The operation of existing activated sludge treatment plants at paper mills in the world is often characterized by disturbances, caused by bad biosolids settling, sludge bulking, and reduction of biomass activity. This paper presents a comparison between the results obtained in a case study based on a full-scale activated sludge treatment (AST) system working as the only biotreatment and the AST working with anaerobic pretreatment by upflow anaerobic sludge blanket (UASB). The study was

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... at paper mills in Hedera, Israel. The data was collected during 8 years. The anaerobic/aerobic system (2002)(2003)(2004) provided steady operation performance, while the AST system without anaerobic pretreatment (1997)(1998)(1999)(2000)(2001) produced effluent characterized by oscillatory values. The results indicate much lower levels of total suspended solids in the anaerobic/aerobic treatment system effluent, 5-10 mg/L, compared to 50-85 mg/L in the AST system only. Similar improvement was observed in terms of the organic matter removal: 220-250 mg/L vs. 80-120 mg/L as CODt, and 20-40 mg/L vs. 4-7 mg/L as BODt with low corresponding fluctuations. After the operation of the anaerobic pretreatment, the sludge age increased from 8-10 days to 30-40 days and the activated sludge performance substantially improved: the sludge volume index decreased from 200 to 80 mL/g with significant reduction of the excess sludge amount. The data collected during the operation monitoring clearly indicates that the anaerobic process followed by AST considerably reduced electrical and chemicals consumption in the biological treatment plant and provided low operational cost.