Achieving efficient nitrogen removal from real sewage via nitrite pathway in a continuous nitrogen removal process by combining free nitrous acid sludge treatment and DO control

The incomplete denitrification due to insufficient carbon resource in the wastewater treatment plants (WWTPs) resulted in low nitrogen removal efficiency, which has become a widespread problem in China and all around the world. Reducing the requirement of carbon source by manipulating the nitrogen removal pathway from conventional nitrification-denitrification to partial nitrification-denitrification is considered as an efficient solution. In this article, the feasibility of combining free nitrous acid (FNA) sludge treatment and DO control to achieve partial nitrification-denitrification in a continuous flow system (aerobic-anoxic-oxic process) using real sewage was assessed. The nitrite pathway was rapidly established in the experimental reactor within 23 days by simultaneously lowering DO concentration in aerobic zone to 0.5 mg/L and treating 30% of the activated sludge per day from the reactor in the FNA sludge treatment unit with FNA concentration of 1.2 mg N/L and exposure time of 18 h. The nitrite oxidizing bacteria (NOB) were efficiently washed out and the partial nitrification process could maintain stable in the experimental reactor even after cease of FNA treatment and increase of DO concentrations in the main stream to 1.5 mg/L, with an average nitrite accumulation rate of above 78%. In contrast, the nitrite accumulation rate was just around 58% during low DO concentrations phase and declined quickly to below 1% after the DO concentrations were increased to 1.5 mg/L in the control reactor which only utilized single strategy of DO control to achieve nitrite pathway. Moreover, a better sludge settleability and nitrogen removal performance could also be realized in the experimental reactor. The results of nitrifying bacteria activities and quantities detection demonstrated that although NOB activities in both reactors were effectively inhibited, a certain amount of NOB (6.26 × 106 copies/g MLSS) were remained in the control reactor and multiplied rapidly as the DO concentration increased, which might break down the partial nitrification. Furthermore, the quantity results of nitrogen cycling related functional genes showed that the increment of the ratio of nitrate reduced bacteria to total bacteria was 0.35% larger than that of nitric oxide bacteria in the control reactor, while those two ratios increased similarly by 1.11% and 1.12% in the experimental reactor, respectively, which might be one potential cause of reduction in N2O emission of nitrite pathway achieved by FNA-based technologies.