Chunhai Wei, Xia Huang and Xianghua Wen
Environment Simulation and Pollution Control State Key Laboratory, Department of Environmental Science and Engineering, Tsinghua University, Beijing 100084, China (E-mail: firstname.lastname@example.org) Abstract A pilot-scale modiﬁed submerged membrane bioreactor (SMBR) withthe capacity of 18.1 m3d21 was developed on the basis of the principle of air-lift internal-loop reactor. Economical aeration intensity of the SMBR was determined as 96 m3m22h21 according to hydrodynamic investigation. Corresponding economical air-ﬂow rate was selected as the working air-ﬂow rate in the long-term run. Under economical aeration intensity, the critical ﬂux zone of the modiﬁed SMBRwas as high as 30 –35 Lm22h21 when MLSS was less than 13 gL21. Therefore, a sub-critical ﬂux of 30 Lm22h21 was selected as the working membrane ﬂux in the long-term run. Membrane fouling was effectively controlled by sub-critical ﬂux operation and periodic on-line chemical cleaning in the long-term run. When the average inﬂuent CODCr, NH3-N and turbidity were 310 and 44.3 mgL21 and 161 NTU,respectively, the average permeate were 38.5 and 19.5 mgL21 and 0.96 NTU under hydraulic retention time (HRT) was only 2.8 h. Corresponding removal was 86, 58.2 and 99.4%. DO deﬁciency caused by high MLSS was demonstrated as the main reason for low NH3-N removal. Keywords Economical aeration intensity; municipal wastewater; on-line chemical cleaning; sub-critical ﬂux; submerged membrane bioreactorWater Science & Technology Vol 53 No 9 pp 103–110 Q IWA Publishing 2006
The membrane bioreactor (MBR) is a promising technology for wastewater treatment and reuse. Specially, the submerged membrane bioreactor (SMBR), in which membrane modules are submerged, has been paid more and more attention in recent years. Although SMBR has some advantages over conventional activated sludgeprocess (CAS), such as more excellent efﬂuent, small footprint and less excess sludge, higher unit investment and operational costs than CAS blocked its wide application. Membrane fouling is considered to be the most important reason. Therefore, how to control membrane fouling effectively and economically has been a concern. Membrane fouling is mainly attributed to sludge cake layer deposition onthe membrane surfaces caused by bioparticles, and gel layer and membrane pore blockage caused by colloids and organic macromolecules. Recent studies (Cho and Fane, 2002; Ognier et al., 2004) showed that the sludge cake layer deposition on the membrane surface largely depended on the hydrodynamic condition, and sub-critical ﬂux operation could avoid rapid fouling caused by cake deposition. Ingeneral, critical ﬂux rises with crossﬂow velocity along the membrane surface (Tardieu et al., 1998; Defrance and Jaffrin, 1999). However, little information was available for crossﬂow velocity and critical ﬂux in the pilot-scale or industrial-scale SMBR. Membrane fouling caused by gel layer and pore blockage could be removed by off-line chemical cleaning, i.e. membrane modules were taken out of thebioreactor and immersed in a tank full of cleaning reagent. However, this added operational complexity and costs. On-line chemical cleaning, i.e. cleaning reagent is injected into membrane ﬁbres in reverse to normal ﬁltration while membrane modules are still submerged in the bioreactor, has some advantages over
off-line chemical cleaning. Similarly, littleinformation regarding on-line chemical cleaning was available in the pilot-scale or industrial-scale SMBR. In this study, a pilot-scale modiﬁed SMBR was developed on the basis of the principle of an air-lift internal-loop reactor to create a more reasonable hydrodynamic condition for membrane fouling control. Hydrodynamic characteristics and critical ﬂux of the SMBR were investigated. The performance...