Anaerobic digestion (AD) is the most widely adopted sewage sludge treatment in the UK, but uncertainties remain on best operational practice. The interactive effects of three critical process control parameters on the conventional mesophilic AD of sewage sludge have been tested using an automated digestion apparatus to simulate the operational process and determine optimal treatment conditions. The first parameter chosen was the ratio of primary to surplus activated sludge (PS: SAS). Because of the difference in origin, the physico-chemical characteristics and anaerobic biodegradability of the two types of sludge were evaluated. The second control parameter was the dry solids concentration of the feed sludge, which impacts the rate of addition of substrate into the reactor, and determines the organic loading rate (OLR). The third parameter was temperature, which fundamentally impacts rate of physico-chemical and biological reactions. Results indicate that PS: SAS has the largest impact on biogas yield and OLR impacts reactor alkalinity. Significant interactive effects have been determined for PS: SAS and OLR, on ammoniacal nitrogen levels, and PS: SAS and temperature, influencing pathogens reduction potential.
Anaerobic Digestion, Operational Parameters, Process Optimisation