Gamma ray irradiation for sludge solubilization and biological nitrogen removal

Radiation Physics and Chemistry 80 (2011) 1386–1390

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Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem

Review

Gamma ray irradiation for sludge solubilization and biological nitrogen removal Tak-Hyun Kim a,n, Myunjoo Lee a, Chulhwan Park b a b

Radiation Research Division for Industry and Environment, Korea Atomic Energy Research Institute, Jeongeup 580-185, Republic of Korea Department of Chemical Engineering, Kwangwoon University, Seoul 139-701, Republic of Korea

a r t i c l e i n f o

a b s t r a c t

Article history: Received 9 March 2011 Accepted 22 June 2011 Available online 7 July 2011

This study was conducted to investigate the effects of gamma ray irradiation on the solubilization of waste sewage sludge. The recovery of an organic carbon source from sewage sludge by gamma ray irradiation was also studied. The gamma ray irradiation showed effective sludge solubilization efficiencies. Both soluble chemical oxygen demand (SCOD) and biochemical oxygen demand (BOD5) increased by gamma ray irradiation. The feasibility of the solubilized sludge carbon source for a biological nitrogen removal was also investigated. A modified continuous bioreactor (MLE process) for a denitrification was operated for 20 days by using synthetic wastewater. It can be concluded that the gamma ray irradiation was useful for the solubilization of sludge and the recovery of carbon source from the waste sewage sludge for biological nitrogen removal. & 2011 Elsevier Ltd. All rights reserved.

Keywords: Gamma ray Sewage sludge Carbon source Denitrification

Contents 1. 2.

3.

4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1386 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1387 2.1. Gamma ray irradiation for sludge solubilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1387 2.2. Biological denitrification using solubilized sludge carbon source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1387 2.3. Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1387 Results and discussions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1387 3.1. Chemical characteristics of waste sewage sludge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1387 3.2. Sludge solubilizaton and carbon source recovery by gamma ray irradiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1387 3.3. EPS production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1388 3.4. Release of nitrogenous compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1388 3.5. Biological nitrogen removal using solubilized sludge carbon source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1389 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1390 Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1390 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1390

1. Introduction Common practice in wastewater is the incorporation of biological processes. These processes produce residues (sludge) which require disposal. Therefore, it is necessary to find more efficient treatment methods in order to reduce sludge production of wastewater treatment plants. Sewage sludge contains a significant amount of organic substances and nitrogenous components.

n

Corresponding author. Tel.: þ82 63 570 3343; fax: þ82 63 570 3348. E-mail address: [email protected] (T.-H. Kim).

0969-806X/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2011.06.011

For nitrogen removal by biological process, ammonia is aerobically oxidized to nitrate (nitrification), and the nitrate is then reduced to nitrogen gas (denitrification). Since the denitrifiers need an electron donor, an external organic compound is usually added to the denitrification reactor. Recently, external carbon sources are being used at domestic wastewater treatment plants, which incur an extra cost. Many commercially available organic compounds are applied as carbon source for a biological denitrification. Of these, methanol is the most commonly used due to its cost effectiveness (Quan et al., 2005). However, the use of such external carbon sources results in an increase of the operational costs. Therefore, for sludge reduction and carbon source recovery

T.-H. Kim et al. / Radiation Physics and Chemistry 80 (2011) 1386–1390

by chemical, thermal and thermochemical methods (Shanableh, 2000; Aravinthan et al., 2001), the aim was to solubilize waste sludge and produce a readily biodegradable carbon sources. Alkaline or acid hydrolysis, and ozone oxidation are often studied for a carbon source recovery from a waste sewage sludge (Lin et al., 1998; Neyens et al., 2003; Song et al., 2003; Cui and Jahng, 2004; Dytczak et al., 2007). Radiation processing such as gamma ray and electron beam has been considered as a promising technology for the treatment of wastewater or sludge. Electron beam treatment has been used to enhance the biodegradability of wastewater containing various biologically refractory organic compounds such as textile wastewater, landfill leachate, paper mill wastewater, and effluent from a petroleum production (Kim et al., 2007; Auslender et al., 2002; Bae et al., 1999; Duarte et al., 2004). Gamma ray irradiation as a pretreatment process has been studied to release soluble carbohydrates from an activated sludge (Mustapha and Forster, 1985), and electron beam irradiation has been applied for the wastewater sludge volume reduction (Zheng et al., 2001). However, the feasibility of the solubilized sludge carbon source by gamma ray irradiation on the biological nitrogen removal has seldom been investigated. This study was designed to investigate sludge solubilization by gamma ray irradiation and the use of sludge as a carbon source for a biological denitrification.

2. Experimental 2.1. Gamma ray irradiation for sludge solubilization The waste sewage sludge was collected from a municipal wastewater treatment plant located in Jeongeup, Korea. The sludge sample was collected from the secondary settling tank. For gamma radiolysis, the sludge samples were placed into 1 L glass screw cap bottles and irradiated at the condition of atmospheric pressure and room temperature (25 71 1C). Gamma ray was irradiated using a high-level 60CO source (Nordion, Inc., Canada) at the Korea Atomic Energy Research Institute (Jeongeup, Korea). The absorbed doses were measured using the alanine-EPR dosimetry system (ISO/ASTM 51607:2003) (ASTM, 2004). 2.2. Biological denitrification using solubilized sludge carbon source A modified Ludzack–Ettinger (MLE) process was used for the biological denitrification test. Two MLE systems were operated at room temperature (25 71 1C). The solubilized sewage sludge was used as an external carbon source for denitrification. For a comparison of the biological nutrient removal system with the solubilized sludge carbon source, a parallel system using methanol (CH3OH) was carried out. The total working volume was about 30 L (anoxic tank 6 L, oxic tank 14 L and settling tank 10 L), and the hydraulic retention time (HRT) was 2 days. Wastewater was continuously pumped into the anoxic reactors, at a rate of about 10 L/d. Synthetic wastewater was prepared to resemble common sewage wastewater (Quan et al., 2005). Methanol and the solubilized sludge carbon source were added to individual reactors at an initial COD value of 300 mg/L. The influent SCOD:T-N ratio was adjusted to 300:50. The influent pH was 7.0–7.5, therefore, it was used without pH adjustment. Initial MLSS was set to be about 3000 mg/L using an activated sludge from a municipal wastewater treatment plant. 2.3. Analysis Waste sewage sludge samples after gamma ray irradiation were centrifuged at 4000 rpm for 20 min and filtered through

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a glass microfiber filter (1.2 mm pore size, Whatman GF/C). The filtrate was analyzed for soluble chemical oxygen demand (SCOD), total chemical oxygen demand (TCOD), biochemical oxygen demand (BOD5), total nitrogen (T-N), ammonia nitrogen (NH3-N), organic nitrogen (Org-N), nitrite nitrogen (NO2-N) and nitrate nitrogen (NO3-N). SCOD and TCOD concentrations were measured using a closed reflux-colorimetric method (colorimeter: Hach DR/4000). NH3-N, NO2-N and NO3-N were analyzed by ionchromatograph equipped with an AS-18 4 mm column (model ICS-2000; Dionex Corp.). The pH was measured using a pH meter (Orion Ross ultra pH, Thermo Electron Corporation). Total solids (TS), volatile solids (VS) and BOD5 were measured by Standard Methods (APHA, 1995). The carbohydrate content of solubilized sludge was measured by the anthrone method (Gaudy, 1962) using glucose as the standard. The content of protein and humic substance were measured by the modified Lowy method (Frolund et al., 1996).

3. Results and discussions 3.1. Chemical characteristics of waste sewage sludge Table 1 shows the chemical characteristics of the raw waste sewage sludge. The waste sewage sludge contained 16,200 mg/L of total solids (TS), 14,740 mg/L TCOD and 700 mg/L SCOD. The SCOD of the original sludge was only 4.7% of the TCOD. These results indicated that the sludge had the potential for converting particulate COD into SCOD. The concentration of T-N in the raw sludge was 92 mg/L. NH3-N and Org-N were predominant among the nitrogen components. SCOD/T-N ratio of the raw sewage sludge was 7.5. 3.2. Sludge solubilizaton and carbon source recovery by gamma ray irradiation Water radiolysis by gamma ray irradiation forms some ionized and very reactive molecules such as OH, e-aq and H (Getoff, 1996). The reducing species H-atoms and the solvated electrons (e-aq) are converted to oxidizing species (HO2, O2-) in the presence of oxygen and water. These oxidizing radicals with the OH-radicals can enhance the production of SCOD from waste sewage sludge. The production of SCOD from waste sewage sludge by gamma ray irradiation has potential to provide a good carbon source for denitrification, leading to substantial cost savings for wastewater treatments. The SCOD/TCOD ratio and the BOD5/SCOD ratio have been used as indicators to evaluate the enhancements of solubilization and biodegradability (Bougrier et al., 2006). Fig. 1 shows the trends of the sludge solubilization by a gamma ray irradiation. From Fig. 1(a), the SCOD concentration and the solubilization ratio (SCOD/TCOD) increased in proportion to gamma ray dose. As the gamma ray dose was increased from 0 kGy (non-irradiated) to Table 1 Chemical characteristics of the waste sewage sludge. Item

Range

Average

pH TS (mg/L) TCOD (mg/L) SCOD (mg/L) NH3-N (mg/L) NO2-N (mg/L) NO3-N (mg/L) Org-N (mg/L) T-N (mg/L)

6.3–6.8 12,300–23,700 9940–17,360 40–810 18–52 0.008–0.032 0.2–2 15–70 34–98

6.6 16,200 14,740 700 48 0.021 0.3 29 92

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25

4000 SCOD SCOD/TCOD

SCOD, mg/l

15 2000 10 1000

0

SCOD/TCOD, %

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0 60

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BOD5 , mg/l

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10

BOD5 /SCOD

0

0

10

20

30

40

50

0 60

Dose, kGy Fig. 1. Effects of gamma ray irradiation on the sludge solubilization (SCOD, SCOD/ TCOD) (a), and the sludge biodegradability (BOD5, BOD5/SCOD) (b).

50 kGy, the SCOD of sludge had also increased from 700 to 2850 mg/L. However, the TCOD concentration remained almost constant at different gamma ray doses. Therefore, the SCOD/TCOD ratio showed a similar pattern of change in the SCOD concentration. SCOD/TCOD ratio increased from 4.8% to 19.0%, as the gamma ray dose was increased from 0 to 50 kGy. Fig. 1(b) shows the changes in the BOD5 and BOD5/SCOD ratio after the different gamma ray doses from 0 to 50 kGy. The BOD5 concentration increased from 160 to 787 mg/L, as the gamma ray irradiation converts non-biodegradable organics to biodegradable compounds (Kim et al., 2007). The BOD5/SCOD ratio indicates the change of the amount of biodegradable compounds in the waste sewage sludge. In Fig. 1(b), the BOD5/SCOD ratio increased from 22.9% to 40.0% up to 10 kGy. However, it decreased to 26.6%, as the gamma ray dose was increased from 10 to 50 kGy. It can be inferred that gamma ray irradiation could produce a greater amount of non-biodegradable SCOD from a sludge solid than that of a converted non-biodegradable compound into biodegradable SCOD after applying specific gamma ray dose value. 3.3. EPS production Sludge extracellular polymeric substances (EPS) has been reported as a major sludge floc component (Liu and Fang, 2002). Extracellular polymeric substances are metabolic products that accumulate on the bacterial cell surfaces. EPS are composed of various organic substances such as protein, carbohydrate and humic substances. It can be used as a degree of sludge

solubilization. Gamma ray irradiation would disrupt the complex sludge floc structure and release extracellular substances such as proteins, carbohydrates, and humic substances from the floc structure into the soluble phase. It would enhance the solubilization of particulate COD. The concentrations of proteins, carbohydrates and humic acids were analyzed after the gamma ray irradiation. Fig. 2 shows the effects of gamma ray irradiation on the production of EPS. In Fig. 2, carbohydrates and humic acid were increased with gamma ray doses. The carbohydrate concentration of the non-irradiated sludge was 4.9 mg/L, it increased to 85.7 mg/L with 50 kGy gamma ray irradiation. The concentration of humic acid increased from 22.4 to 260.2 mg/L by 50 kGy gamma ray dose. However, the concentration of protein was increased from 5.8 to 53.9 mg/L up to 10 kGy, but showed no further increase. The increase of carbohydrates may be due to the disruption of cell wall polysaccharides, which are composed of sugar-ring monomer units connected by covalent glycosidic bonds. These glycosidic linkages within the polysaccharides may have been broken during gamma ray irradiation. As the gamma ray dose increases, the hydrogen bonds and non-polar hydrophobic interactions in the protein structure may begin to break and destabilize the protein by the oxidizing radicals such as OH-radical produced during gamma ray irradiation. However, protein is known to be the most difficult compound to extract (Frolund et al., 1996). 3.4. Release of nitrogenous compounds It should be noted that about 50% of a dry cell weight of sludge is occupied by proteins that contain nitrogen in the molecular structure. Since the waste sewage sludge was treated by an oxidation process, some nitrogenous compounds such as ammonium, nitrite, nitrate and organic nitrogen, could be released from the sludge and be accumulated in the system. If the supernatant of a solubilized sludge were to be recycled in a biological nitrification–denitrification process, the resultant additional nitrogen loading should be taken into consideration (Aravinthan et al., 2001; Cui and Jahng, 2004). Fig. 3(a) shows the amounts of solubilized total nitrogen at different gamma ray doses. It increased with an increasing gamma ray dose. At 0 kGy of a gamma ray dose (non-irradiated), the concentration of the solubilized total nitrogen (T-N) was 92 mg N/L. When the experimental condition was changed to 50 kGy of a gamma ray dose, the concentration of the T-N 300 250 Concentration, mg/l

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Carbohydrate Protein Humic acid

200 150 100 50 0

0

1

10 Dose, kGy

20

50

Fig. 2. Effects of gamma ray irradiation on the release of extracellular polymeric substances (carbohydrate, protein, humic acid) from waste sewage sludge.

T.-H. Kim et al. / Radiation Physics and Chemistry 80 (2011) 1386–1390

500

250 Org. N NH3 -N

400

NO2 -N

COD, mg/l

200

NO3 -N

Nitrogen, mg/l

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300 200 100

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Time, day

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NH3-N, mg/l

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60 40 20

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Fig. 3. Distributions of the nitrogenous compounds and total nitrogen concentration (a) and SCOD/T-N ratio (b) after gamma ray irradiation.

0

5

10 Time, day

40

3.5. Biological nitrogen removal using solubilized sludge carbon source Lab-scale biological nitrogen removal experiments were carried out using MLE system to demonstrate the feasibility of sludge carbon source recovered by gamma ray. Gamma ray dose of

30 NO3-N, mg/l

solubilization increased to 210 mg N/L. Fig. 3(a) also illustrates the distribution of various nitrogenous compounds under different gamma ray doses. Organic nitrogen (Org-N) was found to be the major product in all the experimental conditions. The percentage of Org-N distribution ranged 46–65%. The percentage of ammonia nitrogen (NH3-N) and nitrate nitrogen (NO3-N) distribution ranged 32–54% and 0.3–2.1%, respectively. Nitrite nitrogen (NO2-N) was the lowest product, with a percentage of less than 0.1%. All concentrations of NO2-N were less than 0.2 mg/L. Fig. 3(b) shows the changes of SCOD/T-N ratio at different gamma ray doses. The SCOD/T-N ratio increased with an increase of the gamma ray dose. The SCOD/T-N ratio increased from 7.5 to 13.6 as the gamma ray dose was increased from 0 kGy (nonirradiated) to 50 kGy. The chemical oxygen demand to total nitrogen ratio (COD/T-N) is one of the most critical parameters of the biological nitrogen removal process because it affects the growth competition between autotrophic and heterotropic microorganism populations (Carrera et al., 2004).

20

10

0

0

5

10 Time, day

Fig. 4. Comparisons of COD (a), T-N (b), NH3-N (c) and NO3-N (d) removals in the MLE reactors for a synthetic sewage wastewater between methanol and the solubilized sludge carbon source, MeOH-in, K; MeOH-out, J; sludge-in, .; sludge-out, X.

50 kGy is significant additional energy and would not likely be economically viable when compared to methanol. Therefore, the sludge carbon source treated at 10 kGy gamma ray dose was used for biological nitrogen removal. The solubilized sludge and methanol were used as the carbon sources at a SCOD:T-N ratio of 300:50. Fig. 4 shows the changes of removal level of COD, T-N, NH3-N and NO3-N in two MLEs. From Fig. 4(a), the average COD values of the effluents from two reactors were 5.7 mg/L for methanol and 45.7 mg/L for the solubilized sludge carbon source.

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T.-H. Kim et al. / Radiation Physics and Chemistry 80 (2011) 1386–1390

When methanol was applied as a carbon source, most of the COD was consumed and the average effluent COD value was observed at very low concentrations of around 5.7 mg/L, corresponding to a 98% COD removal efficiency. However, when the solubilized sludge carbon source was used, the unused carbon compound was accumulated in the effluent and resulted in a high COD level, 45 mg/L. About 15% of the COD was not used and accumulated in the effluent. In the application of a solubilized sludge to improve the biological denitrification efficiency, the important drawbacks could be the facts that the addition of a carbon source includes not only the addition of nitrogenous compounds but also the possibility of the presence of a slowly biodegradable residual COD in the effluent (Aravinthan et al., 2001). Fig. 4(b) shows the behaviors of the T-N removal at SCOD:T-N ratio of 300:50. When methanol was applied as a carbon source, the T-N removal efficiency was 55.6%. When the solubilized sludge was used as a carbon source, the T-N removal efficiency was 51.1%. Fig. 4 also shows the ammonia nitrogen (c) and nitrate nitrogen (d) removals in two MLEs. Ammonia was completely oxidized and not detected in the effluent of both systems. However, the concentration of nitrate nitrogen was 20.6– 35.8 mg NO3-N/L with methanol, and 33.3–51.4 mg NO3-N/L with the solubilized sludge carbon source. The nitrogenous compounds in the effluent appeared mostly in the form of nitrate nitrogen in both processes. It means that a nitrification had almost been perfectly completed. Conclusively this data indicated that the solubilized sludge was capable of denitrifying nitrate but was insufficient in completely removing nitrogen originating from the solubilized sludge itself. Therefore, when the solubilized sewage sludge is used for the biological nitrogen removal, additional nitrogen loading must be considered (Lin et al., 1998).

4. Conclusions The solubilization of sludge for the recovery of a carbon source was investigated by gamma ray irradiation. Gamma ray irradiation effectively disrupted the sludge floc and degraded biological cells, which increased the SCOD concentration. The production of EPS was also enhanced by the gamma ray irradiation. However, the nitrogenous compounds were released to the supernatant and brought an additional nitrogen load on the system. Operating for 20 days the MLE processes using a synthetic wastewater, the solubilized sludge carbon source had similar denitrification efficiencies to methanol. These results show that the sewage sludge carbon source recovery by a gamma ray irradiation for a biological nitrogen removal can be one of options for sewage sludge management.

Acknowledgment This research was supported by the Nuclear R&D program and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology. References APHA, AWWA, WPCF, 1995. Standard Methods for the Examination of Water and Wastewater, 19th ed. APHA, AWWA, WPCF, Washington, DC. Aravinthan, V., Mino, T., Takizawa, S., Satoh, H., Matsuo, T., 2001. Sludge hydrolysate as a carbon source for denitrification. Water Sci. Technol. 43, 191–199. ASTM, 2004. Standards on Dosimetry for Radiation Processing: ISO/ASTM 51607:2004 2nd ed. ASTM International, West Conshohocken, PA, USA. Auslender, V.L., Ryazantsev, A.A., Spiridonov, G.A., 2002. The use of electron beam for solution of some ecological problems in pulp and paper industry. Radiat. Phys. Chem. 63, 641–645. Bae, B.-U., Jung, E.-S., Kim, Y.-R., Shin, H.-S., 1999. Treatment of landfill leachate using activated sludge process and electron-beam radiation. Water Res. 33, 2669–2773. Bougrier, C., Albasi, C., Delgens, J.P., Carrre, H., 2006. Effect of ultrasonic, thermal and ozone pre-treatments on waste activated sludge solubilization and anaerobic biodegradability. Chem. Eng. Proces. 45, 711–718. Carrera, J., Vicent, T., Lafuente, J., 2004. Effects of influent COD/N ratio on biological nitrogen removal (BNR) from high-strength ammonium industrial wastewater. Process Biochem. 39, 2035–2041. Cui, R., Jahng, D., 2004. Nitrogen control in AO process with recirculation of solubilized excess sludge. Water Res. 38, 1159–1172. Duarte, C.L., Geraldo, L.L., Junior, O.A.P., Borrely, S.I., Sato, I.M., Sampa, M.H.O., 2004. Treatment of effluents from petroleum production by electron beam irradiation. Radiat. Phys. Chem. 71, 443–447. Dytczak, M.A., Londry, K.L., Siegrist, H., Oleszkiewicz, J.A., 2007. Ozonation reduces sludge production and improves denitrification. Water Res. 41, 543–550. Frolund, B., Palmgren, R., Keiding, K., Nielsen, P.H., 1996. Extraction of extracellular polymers from activated sludge using a cation exchange resin. Water Res. 30, 1749–1758. Gaudy, A.F., 1962. Colorimetric determination of protein and carbohydrate. Ind. Water Wastes 7, 17–22. Getoff, N., 1996. Radiation-induced degradation of water pollutants-state of the art. Radiat. Phys. Chem. 47, 581. Kim, T.-H., Lee, J.-K., Lee, M., 2007. Biodegradability enhancement of textile wastewater by electron beam irradiation. Radiat. Phys. Chem. 76, 1037–1041. Lin, J.-G., Ma, Y.-S., Huang, C.-C., 1998. Alkaline hydrolysis of the sludge generated from a high-strength, nitrogenous-wastewater biological-treatment process. Bioresource Technol. 65, 35–42. Liu, H., Fang, H.H.P., 2002. Extraction of extracellular polymeric substances (EPS) of sludges. J. Biotechnol. 95, 249–256. Mustapha, S., Forster, C.F., 1985. Examination into the gamma irradiation of activated sludge. Enzyme Microb. Technol. 7, 179–181. Neyens, E., Baeyens, J., Creemers, C., 2003. Alkaline thermal sludge hydrolysis. J. Hazard. Mater. 97, 295–314. Quan, Z.-X., Jin, Y.-S., Yin, C.-R., Lee, J.J., Lee, S.-T., 2005. Hydrolyzed molasses as an external carbon source in biological nitrogen removal. Bioresource Technol. 96, 1690–1695. Shanableh, A., 2000. Production of useful organic matter from sludge using hydrothermal treatment. Water Res. 34, 945–951. Song, K.-G., Choung, Y.-K., Ahn, K.-H., Cho, J., Yun, H., 2003. Performance of membrane bioreactor system with sludge ozonation process for minimization of excess sludge production. Desalination 157, 353–359. Zheng, Z., Kazumi, J., Waite, T.D., 2001. Irradiation effects on suspended solids in sludge. Radiat. Phys. Chem. 61, 709–710.