Performance and recovery of nitrifying biofilm after exposure to prolonged starvation

Achieving consistent ammonia removal in post-lagoon processes faces two major challenges impacting nitrifiers due to the unique seasonal variation of lagoon-based systems: summer to winter temperature drop and summer to fall ammonia starvation period while lagoon is removing ammonia. The objective of this study was to follow microbial diversity and define conditions that could overcome these challenges in a post-lagoon moving bed biofilm reactor (MBBR) operated at an initial surface area loading rate (SALR) of 0.3 g-NH4-N m^-2d^-1 from mesophilic (20 °C) to psychrophilic (4 °C). Initially the temperature was maintained at 20 °C and decreased to 10 °C until steady state was achieved. During starvation conditions (i.e., continuous, intermittent and no aeration without inflow; decanted media; and intermittent and continuous ammonia supplement) the temperature was decreased by 2 °C per week until 4 °C. The results indicated that operational procedures, such as intermittent ammonia supplement with SALR of 0.15 g-NH₄-N m^-2d^-1 could improve performance with 80% ammonia removal achieved immediately after starvation period. Intermittent ammonia supplement had produced the greatest biofilm preservation comparable to the initial load with the highest specific and surface area removal rates. In the recovery phase (initial load restoration) 10 days were required to reestablish performance above 95% ammonia removal. When temperature was decreased from mesophilic to psychrophilic, the microbial diversity was found higher when starving biofilm compared to the control operated at the initial load while it converged to a similar population over recovery. The main actors associated to nitrification enriched at psychrophilic conditions were Proteobacteria and Bacteriodotes at phyla level. Ammonia oxidation to nitrite was mainly driven by the order Burkholderiales and nitrite oxidation to nitrate by Pseudomonadales. This procedure should be considered in the implementation of full-scale post-lagoon MBBR technologies to ensure reliable, robust, and consistent performance despite the inherent seasonal variability of lagoon-based processes.