 % more water could be extracted compared to the control. The dewaterbility of ferrate treated sludge could be further improved by combining it with cationic polymer. Although centrate from the sludge centrifugation was reddish colored and contained high TSS, it should not be a significant issue if the centrate is sent back to the headworkofthe WWTP. From these results, it appears that ferrate alone is not adequate to provide the dewatering capability required to achieve sufficient water removal from blended sludge. Considering the exceptional ability of ferrate to destroy organic


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compounds with oxidizable functional groups and disinfecting microorganisms, however, it appears that ferrate(VI) has the potential as a multi-purpose chemical conditioner for wastewater sludge, if it is applied as a part of a conditioning program prior to centrifuge dewatering. Therefore, further research should be conducted to combine ferrate with other dewatering tools to gain the positive effects of oxidation and disinfection, which ferrate can provide. Research should focus on the means to tie up the Fe(III) generated from ferrate degradation to reduce the gel-like transition of the sludge material. This may drastically increase the dewatering effectiveness of ferrate for thickened sludge.

Acknowledgement This study was funded by National Science Foundation (Award number: 0406255), which is greatly appreciated.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

Krogmann, U.; Boyles, L.S.; Bamka, W.J.; Chaiprapat, S.; Martel, C.J. Water Environ. Res. 1999, 71(5), 692-714. Dentel, S. K. Water Sci. Technol. 2001, 44(10), 9-18. Oleszkiewicz, J. A.; Mavinic, D. S. Can. J. Civ. Eng. 2001, 28(SuppL 1), 102-114. Hertle, A. Water (Australia) 2003, 30(4), 70-73. Neyens, E.; Baeyens, J.; Weemaes, M.; De Heyder, R. Environ. Eng. Sci. 2002, 19(1), 27-35. Amokrane, A.; Cornel, C ; Veron, J. Water Res. 1997, 31(11), 2775-2782. Diamadopoulos, E. Water Res. 1994, 28(12), 2439-2445. Neyens, E.; Baeyens, J.; De Heyder, B.; Weemaes, M. Management Environ. Quality 2004, 15(1), 9-16. WRC "Permissible Utilization and Disposal of Sewage Sludge Edition 1." TT 85/97, Water Research Commission: Pretoria, South Africa. 1997. U.S. EPA "A plain English guide to the EPA Part 503 Biosolids Rule." EPA832-R-93-003, USA EPA DC, USA. 1994. Ma, J.; Liu, W. Water Res. 2002, 36, 4959-4962. Thompson, G. W.; Ockerman, L. T.; Schreyer, J.M. J. Am. Chem. Soc. 1951, 73, 1379-1381. Dedushenkol, S. K.; Perfiliev, Yu. D.; Goldfield, M. G.; Tspin, A. I. Hyperfine Interact. 2001, 136/137, 373-377 Bouzek, K.; Schmidt, M.J.; Wragg, A. A. Collect. Czech Chem. Commun. 2000, 65, 133-140. Ayol, A.; Dentel, S. K.; Filibeli, A. Water Sci. Technol. 2004, 50(9), 9-16. Kim, H.; Millner, P.; Sharma, V.K.; McConnell, L.L.; Torrents, A.; Ramirez, M.; Peot, C. Water Environ. Lab. Solutions 2006, 12(6), 1-6. U.S. EPA 40 CFR Part 503: The Standard for the Use or Disposal of Sewage Sludge, 1993, 58, 9248-9404.


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