A bench scale anaerobic activated sludge digestion (AASD) process was developed to study the effect of biomass recycling at different amounts of digested sludge recirculation and various solids concentrations in feed sludge. It was selected to observe how the anaerobic digestion can be improved and stabilized by using municipal wastewater sludge. The mathematical model of AASD process was developed. The positive effect of biomass recycling on the sludge concentration depends on the settling rate of the sludge. The performance parameters such as SRT, methane yield and DHA of the reactor with R=0.5 were found best among those of the other recycle ratio reactors. With the reactor with sludge loading of 1.1 kg TVS/m3 day, SRT/HRT = 1.493 can be obtained.
