Influence of nitrite on sludge settleability under anoxic and aerobic conditions

Three lab-scale automatic control sequencing batch reactors (SBR) were operated by using acetate as the single carbon source, nitrite was dosed to SBR-1 and SBR-2 under anoxic and aerobic conditions, respectively, and SBR 3 was operated as control without dosing. The results show that, the presence of nitrite under anoxic and aerobic conditions would both cause filamentous sludge bulking. A more serious sludge bulking is observed when the nitrite is presented under the anoxic conditions. Nitrite could stimulate the increase of carbohydrates in extracellular polymeric substances, weaken the storage capability of floc-formers and decrease the phosphorous removal efficiency, thus cause the deterioration of sludge settleability. The dominant filaments are both Thiotrix nivea for SBR-1 and SBR-2 when sludge bulking occurred.