Journal of Environmental Sciences 21(2009) 1639–1645 Simulation and assessment of sludge concentration and rheology in the process of waste activated sludge treatment XIA Mingfang 1 , WANG Zhiwei 2, ∗ , WU Zhichao 2 , WANG Xinhua 2 , ZHOU Zhen 2 , LU Jilai 1 1. State Key Laboratory of Pollution Control and Resource Reuse, Nanjing University, Nanjing 210093, China. E-mail: mingfang xia@sina.com 2. State Key Laboratory of Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Tongji University, Shanghai 200092, China Received 10 February 2009; revised 20 April 2009; accepted 27 April 2009 Abstract The process of using flat-sheet membrane for simultaneous sludge thickening and digestion (MSTD) was employed. The variations of sludge concentration and rheology were characterized and simulated. Based on mass balance analysis, mathematical models were developed and successfully used to predict and evaluate the variations of sludge concentration and the digestion efficiency in the MSTD process. The apparent viscosity of sludge could be modeled as functions of mixed liquor suspended solids and shear rates. The sludge in the MSTD process showed both shear-thinning and viscoplastic behaviour, and under various shear rates different rheological models could be chosen to predict their flow behaviour. It was also found that sludge concentration and viscosity had significant correlations with membrane fouling in the MSTD process. Key words: membrane filtration; membrane fouling; sludge thickening; sludge digestion; waste activated sludge DOI: 10.1016/S1001-0742(08)62467-5 Introduction It is widely accepted that conventional activated sludge process is a cost-effective and efficient method for wastew- ater treatment; however, it produces excess biomass as waste activated sludge (WAS) that is difficult and expen- sive to handle and dispose of, particularly when wastewater treatment plants (WWTPs) were adapted for biological nutrient removal. Rai et al. (2004) reported that the cost of WAS treatment and disposal accounted up to 60% of the total operating cost in WWTPs. WAS treatment became a more difficult and complex problem in WWTPs attributed to the stringent effluent criteria and restrictions to landfill WAS. Thickening and dewatering are usually performed for volume reduction of WAS in WWTPs. After thickening, a stabilization step, such as sludge digestion, is practiced to achieve stabilization, detoxification and minimization of WAS, especially for medium and large-scale WWTPs (Zhang et al., 2000). Sludge thickening is generally conducted by physical means, including gravity thicken- ing, dissolved air flotation (DAF) thickening, centrifugal thickening, etc. Among the typical sludge thickening tech- nologies, several problems and disadvantages are existing, e.g., the large footprint and low thickening efficiency and the release of phosphorus under long sludge retention * Corresponding author. E-mail: zwwang@tongji.edu.cn time (SRT) with gravity thickening process, lower quan- tity of sludge storage and higher energy cost with DAF thickening compared with gravity thickening, and much higher energy cost and advanced maintenance require- ments with centrifugal thickening technology (Zhang et al., 2000; Metcalf & Eddy Inc., 2003; Hua et al., 2005). In order to solve some problems dealing with conventional thickening technologies and to incorporate thickening and digestion into a single reactor, an innovative process of employing flat-sheet membrane for simultaneous sludge thickening and digestion (MSTD) was proposed in our previous research (Wang et al., 2008). The preliminary study demonstrated its feasibility for WAS treatment, in which good sludge thickening and digestion efficiency and superior effluent quality were achieved. Although micro- filtration, ultra-filtration, nanoflitration and other processes coupled membrane solid-liquid separation such as mem- brane bioreactor (MBR) have been intensively studied and widely used for the treatment of municipal wastewater, industrial wastewater and surface water/drinking water in past decades (Mehta and Zydney, 2005; Melin et al., 2006; Kim et al., 2007; Wu et al., 2007), the application of membrane to WAS thickening and digestion is a new attempt and worth investigating. In this study, specific attention was paid to simulate and assess the variations of sludge concentration and rheolo- gy in the membrane for simultaneous sludge thickening