Biomass types of nitrate-reducing denitrifiers to differentiate the hybrid community allocation of partial denitrification (PD) and anammox: IFAS vs MBBR

Partial denitrification (PD) and anaerobic ammonia oxidation (anammox) are modern treatment processes that can be combined to provide energy- and resource-efficient nitrogen removal methods; however, these processes are challenging to implement as a one-stage hybrid system. The coupling forms to achieve steady long-term PD/anammox operation need to be investigated. In this study, two reactors were operated for 300 days at 25 ± 1℃, in which one was inoculated with PD biofilms and anammox biofilms in a moving bed biofilm reactor (MBBR), and the other was inoculated with PD flocs and anammox biofilms in an integrated fixed-film activated sludge (IFAS) reactor. The results showed that the IFAS reactor had suprerior nitrogen removal performance, with the effluent total nitrogen (TN) concentration in the MBBR and IFAS of 5.17 ± 0.94 mg∙L−1 and 1.52 ± 0.75 mg∙L−1, respectively. Illumina MiSeq sequencing showed that Thauera and OLB8 were the dominant nitrate-reducing denitrifiers in PD biofilms in MBBR and flocs in IFAS, respectively. Since Thauera tended to selectively immigrate to the anammox biofilms, nitrate-reducing denitrifiers and anammox bacteria (AnAOB) coexisted in the anammox biofilms in MBBR. This decreased the PD activity (486.14 ± 23.04 mg∙(L∙d)−1 to 249.66 ± 26.90 mg∙(L∙d)−1) in the PD biofilms. On the contrary, nitrate-reducing denitrifiers and AnAOB were separately allocated in flocs and anammox biofilms in the IFAS reactor. The retention of PD biomass and superior mass transfer performance of the activated sludge flocs maintained a higher PD activity (2193.20 ± 26.62 mg∙(L∙d)−1), which provided sufficient NO2−-N for AnAOB and was beneficial to the growth of AnAOB. Hence, IFAS may be a potentially superior coupling mode for the PD/anammox process for engineering applications.