Functional microbial competition and evolution driven by increased chemical oxygen demand/nitrogen ratio and reduced...

Increased COD/N (chemical oxygen demand/nitrogen) ratios and reduced temperatures critically limit mainstream partial nitritation-anammox (PN-A) by disrupting functional microbial dynamics, but their specific impacts on microbial competition and evolution remain poorly understood. This study addressed this gap in a mainstream PN-A reactor operated under precise oxygen-input control. Increasing COD/N from 0.5 to 1.5 enhanced Anammox and heterotrophic denitrification (HD), while suppressing nitrite-oxidizing bacteria (NOB) at a higher rate of 0.0098 d^-1 than ammonia-oxidizing bacteria (AOB, 0.0021 d^-1). These led to a better total nitrogen removal performance. Sudden cooling from 20 °C to 15 °C decreased all studied activities, yet Anammox showed lower reduction rate of 0.0014 d^-1 than other microbes. Anammox's competitiveness was defined as the percentage of Anammox's activities when competing with HD (Anammox _HD) and nitrifiers (Anammox _Nitrifiers) to the contemporaneous maximum Anammox activity (Anammox _max) to investigate the competition of Anammox with HD and nitrifiers. Increased COD/N didn't affect Anammox's competitiveness against nitrifiers, but reduced its competitiveness against HD from 95 % to 56 %. Surprisingly, the reduced temperatures to 15 °C improved Anammox's competitiveness over both nitrifiers and HD by 63 % and 67 %, respectively. Thus, increased COD/N and reduced temperatures had opposite effects on Anammox: increased COD/N enhanced Anammox _max, but reduced its competitiveness against HD, while reduced temperatures dramatically reduced Anammox _max, but increased its competitiveness over other microbes. These findings reveal that COD/N and temperature regulate mainstream PN-A performance through the reshaping of both the activities and the competitive landscape of the core microbes.