Pilot-scale research was conducted to quantify the effects of stabilizing combined wastewater sludges in side by side continuous aeration or cycled aeration (anoxic-aerobic) digestion operation at ambient temperatures. Varying ratios of raw primary sludge (RPS) and biological nutrient removal waste activated sludge (WAS) were digested at a number of fixed solids residence times (SRTs); digester operational efficiency was assessed on the basis of the amount and rate of volatile mass reduction, and the need and amount of required chemical for mixed liquor pH (MLpH) control. Volatile mass reduction efficiencies ranged from 30#x2013;55, and depending on operational variables SRT, MLpH and WAS:RPS ratio, some of these values exceeded those from comparable WAS-only aerobic digesters. The WAS:RPS ratio had a significant effect on digester conditions and subsequent digestion behaviour and efficiency; the 60:40 WAS:RPS ratio reactors consistently outperformed the 80:20 ratio units. It is proposed that this was due to the RPS serving as external substrate to the digester biomass, allowing the organisms to maintain higher levels of activity, which resulted in better mass reduction at decreased SRT, and resistance to nitrification-induced MLpH decreases. It is suggested that some cost savings may be realised with this system, through possible digester downsizing and reduced chemical use, when operating in either configuration; however, site-specific examination is required to ensure operational compliance.
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