STABILIZATION OF COMBINED WASTE-WATER SLUDGE - ANAEROBIC PROCESSES

Pilot-scale research was conducted to quantify the effects of stabiliz ing combined wastewater sludges (primary and biological nutrient remov al waste activated sludges), in a common high-rate, single-stage anaer obic 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 pro duction, and digester supernatant quality. It was found that, dependin g on the sludge ratio, addition of the biological nutrient removal was te activated sludge decreased digester operational efficiency; for exa mple, 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 membr ane encapsulating the fermentable substrates of waste activated sludge , making them less available in the digestion process. It was conclude d that, unless some type of pretreatment operation is utilized to libe rate these substrates, this type of commonly used codigestion system w ill be of less benefit to a wastewater treatment plant, especially the smaller facilities which will lose a ready source of power in the for m of biogas production, and the full value of the waste activated slud ge as a resource will not be realized.