High-solids anaerobic digestion of sewage sludge in China
Dai, X.1 and Zhou, X.2, 1Tongji University, 2National Engineering Research Center for Urban Pollution Control, China
(free)
Because of the rapid development of economy and urbanization in China, a huge number of waste water were discharged, more and more waste water treatment plants were built to treat the waste water. The sludge production increases also year by year. The treatment and disposal of sludge from WWTPS becomes one of the most important environmental topics in China. This paper introduced the current situation of sludge treatment and disposal in China, and compared the situation of sludge anaerobic digestion between China and other countries. Feasibility of high-solids anaerobic digestion of sewage sludge was investigated. The result shows, that high-solids anaerobic digestion of sludge can improve the efficiency of system, and is proved to be feasible. At the end, the prospect of high-solids anaerobic digestion in China will be mentioned.
Keywords: Anaerobic digestion; High-solid; Mesophilic; Sewage sludge
Introduction
Sewage sludge, the by-product of biological wastewater treatment processes, is expected to increase continuously in the next decade, due to increasing population connected to sewage networks, building new waste water treatment plants (WWTPs) and upgrading existing plants to meet the more stringent local effluent regulations. In China, over 3 x 1010 kg dewatered sludge (80% moisture content) from municipal
wastewater treatment plants is generated in 2011, and almost 80% of it has not be sufficiently stabilized (Duan et al., 2012). Most of the dewatered raw sludge is directly disposed in landfills (Figure 1). The bio-energy in sludge was rarely utilized. It is estimated that sludge production might reach 6 x 1010 kg per year by 2020. Thus, it is essential to develop proper treatment processes to reduce the amount of sludge. Anaerobic digestion and landuse of sewage sludge is adopted to be one of the key technology roadmap in China. It can realize sludge stabilization by converting a part of its organic matter into biogas which is a renewable energy source. The volume reduction rate could reach 30%-50%. This technology has been successfully implemented in the treatment of agricultural wastes, food wastes, and sewage sludge (Chen et al., 2008).
Figure 1: Ratio of sludge disposal in China
Sludge anaerobic digestion is the most applied stabilized technology in EU. There are more than 50,000 municipal wastewater treatment plants in EU countries, producing more than 50 million tons of sludge every year (estimated with water content of 80%). Over 50% of sludge is stabilized with anaerobic digestion (European Commission, 2010). Using anaerobic digestion to produce methane can not only reduce the pollution load of sludge to the environment but also produce biogas, which could supply energy for sewage treatment plants and surroundings to a certain degree (Bungay, 2004; RubioLoza and Noyola, 2010).
However, the use of traditional anaerobic digester is not always feasible in China. Until now about 60 anaerobic digestion facilities of sludge were built, and only about 15 Plants among them are in operation. Anaerobic digestion was not well applied in China mainly
due to sludge characteristics. Low organic matter of sludge causes less biogas production; high content of inorganic sands causes easy abrasion of equipment. Low efficiency of sludge anaerobic digestion is also due to poor management, unprofessional operation, economical limitation and inadequate planning. Hence, how to improve the efficiency of sludge anaerobic digestion became a key issue to be
researched in China.
