Introduction of Exogenous Activated Sludge as a Way to Enhance the Efficiency of Nitrogen Removal in the Anammox Process

— The effect of introduction of exogenous activated sludge (bioaugmentation) on the activity and composition of the microbial consortium carrying out the nitritation-anammox process in an SBR bioreactor was investigated. Two bioaugmentation strategies were tested: the exogenous sludge was added either immediately after inoculation with the anammox activated sludge or when the stable mode of nitrogen removal was achieved. The share of introduced sludge (by the amount of volatile dry matter) was 28‒35% of the total activated sludge mass. The growth conditions and community composition for activated sludges differed significantly: members of the genera “ Candidatus Brocadia” and “ Ca. Jettenia” were predominant in the aboriginal sludge, while “ Ca . Kuenenia” and “ Ca . Jettenia” predominated in the exogenous one. While bioaugmentation at the time of launching resulted in a 15% increase in nitrogen removal efficiency, this positive effect was short-lived (by day 46 of cultivation, the values of nitrogen removal efficiency in the control and experimental reactors were the same). Addition of exogenous activated sludge after the stable nitrogen removal mode was reached (day 53) increased the efficiency of nitrogen removal by 21‒35%, and this difference was maintained until the end of the experiment (90 days). The introduced anammox bacteria did not get acclimatized in the community of the SBR reactor; whatever was the method of their introduction, their abundance decreased to the minimum values. Our data indicate that the efficiency of nitrogen removal by the nitritation-anammox process may be enhanced by introduction of activated sludges differing in both the composition and the cultivation conditions from the aboriginal ones, either at reactor launching or into an actively operating bioreactor.