Temperature-Tolerated Mainstream Nitrogen Removal by Anammox and Nitrite/Nitrate-Dependent Anaerobic Methane Oxidation in a Membrane Biofilm Reactor

The mainstream anaerobic ammonium oxidation (anammox) process provides strong support to the on-going paradigm shift from energy-negative to energy-neutral in wastewater treatment plants. However, the low temperature (e.g., below 15 °C) represents one of the major challenges for mainstream anammox in practice. In this study, a stable nitrogen removal rate (0.13 kg m^-3 day^-1), together with a high-level effluent quality (<5.0 mg N L^-1), was achieved in a lab-scale upflow membrane biofilm reactor (MBfR) by coupling anammox with nitrite/nitrate-dependent anaerobic methane oxidation (n-DAMO) microorganisms, at a temperature as low as 10 °C. With the temperature being progressively decreased from 25 to 10 °C, the total nitrogen removal efficiency was maintained in the range of 90-94% at a constant hydraulic retention time of 9 h. The impact of temperature on the biofilm system coupling anammox and n-DAMO reactions increased at a lower temperature range with higher Arrhenius coefficients. Additionally, 16S rRNA gene sequencing results showed that anammox bacteria, n-DAMO bacteria, and n-DAMO archaea jointly dominated the biofilm, and their respective abundances remained relatively stable when the temperature was decreased. The major reason for this temperature-tolerated performance is the overcapacity developed, which is indicated by biofilm thickness measurements and mathematical modeling. The stable performance obtained in this study shows promise for the n-DAMO application in domestic wastewater.