Aerobic digestion is commonly used at small plants to stabilize the organic matter in the sludge. The objective of this research was study on the efficiency of the aerobic digester for stabilization of disposal sludge from Serkan city sewage treatment plant.

          This study was conducted in four stages over seven months from July 2001 to March 2002. Aerobic digestion of sludge was accomplished in the bench scale with 30-liter volume on the environmental health laboratory of Hamadan Medical Sciences University. In each stage, a sample of disposal sludge from sewage treatment plant was analyzed firstly, then a number of sludge samples from bioreactor were considered for 3 weeks. Sludge
   samples were tested for stabilization and reuse indexes such as: the ratio of volatail solids to total solids (VS/TS), decresing percent of volatail solids, specific oxygen uptake ratio (SOUR), total and fecal coliform(TC and FC) number.

          The results showed that the average amounts of VS/TS ratio, SOUR and FC density in disposal sludge of sewage treatment plant were 0.754, 3.395 and 1.93´10⁸ respectively. The amounts of sludge stabilization indexes in bioreactor were60% for VS/TS ratio after 15 days, 2 mgo₂/gr.vs.h for SOUR after 17 days and 40% decrease of VS after 11 dayes. Also, the average of 
   FC density in bioreactor sludge was 1.08 × 10⁶ after 22 days.

          These result indicated that the disposal sludge from Serkan sewage treatment plant was raw and destabilized and aerobic digester could stabilized these, after 17 days. In spite of, the maximum level of microbial quality of digested sludge was in class B of USEPA regulation after 22 days. Therefore, the digested sludge may be used for reconditioning the poor soil or forest areas, but never should be used as a fertilizer for the agriculture.