Low temperature removal of nitrate by bacterial denitrification

The efficiency of two denitrifying sludges enriched at 5 and 20°C were compared using methanol as an electron donor. Both sludges were exposed to the same hydraulic and chemical conditions using an influent containing methanol and mineral salts. The low temperature sludge seemed to have several advantages over the sludge selected for at the higher temperature. In the range 0–17°C, the specific denitrification rate was 1.5–4 times the rate for the high temperature sludge, temperatures below 8°C being the most favourable. At 2°C, under nitrate limiting conditions, 98% nitrate reduction was obtained at a hydraulic residence time of 3.5 h, with an effluent concentration of 0.8 mg NO₃---Nl⁻¹. Sedimentation characteristics were always better for the low temperature sludge, and the utilization of methanol equally good as the high temperature sludge. The low temperature sludge appeared to be biochemically and microbiologically stable to temperature changes within the range 0–17°C, the latter temperature being close to the limit for maintaining the psychrophilic characteristics of the sludge. Studies on pure culture isolates of the denitrifying bacteria showed >90% dominance of one bacterial strain in both sludges. Studies of the isolates also showed that the low-temperature sludge consisted predominantly of psychrotrophs/psychrophiles, and not well-adapted mesophiles, which were only present in low concentrations. The dominant strain in both sludges was unable to grow on methanol in pure culture without access to nutrient growth factors. Only a few minor strains were obligate methylotrophs.Low temperature sludges were tested in a 3-stage biological process receiving domestic sewage. Each stage; carbon oxidation, nitrification and denitrification had separate sludge recycle, and methanol was added to the denitrification stage. These sludges were grown and selected for at temperatures 5°C. At 5°C the laboratory scale process gave 90% removal of total nitrogen at hydraulic residence times of 1.5, 9 and 4 h for the two aeration stages and the anaerobic stage respectively. Overall nitrification/denitrification was 95%, while denitrification separately was 98%. The effluent contained 0.4 mg NO₃---Nl⁻¹. The critical step in the process was unquestionably nitrification. Oxidation of ammonium was satisfactory at low temperature, but the reaction was somewhat vulnerable to changes in external conditions. The low temperature denitrifying sludge was originally enriched on synthetic waste but did not appear to change its microbial composition or characteristics by exposure to municipal wastewater.