Understanding the effect of free ammonia on microbial nitrification mechanisms in suspended activated sludge bioreactors.

ABSTRACT

During nitrification, the varieties of microbial structures, metabolic pathways and functional profiles in four parallel laboratory-scale sequencing batch reactors (SBRs) with 0.5, 5, 10 and 15 mg/L of free ammonia (FA) concentrations were analyzed by high-throughput sequencing of the 16S rRNA gene. The SBRs were named S0.5, S5, S10 and S15, respectively. Ammonia removal via the nitrate pathway was achieved in S0.5 and S5 throughout the whole experimental period, while ammonia removal via the nitrite pathway was established in S10 and S15 after 89 and 146 day, respectively. The key finding of this study is that both the microbial diversity and richness were significantly affected (p < 0.05) by the FA concentration at different taxonomic levels. The most dominant taxa of S5, S10 and S15 were same, and mainly included Thauera while S0.5 was mainly composed of Zoogloea. Linear discriminant analysis (LDA) effect size (LEfSe) analysis was used to identify unique biomarkers in SBR activated sludge (AS) sample. The functional genera and enzyme in the four SBRs are similar but different in abundance and they are responsible for the removal of organics and nitrogen. Moreover, metabolic pathways are similar by PICRUSt analysis. The relative proportions of pathway-specific genes involved in some metabolic pathways differed to some extent. The ammonia oxidation rate was positively linked to Nitrosomonas and amo (both Spearman correlation coefficients (ρ) = 0.777) while the nitrite oxidation rate was positively linked to Nitrospira (ρ = 0.777) by co-occurrence network analysis. This work deciphered the response of microbial characteristics to different FA constraints in AS process and could provide helpful information for revealing the biological mechanism of FA inhibition on nitrogen removal.