Biomass segregation between biofilm and flocs improves the control of nitrite-oxidizing bacteria in mainstream partial nitritation and anammox processes

The control of nitrite-oxidizing bacteria (NOB) challenges the implementation of partial nitritation and anammox (PN/A) processes under mainstream conditions. The aim of the present study was to understand how operating conditions impact microbial competition and the control of NOB in hybrid PN/A systems, where biofilm and flocs coexist. A hybrid PN/A moving-bed biofilm reactor (MBBR; also referred to as integrated fixed film activated sludge or IFAS) was operated at 15 {degrees}C on aerobically pre-treated municipal wastewater (23 mgNH4-N{middle dot}L-1). Ammonium-oxidizing bacteria (AOB) and NOB were enriched primarily in the flocs, and anammox bacteria (AMX) in the biofilm. After decreasing the dissolved oxygen concentration (DO) from 1.2 to 0.17 mgO2{middle dot}L-1 - with all other operating conditions unchanged - washout of NOB from the flocs was observed. The activity of the minor NOB fraction remaining in the biofilm was suppressed at low DO. As a result, low effluent NO3- concentrations (0.5 mgN{middle dot}L-1) were consistently achieved at aerobic nitrogen removal rates (80 mgN{middle dot}L-1{middle dot}d-1) comparable to those of conventional treatment plants. A simple dynamic mathematical model, assuming perfect biomass segregation with AOB and NOB in the flocs and AMX in the biofilm, was able to qualitatively reproduce the selective washout of NOB from the flocs in response to the decrease in DO-setpoint. Similarly, numerical simulations indicated that flocs removal is an effective operational strategy to achieve the selective washout of NOB. The direct competition for NO2- between NOB and AMX - the latter retained in the biofilm and acting as a \"NO2-sink\" - was identified by the model as key mechanism leading to a difference in the actual growth rates of AOB and NOB (i.e., NOB \nView larger version (21K):\norg.highwire.dtl.DTLVardef@cad5deorg.highwire.dtl.DTLVardef@9efef3org.highwire.dtl.DTLVardef@7409feorg.highwire.dtl.DTLVardef@3c1043_HPS_FORMAT_FIGEXP M_FIG C_FIG