Evaluation of nitrite oxidizing bacteria (NOB) activity in the nitritation reactor of a two-stage partial nitritation-anammox (PNA) system operating at mainstream conditions

Abstract: Nitrification-denitrification process, which is the most commonly used method for nitrogen removal from wastewaters, is energy demanding and contributes very much to the the carbon footprint of wastewater treatment. One of the alternative methods for nitrogen removal is the nitritation-anammox process. Nitritation-anammox has been implemented successfully in sludge liquor streams and industrial wastewaters with high temperature and high ammonium concentrations. However, implementation of nitritation-anammox is challenging in mainstream municipal wastewater treatment due to low temperature and low ammonium concentrations. Competition between different microbial groups determines the fate of nitritation-anammox process. It is of great importance to maintain nitrite oxidizing bacteria (NOB) repression for the success of operation. The aim of this study was to evaluate the activity of the NOB in the nitritation reactor of a two-stage nitritation-anammox pilot system operating under mainstream conditions at Sjölunda Wastewater Treatment Plant, Malmö, Sweden. With this aim, ex-situ batch tests were designed and performed to determine the microbial acitivity on Anox KaldnesTM Z-200 carriers collected from the pilot plant moving bed biofilm reactor (MBBR). Laboratory scale activity test results were further correlated to results obtained from the Nanammox pilot plant operation. Activity test results showed that 50 mg N L-1 of nitrite and ammonium concentrations were suitable to perform lab scale experiments with Z-200 carriers. A better mixing was provided with 27 carriers in the lab scale batch reactor whereas the activity results were more stable with 107 carriers. NOB repression was observed for a very short period of time at the beginning of the pilot operation when ammonium concentration in the inlet stream of the nitritation reactor was between 50-60 mg L-1 and the dissolved oxygen concentration to ammonium concentration ratio (DO:NH4 + ) was around 0.08. A decrease in ammonium concentration (without decreasing the DO concentration) resulted in an increase in NOB activity which may indicate the high sensitivity of microbial population structure to a change in substrate concentration.