The biomass concentration has an important effect on treatment performance. Previous studies
showed that a minimum of mixed liquor suspended solids (MLSS) in the activated sludge process for
of paper mill effluent treatment is 2,000-2,500 mg L-1 [1]. Tsang et al. [4] showed that a higher
concentration of MLSS promoted the activated sludge performance in organic parameters removals,
however the improvement of COD removal efficiency from increasing MLSS was limited within a
certain range. The mixed liquor suspended solids (MLSS) in this study, was 5,300 – 6,200 mg L- 1 %.
Some negative effects on the effluent quality, such as excess air supply for aeration, long sludge
settling time and high concentration of suspended solids in the effluent can occur due to high biomass
concentration in the reactor [4] For this range of COD removal the F/M ratio (food /microorganism)
varied from 0.21 to 0.80 g COD/gVSS d (volatile suspended solids). According to the literature for
activated sludge systems with conventional aeration, for treatment of paper mill effluent, the optimal
F/M ratios falls beween 0.09 and 0.98 g COD/gSSV d [36].