Anaerobic digestion is an effective process for sewage sludge reduction. The production of sludge is increasing worldwide and anaerobic digestion presents limitations in terms of solid destruction and long sludge retention time. This study investigates the possibility of reducing the sludge retention time via increasing the Solid Retention Time (SRT). The effect of sludge recirculation rate on digester performance operated at 20 day Hydraulic Retention Time (HRT) was evaluated. Three different SRTs (20, 35, and 48 days) were evaluated. The experimental set up consists of three similar pilot scale mesophilic (37OC) anaerobic digesters with a capacity of 0.24 m3. The digesters were fed with thickened sludge (mixed primary and trickling filter humus) once daily. Also the effect of sludge recirculation at different HRTs was evaluated. The experimental results showed that for reactors operated at 20 day HRT; the chemical oxygen demand (COD) mass balance indicated; removal ratio increased from 43% to 60% as the SRT increased from 20 to 48 days. Also the methane generation increased as SRT increased. At lower HRTs the digester performance was decreased for all different SRTs. Results also showed that at high SRT the digester could be operated with higher loading rates (more than double) without affecting the efficiency. Finally the process showed stability for all operating conditions. Sludge recirculation is an easy, simple and reliable method for increasing anaerobic digester performance.