Sequential adaptation strategies of SPDA systems to low temperature: EPS mediation and community structure evolution

Temperature stands as a vital factor in the practical application of simultaneous partial-denitrification/anammox (SPDA) systems in mainstream wastewater treatment, thus it is necessary to understand the adaptation strategy of SPDA systems to low temperature. To address this, the potential adaptation of different coupling forms to low temperatures was investigated in a moving bed biofilm reactor (MBBR) and an integrated fixed-film activated sludge (IFAS) reactor for 182 days. The results showed that IFAS reactor achieved higher total nitrogen removal rates (TNRRs) at 15 °C (89.67 ± 0.01%) than MBBR system (77.28 ± 0.02%). Nitrogen conversion in the typical cycles showed that PD process was more sensitive to low temperatures than anammox process in SPDA systems. Meanwhile, the significant increase of total extracellular polymeric substances (EPS) content in each biophase and the change of community structure indicated that EPS mediation and community structure evolution in different biophases are two adaptive strategies of SPDA system to temperature decreasing. Biofilm phase tends to produce more EPS to resist the temperature decrease than floc phase. The highly negative relationship between EPS content at 15 °C and the initial temperature of change in community structure (p = 0.036＜0.05, r = -0.964) suggested that microorganisms always tend to resist the adverse environment through EPS mediation first, and then adapt to the low-temperature environment through community structure evolution with the temperature further decreased. This study guided the selection of SPDA coupling system in practical engineering applications, and laid a foundation for efficient biological nitrogen removal at low temperatures.