The start-up of anammox system inoculated with sludge overwhelmingly dominated by Candidatus Competibacter: System performance and microbial community succession

In situ start-up of the anaerobic ammonia oxidation (anammox) process using ordinary sludge is quite time-consuming, and the existing denitrifying bacteria can compete for nitrite during the initial start-up of the reactor, resulting in inadequacy of substrates. In this study, anaerobic sludge with the relative abundance of Candidatus Competibacter up to 40.27 % was used as a seed, and sufficient nitrite was provided to maintain the levels of electron acceptor for denitrifiers during the initial start-up of the reactor. After 176 days of operation, the anammox reactor achieved removal efficiencies of ammonia and nitrite at 93.75 % and 99.88 %, respectively, and the total nitrogen removal was 73.16 %. The refined ratios of influent nitrite to ammonia in the range of 1.33–2.66 were found to minimize the effect of the large consumption of nitrite by denitrifying bacteria. High-throughput sequencing results showed that Candidatus Brocadia was the only anammox genus obtained, which accounted for 20.31 % at the end of operation. Functional bacteria for denitrification, assumed to be the Candidatus Competibacter, were decreased to 0.88 %. The predicted functions confirmed the dominance of nitrate reductase gene napA, which was positively correlated with Candidatus Competibacter and Thermovirga (Spearman correlation, p < 0.05). Therefore, the speculated molecular mechanism was the Thermovirga-dominated syntrophic metabolism coupled with the endogenous denitrification process of Candidatus Competibacter. This study revealed the microbial ecology and evolution responsible for nitrogen removal during the rapid start-up of anammox, and provided an extra line of guidance for the start-up of anammox reactors inoculated with denitrifying sludge.