Simultaneous nitrification and denitrification in an oxidation ditch plant

The performance of a full-scale oxidation ditch plant that was aerating at a low level of oxygen concentration is presented. The plant receives an influent wastewater with high COD (around 1500 mg l(-1)) and nitrogen (about 200 mg l(-1)) concentrations. The plant achieved high levels of COD and nitrogen removal at low dissolved oxygen (DO) mass concentration without alternating aerobic and anoxic phases, either in time or in space. Simultaneous nitrification and denitrification was responsible for such high nitrogen removal, about 90%. The plant achieved higher nitrogen removal at this low level of DO than when it was intermittently aerating at higher dissolved oxygen concentration. BOD removal, however, was not affected. Bench scale studies confirmed that fact and simultaneous nitrification and denitrification occur-red at DO concentrations as low as 0.2 mg l(-1). Nitrogen removal rates were found between 12.2-19.5 mg l(-1) h(-1) depending on MLVSS and DO concentrations. Specific removal rates were found to lie between 0.14-0.19 mg N per mg MLVSS per day, with higher removal rates for lower DO mass concentrations. With every mg N removed, between 5.5-6.3 mg COD was removed.