Stabilization of combined wastewater sludge: anaerobic processes

Abstract

Pilot-scale research was conducted to quantify the effects of stabilizing combined wastewater sludges (primary and biological nutrient removal waste activated sludges), in a common high-rate, single-stage anaerobic digestion operation. Various ratios of primary to waste activated sludge were used, and digester operational efficiency was assessed on the basis of the amount and rate of volatile mass removal, biogas production, and digester supernatant quality. It was found that, depending on the sludge ratio, addition of the biological nutrient removal waste activated sludge decreased digester operational efficiency; for example, almost 20% less volatile mass reduction, up to 25% reduction in metabolic reaction rates, and reduced gas production rates of up to 40% were observed. This was attributed to the presence of the cell membrane encapsulating the fermentable substrates of waste activated sludge, making them less available in the digestion process. It was concluded that, unless some type of pretreatment operation is utilized to liberate these substrates, this type of commonly used codigestion system will be of less benefit to a wastewater treatment plant, especially the smaller facilities which will lose a ready source of power in the form of biogas production, and the full value of the waste activated sludge as a resource will not be realized. Key words: anaerobic sludge digestion, biogas production, biological nutrient removal, biosolids, combined sludge, fermentation, volatile mass reduction.