Tetrasphaera, rather than Candidatus Accumulibacter as performance indicator of free ammonia inhibition during the enhanced biological phosphorus removal processes

Abstract

Free ammonia (FA) has been proposed to inhibit phosphorus (P) removal efficiency and reconfigures the microbial community in the enhanced biological phosphorus removal (EBPR) systems. However, in response to FA exposure, the microbial kinetics and the functional bacteria belonging to polyphosphate accumulating organisms (PAOs) i.e., Tetrasphaera and Candidatus Accumulibacter within the mixed communities are generally ignored. This work was conducted to characterize the effects of FA on the biological processes of P-release and its uptake, and the virtual role of PAOs in EBPR systems. Experimental data demonstrated that the FA strongly inhibited the P-release and uptake rates which was described well by Hellinga and Vadivelu models, and the determined R2 in the anaerobic and aerobic period were about 0.91 and 0.96. Moreover, the abundance of key microbial communities differed significantly with various FA concentrations. Structural equation model (SEM) revealed that FA was positively associated with Ca. Accumulibacter (path coeff. = 0.30; P = 0.13) and Ca. Competibacter (path coeff. = 0.43; P < 0.05), while it was negatively associated with Tetrasphaera (path coeff. = − 0.45; P < 0.001). Furthermore, Tetrasphaera was found to strongly and positively drive the rates of P-release and uptake, and was confirmed as the only virtual biomarker for indicating the response of EBPR performance towards FA inhibition. Two hypothetical mechanisms indicated that Ca. Accumulibacter under higher FA concentration could not synthesize PHA to support the EBPR process. In addition, the release of P was hindered due to the FA toxicity on the urea cycle of Tetrasphaera microbial cells. This work gives comprehensive and deep insight into the effect of FA on the EBPR system and provides a useful guide for process optimization.