Temperature effects on wastewater treatment under aerobic and anoxic conditions

Current evaluations find savings in operating costs of greater than 25% when denitrification is added to a system already achieving nitrification. The economic evaluations performed were based on a sludge reduction of 25% under anoxic conditions compared to aerobic conditions. Conflicting reports of biosolids production for reactors operating under aerobic, anoxic and alternating aerobic conditions exist. This study quantifies and compares biosolids production rates for SBRs achieving carbon removal under aerobic or anoxic conditions as a function of temperature (14°C and 20°C). Proteins were chosen as model substrates because they are a major contributor to the total COD of raw sewage and by virtue of their high molecular weight, require hydrolysis. This study is unique in that the bacterial population was also enumerated using acridine orange direct counts (AODC). Observed yields were 35–52% higher for the anoxic reactors than they were for the aerobic reactors. The observed yield for the aerobic reactor decreased by 4% at a lower temperature whereas the anoxic yield increased by 8%. Further analysis discounted that differences in observed yield were caused by an accumulation of influent proteins. A factorial analysis revealed that the anoxic reactor contained more bacteria than its aerobic counterpart. Microscopic examination of the mixed liquor showed the presence of protozoa in the aerobic reactor while none were noted in the anoxic reactor. Predation is thought to be responsible for differences in the observed sludge production between anoxic and aerobic conditions.