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108The increased use of the activated sludge system to treat municipal and industrial wastewater has led to a considerable production of sludge, which is potentially harmful to both the environment and human beings. Anaerobic digestion (AD) of waste-activated sludge (WAS) is a widely accepted method for sludge stabilization and biogas production. The enhancement of the AD process has been studied by implementing different pretreatments of WAS, codigestion of WAS, and other substrates and applying multiphase systems. The current study applied compost material from aerobic composting of plant litter as inoculants to the WAS digesters. The studies were carried out in two phases. First, compost with different maturations was tested. During the process of compost production, the temperature and the microbial community evolve with time. The phases of composting are indicated by temperature difference. In this test, the compost was sampled at its initial, hot and stable phases, respectively. The stable compost was found to have the strongest stimulatory effect on the biogas production. The reactor with an inoculation of 0.1 g compost DW/g COD could achieve 42% more biogas production than the control reactor at the 10th day. Subsequent experiments further investigated this effect by distinguishing it into biological and nonbiological aspects. After 60 days, the biological aspects showed a stronger stimulatory effect of biogas production and a better removal of effluent chemical oxygen demand (COD) than the control. Furthermore, an addition of 0.1 g compost DW/g COD, with a thermo-alkaline pretreatment and kitchen waste as co-substrate, would give 19% more specific biogas production (mL biogas/g COD loaded) than the control.