Sewage Sludge in China: Challenges Toward a Sustainable Future

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1 Sewage Sludge in China: Challenges Toward a Sustainable Future P. J. He*, F. Lü*, H. Zhang*, L. M. Shao* and D. J. Lee** * State Key Laboratory of Pollution Control and Resources Reuse, Tongji University, Shanghai , P.R. China ( solidwaste@mail.tongji.edu.cn) ** Department of Chemical Engineering, National Taiwan University, Taipei 10617, Chinese Taiwan ( djlee@ntu.edu.tw) Abstract: Sewage sludge is of increasing concern in China, due to extended sewerage control and advanced wastewater treatments resulting from urbanization and economic growth. Based on the evolution of municipal sewage generation and treatment technologies in the last decade, as well as the long-term national and local plans in China, the paper elucidates production, distribution and characteristics (organics, nutrients, heat value, heavy metals, trace pollutants, pathogens) of sludge in some typical districts of China. The status and challenges of sludge management are discussed. Keywords: China; management; sewage sludge; status and perspectives; sustainability INTRODUCTION In China, more than one thousand municipal wastewater treatment plants (WWTPs) will be put to operation in the next 10 years, most of which will adopt secondary or tertiary treatment processes. Increasing sewage treatment capacity and enhanced treatment processes will produce huge quantity of sewage sludge. Lee et al. (2006) reviewed the perspectives of sludge management in Beijing and Shanghai of China. This paper overviewed the current status of sludge management in China. SEWAGE AND SEWAGE SLUDGE In the past five years, the collected sewage in China increased in amount by 18.2%, while the sewage treatment capacity increased by 135%. At the end of 2005, 39% of the collected sewage was treated in WWTPs (Figure 1). A total of 792 sewage treatment plants located in 661 cities treated million tons of wastewater per day (Figure 2). 87.4% of these WWTPs were operated with secondary or tertiary treatment processes. Up to June, 2005, 297 cities in China had no WWTPs for treating their sewage, indicating the potential growth of sewage treatment capability. Development in different regions of China is rather unbalanced, as evidenced by the implementations of big projects for sewage infrastructures in major provinces. The eastern regions of China have more WWTPs than the reminder area (Figure 3). The scale of municipal WWTPs is generally small (< m 3 d -1 ) to medium ( m 3 d -1 ), accounting for 70% 80% of all plants (Table 1). The WWTP scale is directly proportional to city scale. Mega-cities like Shanghai and Beijing have super WWTPs (> m 3 d -1 ). Besides, the capacities of these super plants account for more than 50% of all plants. In Shanghai, Bailonggang WWTP ( m 3 d -1 ), Zhuyuan WWTP ( m 3 d -1 ), and Shidongtou WWTP ( m 3 d -1 ) possess 88.2% of m 3 d -1 capacity for Shanghai City in year 2007 (Shanghai City Government, 2006). In Beijing, Gaobeidian WWTP ( m 3 d -1 ), Xiaohongmen WWTP ( m 3 d -1 ), Zhengwangfen WWTP ( m 3 d -1 ), and Qinghe WWTP ( m 3 d -1 ) will account for 73.6% of m 3 d -1 capacity for year 2008 (Lee et al., 2006). Centralized sewage treatment encourages sludge treatments at these super WWTPs. 39

2 Figure 1: Sewage generation and treatment in China (National Bureau of Statistics of China, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006; State Environmental Protection Administration of China, 1999, 2000, 2001, 2002, 2003, 2004, 2005) * The gap between year 2002 and 2003 was owing to more cities accounted in Figure 2: WWTPs in China (National Bureau of Statistics of China, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006; State Environmental Protection Administration of China, 1999, 2000, 2001, 2002, 2003, 2004, 2005) 40

3 Figure 3: The spatial scheme for the ratio of sewage treated in WWTPs in 2005 (State Environmental Protection Administration of China, 1999, 2000, 2001, 2002, 2003, 2004, 2005) Table 1: Scale of the municipal WWTPs in 2005 (Zhejiang Province Government, 2005; Jiangsu Province Government, 2006; Lee et al., 2006; Shanghai City Government, 2006) The existing municipal WWTPs in China adopt numerous treatment processes, including conventional activated sludge, triple oxidation ditch, modified triple oxidation ditch, UNITANK, A2O, SBR, A/O, A2O-oxidation ditch, MSBR, CAST, CASS, PACT, Carrousel oxidation ditch, Orbal oxidation ditch, modified A2O, biofilter, SPF, biological contact oxidation, and wetland. These biological processes are commonly applied with chemical and physical treatment stages. The SBR, modified SBR, A2O, and oxidation ditch are the most popular processes adopted in China (Figure 4). 41

4 Figure 4: Fractions of wastewater treatment processes applied in Jiangsu Province in 2006 (Jiangsu Province Government, 2006) In 2005, 9 million tons of sewage sludge was generated as dewatered cake with water content 80%, accounting for 7% of solid waste in China. Since SBR, A2/O, and oxidation ditch are commonly adopted for sewage treatment, the dry sludge production is only 0.01% 0.014% of the treated sewage (Figure 4). However, in the next 10 years the number of sewage treatment plants will increase to more than 1000, with most of the newly established plants equipped with nutrient removal processes. Hence, the dewatered sewage sludge production is expected to reach 27 million tons per year in the near future. CHARACTERISTICS OF SLUDGES Macro-nutrients and heat values The sludge from municipal WWTPs contains macro-nutrients including organics, nitrogen, phosphorus, and potassium. As shown in Tables 2 and 3, sewage sludge contained more organics, nitrogen and phosphorus than manures, with the latter usually used as fertilizer in countryside. Table 2: Contents of macro-nutrients in sludge from 43 WWTPs in Jiangsu Province (Jiangsu Province Government, 2006) 42

5 Table 3: Macro-nutrients and heat value of sludge from 8 WWTPs in Shanghai City (2006) Heavy metals Tables 4 and 5 list heavy metal contents in sewage sludges collected in Jiangshu Province and Shanghai City. All heavy metals and As had the potential to surpass the limits for agricultural use. In summer, the concentrations of Pb, Cr, Ni, Cu and Zn were higher than those in winter except for Cd (data not shown). It could be attributed to better treatment efficiency in summer. The investigation shows that the WWTPs with higher heavy metals in sludge usually accept more industrial wastewater. Table 4: Heavy metals and As in sludge from 43 WWTPs in Jiangshu Province (Jiangsu Province Government, 2006) Table 5: Heavy metals and As in sludge from 11 WWTPs in Shanghai City in

6 Organic micro-pollutants Organic micro-pollutant contents listed in Table 6 for sewage sludge should be considered preliminary. The organic content, nitrogen, phosphorus and potassium in sludge meet the requirements for agricultural use. However, heavy metals are still the matter of concern. By stringent pollution control of industrial wastewater, the risk introduced by heavy metals can be reduced. Organic micro-pollutants like endocrine disrupters are new emerging considerations. Case studies showed that the risk from micro-pollutants could be limited when less industrial wastewater was mixed into the municipal sewage. It is commonly agreed that the principal problem related to sludge characteristics is too high moisture content of dewatered sludge. Table 6: Micro-pollutants in sludge (mg kg-1) (Jiangsu Province Government, 2006) TREATMENT AND DISPOSAL The sludge treatment in WWTPs has been overlooked. Thickening and dewatering are commonly applied in plants. Most dewatered sewage sludge is open dumped or landfilled with municipal solid waste (Yu et al., 2007). No more than 10% of sewage sludge is safely applied in land (Figure 5). Owing to high water content of dewatered sludge (80% 85%), the sludge has too poor strength to be accepted by landfills. Regulations require the establishment of stabilization units (anaerobic or aerobic digestion/composting) for sludge. However, only 25% of existing plants have sludge stabilization unit. What s more, few of them are in operation owing to lacking of running cost. Figure 5: Ratio of sludge treatment in Jiangsu Province in 2006 (Jiangsu Province Government, 2006) PERSPECTIVES The sludge management practice in China will be significantly changed in the next five years. National regulations are promulgating with construction of specialized treatment plants for sludge, including drying, composting, incineration, and brick manufacture. Main cities in China started drafting plans for sludge treatment and disposal since Guangzhou city will use sludge as fertilizer. Shenzhen city will adopt drying and incineration for treating its sludge. Shanghai city will adopt scheme with anaerobic digestion dewatering drying composting for horticultural use landfill, dewatering drying incineration (with or without coal), and centralized facilities for super WWTPs, dispersed treatment for medium and small WWTPs. Tianjing city will construct three anaerobic digestion plants for sludge treatment. Beijing city will have extensive land use of sludge. Hefei city will incinerate sludge with coal. Wuhan city will take compositing and brick making options. Taiyuan city will compost the sludge to fertilizer. The dewatered sludge of Shenyang city will be incinerated. The first sludge treatment plant was in operation in Guangdong Province in 2004 at 900 t d-1, and now extended to 1200 t d-1, in which deodorization and detoxification was applied followed by dewatering and brick manufacture. 44

7 There were only three regulations related to sludge in China, Control standards for pollutants in sludges for agricultural use (GB ), Criteria for controlling the discharge of sewage and sludge from municipal wastewater treatment plant (CJ ) and Criteria for controlling the discharge of pollutants from municipal wastewater treatment plant (GB ). The standard GB was promulgated in 1984, but never been amended. The limits set for heavy metals need updating. While the micro-pollutants and pathogen control are totally disregarded. The sludge was requested to be stabilized before end use, but lacking quantitative measure of degree of stabilization. New regulations will be announced shortly to meet the mentioned concerns. It is not realistic to search for a universal solution to sewage sludge problem in China. Optimal solutions for local need require site-specific considerations. Different sludge management strategies with hierarchic structure should be evaluated, with regard to the economic level, climate, city planning, social concerns, and soon. ACKNOWLEDGMENTS The authors acknowledge the financial support given by Hi-Tech Research and Development Program of China (863 Program) (No.2006AA06Z384). REFERENCES Criteria for controlling the discharge of pollutants from municipal wastewater treatment plant (2002). GB State Environmental Protection Administration of China, Beijing, China. Jiangsu Province Government (2006). Strategy study on the treatment technologies for sludge from municipal wastewater treatment plant. Lee D. J., Spinosa L., He P.J. and Chen T. B. (2006). Sludge production and management processes: case study in China. Water Science and Technology, 54(5), National Bureau of Statistics of China (1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006). China Statistical Yearbook 1999/2000/2001/2002/2003/2004/2005/2006. China Statistics Press, Beijing. Shanghai City Government (2006). Future plan for sewage treatment system for Shanghai. State Environmental Protection Administration of China (1999, 2000, 2001, 2002, 2003, 2004, 2005). State Environmental Statistic Reports 1999/2000/2001/2002/2003/2004/2005. Beijing, China. Yu J., Tian N.N., Wang K.J. and Ren Y. (2007). Analysis and discussion of sludge disposal and treatment of sewage treatment plants in China. Chinese Journal of Environmental Engineering, 1(1),: Zhejiang Province Government (2005). Plan for the treatment of municipal sewage in the 11th 5 years period. 45