Partial denitrification as nitrite provider for mainstream anammox

Sammanfattning: In traditional wastewater treatment, nitrogen is removed through the biological process combination of nitrification and denitrification. Although it is a well-established method, the need for aeration and possible addition of an external carbon source results in it being an energy-negative and costly process, thus making alternative methods for nitrogen removal compelling to examine. Anaerobic ammonium oxidation (anammox) has been presented as such an alternative, providing a “short-cut” through the nitrogen cycle by anaerobically oxidizing ammonium (NH4+) to nitrogen gas (N2) using nitrite (NO2-) as elector acceptor. By coupling the anammox reaction to partial denitrification (PDA), where nitrate (NO3-) is reduced to nitrite, simultaneous nitrogen removal of ammonium and nitrate is possible under anoxic conditions and low concentrations of organic carbon. To examine the start-up and operation of a PDA process treating synthetic mainstream wastewater, two continuous 2 L moving bed biofilm reactors (MBBRs) were operated in parallel for ten weeks. After being inoculated with K5 biofilm carriers (previously used for partial nitrification/anammox) provided by AnoxKaldnes, the reactors were fed with low concentration synthetic wastewater, with either acetate or propionate as carbon source at a chemical oxygen demand (COD) to NO3--N ratio of 2. Analysis of the influents and effluents were performed three times a week to determine the removal rates of the reactors. Furthermore, four ex-situ activity batch tests were performed during the ten weeks to determine the change of activity in the microbial populations on the biofilm carriers. During the start-up period several modifications were made to the experimental setup, such as separating the carbon source influents, in order to achieve stable conditions and influent concentrations. Subsequently, the reactors displayed a general increase of PDA capabilities during the final five weeks, with total nitrogen removal rates increasing from 0.29 ± 0.01 to 0.47 ± 0.03 g N/(m2'd) in the acetate fed reactor and from 0.17 ± 0.03 to 0.23 ± 0.03 g N/(m2'd) in the propionate fed reactor. Additionally, the anammox contribution to nitrogen gas formation was calculated to approximately 91 % in the acetate fed reactor and 64 % in the propionate fed reactor in the final week of operation. The improved performance of the acetate fed reactor was mainly attributed to a higher denitrification rate achieved by less complex utilization of the carbon source, as well as a larger abundance of partial denitrifiers compared to complete denitrifiers on the biofilm carriers. Furthermore, the nitrate reducing activities, and thus the nitrogen removal rates, appeared to still be increasing during the final weeks, suggesting that the microorganisms had not yet reached a stable co-community of denitrifiers and anammox bacteria. Consequently, further operation is required to establish the full capacity of the process.