Effect of COD on mainstream anammox: Evaluation of process performance, granule morphology and nitrous oxide...

The effect of COD addition into an anammox reactor was assessed during short and long term exposure. The short term exposure was assessed via batch tests and lasted 48 h. Results indicated the presence of a very active denitrifying community able to consume COD using nitrate and nitrite as electron acceptors. However, the presence of COD did not result in an increase of the ammonium concentration at the end of the tests indicating that anammox activity was not suppressed by the addition of COD. Different COD concentrations (125, 225 and 175 mg COD/L) were also added in the reactor during 3 periods within its operation (period II, III and V respectively). Long term COD addition (up to 102 d of continuous addition during period II and III) caused a decrease of the anammox activity and a shift on the microbial community, with a decrease on the anammox fraction. However, the anammox process was never lost and it fully recovered as soon as COD addition stopped. Finally, dissolved N₂O was monitored under periods with and without COD addition, showing higher concentrations during transient periods from COD addition to no addition. The results of this paper provide evidence of how a long term COD exposure into an anammox reactor affect the overall nitrogen removal process, the granular structure of the anammox biomass, its microbial composition and for the first time, its N₂O emissions.