Substrate-related features to maximize bioenergy potential of chemical enhanced primary treatment sludge

Primary treatment is a major component in modern sewage treatment systems; and chemical enhanced primary treatment (CEPT) process can further enhance its performance for energy recovery or process improvement. With significant benefits on small footprint and sludge dewaterability, CEPT process has been selected and applied as the major wastewater treatment technology in Hong Kong. CEPT sludge, similarly in composition to typical primary sludge, is rich in organic contents and therefore can serve as a proper source for bioenergy production after anaerobic digestion (AD). The heating value of CEPT sludge can be higher than that of secondary sludge, which is mainly composed of slowly degradable organics and microorganisms. If neglecting energy consumed to produce the coagulant a CEPT-AD system shall drive the wastewater treatment process from energy-negative practice to energy-neutral or even energy-positive systems. This paper demonstrates some of the most important substrate-related features to increase the bioenergy production in the CEPT-AD system. Attentions have been paid particularly on the analysis of physiochemical characteristics of the CEPT sludge and their impacts to the biological system with heterotrophic species. An innovative sulfide oxidation process was presented with experimental evidence to overcome the potential problems of sulfide induced growth inhibition of methanogens. It was expected that the new process could reduce approximately 0.744 kWh/m³ of energy consumption in comparing with conventional activated sludge process without nutrient removal.