Simultaneous removal of carbon and nitrate in an airlift bioreactor

This paper presents the integrated removal of carbon (measured as chemical oxygen demand i.e. COD) and NOx–N by sequentially adapted sludge, studied in an airlift reactor (ALR). Simultaneous removal of COD and nitrate occurs by denitrification (anoxic) and oxidation (aerobic). Aerobic (riser) and anoxic (remaining part) conditions prevail in different parts of the reactor. Studies were carried out in a 42 L ALR operated at low aeration rate to maintain anoxic and aerobic conditions as required for denitrification and COD removal, respectively. The sludge was adapted sequentially to increasing levels of NOx–N and COD over a period of 45 days. Nitrate removal efficiency of the sludge increased due to adaptation and degraded 900 ppm NO3–N completely in 2 h (initially the sludge could not degrade 100 ppm NO3–N). The performance of the adapted sludge was tested for the degradation of synthetic waste with COD/N loadings in the range of 4–10. The reduction of COD was significantly faster in the presence of NOx–N and was attributed to the availability of oxygen from NOx–N and distinct conditions in the reactor. This hypothesis was justified by the material balance of COD.