Effect of Hydraulic Retention Time on the Efficiency of Vertical Tubular Anaerobic Sludge Digester Treating Waste Activated Sludge

Abstract

Treating waste activated sludge at a temperature of 35 °C was investigated in vertical tubular anaerobic sludge digester and common anaerobic sludge digester, with the working volume of 2.5 m3 each. Compared with common anaerobic sludge digester, the vertical tubular anaerobic sludge digester greatly improved volatile solids and COD removal efficiencies, the methane production and the stability of the anaerobic digestion system under the same condition of hydraulic retention time. The minimum hydraulic retention time of the vertical tubular anaerobic sludge digester was 5 days, when the organic loading remained at 5.28 kg m-3 d-1 (as volatile solids) and the volatile solids removal rate was 41.7%. The suitable hydraulic retention time for vertical tubular anaerobic sludge digester was 5 days, while to get the same COD removal rate it was 10 days for common anaerobic sludge digester. At optimum hydraulic retention time of 5 days, the methane production of the two digesters achieved the maximum difference of 0.06 l g-1 volatile solids (0.26, 0.20 l g-1 volatile solids for vertical tubular anaerobic sludge digester and common anaerobic sludge digester, respectively). The advantage of vertical tubular anaerobic sludge digester was especially obvious when the hydraulic retention time was relatively short. Furthermore, it was found that the vertical tubular anaerobic sludge digester in which more anaerobic bacteria lived has significantly intensified the sub-micro mass transfer between the substrate and the microorganisms, which resulted in a remarkable effect on shortening the hydraulic retention time of anaerobic sludge digestion.