AUTOMATIC REGULATION OF ACTIVATED-SLUDGE AERATION - SINGLE-TANK NITRIFICATION-DENITRIFICATION

Several activated sludge processes using intermittent aeration were developed during the last decade to improve nitrogen removal. Timers, oxidation-reduction or oxygen probes are often used to switch aeration on and off at present times or by upper and lower limits. These regulation systems cannot adjust the oxygen supply to the biological oxygen demand and cannot realize the supply when 'requested' by the influent. A new controller/regulator system (OGAR : Optimized manaGement of Aeration by Redox) based on the variation with time of the oxidation reduction potential was developed : EH = f(time). This OGAR system controls the aeration periods and therefore optimizes the nitrification and denitrification reactions in the sludge processes. The operation of the OGAR programmable controller can be summarized by three functions: Analysis --> Diagnosis --> Action It is a true diagnostic tool and a decision making aid. Its simple design enables it to be adapted easily to any site and to be highly user-friendly. OGAR was applied to an existing wastewater treatment plant located at Nezel-la-Falaise near Paris (west suburb). Overall performance of the plant before and after implementation of automatic aeration control by OGAR yields the following conclusions: removal of COD was improved by 45% reduction in COD from 100 mg/l to 55 mg/l ammonia nitrogen concentration was reduced from 42 mg.l-1 to 2 mg.l-1 and nitrate nitrogen concentration was reduced to less than 5 mg.l-1