STABILIZATION OF COMBINED WASTE-WATER SLUDGE - AEROBIC PROCESSES

Pilot-scale research was conducted to quantify the effects of stabiliz ing combined wastewater sludges in side by side continuous aeration or cycled aeration (anoxic-aerobic) digestion operation at ambient tempe ratures. Varying ratios of raw primary sludge (RPS) and biological nut rient removal waste activated sludge (WAS) were digested at a number o f fixed solids residence times (SRTs); digester operational efficiency was assessed on the basis of the amount and rate of volatile mass red uction, and the need and amount of required chemical for mixed liquor pH (MLpH) control. Volatile mass reduction efficiencies ranged from 30 -55%, and depending on operational variables SRT, MLpH and WAS:RPS rat io, some of these values exceeded those from comparable WAS-only aerob ic digesters. The WAS:RPS ratio had a significant effect on digester c onditions and subsequent digestion behaviour and efficiency; the 60:40 WAS:RPS ratio reactors consistently outperformed the 80:20 ratio unit s. It is proposed that this was due to the RPS serving as external sub strate to the digester biomass, allowing the organisms to maintain hig her levels of activity, which resulted in better mass reduction at dec reased 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 o perating in either configuration; however, site-specific examination i s required to ensure operational compliance.