Improved Sludge Gasification by Two‐Phase Anaerobic Digestion

Abstract

The objectives of this research are to study the relative efficacies of two‐phase and single‐stage, high‐rate anaerobic sludge digestion at mesophilic and thermophilic temperatures at several levels of hydraulic residence time (HRT), organic loading rate, and feed consistency. In addition, the effects of pH, temperature, and HRT on acid‐phase digestion are studied. Data from continuous‐flow digestion studies showed that the two‐phase process is better than single‐stage digestion under all test conditions when compared on the bases of gas yield and production rate, reductions of volatile solids, carbohydratelipid‐protein conversions, buffer‐capacity, and unconverted volatile acids in the effluent. A mesophilic two‐phase system exhibits about the same methane yield and solids reduction at a 3‐day HRT as those of single‐stage high‐rate digestion at 15‐ and 17‐day HRT's. The enhanced stability of two‐phase digestion relative to single‐stage digestion increases as the system loading and hydraulic dilution rates increased. Optimum hydrolysis and acidification occur at pH 6. Lipid and protein degradation products generated at 55 °C inhibit acetogenesis and methanogenesis.