Journal of Biotechnology 127 (2007) 443–451 Excess sludge reduction induced by Tubifex tubifex in a recycled sludge reactor Xia Huang ∗ , Peng Liang, Yi Qian Environment Simulation and Pollution Control State Key Laboratory, Department of Environmental Science and Engineering, Tsinghua University, Beijing 100084, PR China Received 11 April 2006; received in revised form 6 July 2006; accepted 20 July 2006 Abstract Sludge reduction in a conventional activated sludge (CAS) process combined with a recycled sludge reactor where Tubifex tubifex (one of Oligochaeta) was inoculated was investigated in this study. The results showed sludge production could be reduced through T. tubifex’s predation on sludge in the recycled sludge reactor. The sludge reduction rate of T. tubifex (R) was from 0.18 to 0.81 mg-VSS mg-Tubifex -1 d -1 . The sludge reduction capacity of the recycled sludge reactor E was from 650 to 1080 mg-VSS L -1 d -1 . The optimum density of T. tubifex was 2500 mg L -1 and the optimum sludge recycled ratio was 1. The existence of T. tubifex did not affect COD and NH 4 + -N removals in the process, but led to a slight decrease in TP removal. SVI almost did not change when the T. tubifex density was lower than 3300 mg L -1 . The LC50 values on T. tubifex of copper and ammonia were 2.5 and 880 mg L -1 , respectively, both of which were higher than those on Aeolosoma hemprichi. © 2006 Elsevier B.V. All rights reserved. Keywords: T. tubifex; Predation; Sludge reduction; Wastewater treatment 1. Introduction The conventional activated sludge (CAS) process is the most widely used biological wastewater treatment process for both domestic and industrial wastewaters. One of the drawbacks of the CAS process is, however, its high sludge production. With the strictly enforced environmental and legislative requirements on the dis- ∗ Corresponding author. Tel.: +86 10 62772324; fax: +86 10 62771472. E-mail address: xhuang@tsinghua.edu.cn (X. Huang). charge of excess sludge, the cost of excess sludge treat- ment and disposal has accounted up to about 60% of the total operating cost in municipal wastewater treat- ment plants (Low and Chase, 1999; Wei et al., 2003a,b). This leads to an impetus to develop strategies for excess sludge reduction in biological wastewater treatment processes. Until currently different strategies have been investigated for reducing excess sludge production in biological wastewater treatment processes. According to the production principle of excess activated sludge in biological treatment processes, possible approaches to sludge reduction may include three aspects: uncoupling 0168-1656/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jbiotec.2006.07.035