The authors regret that there is an error in one stoichiometric coefficient and in the process rate of NOB in Table 1 of the original publication. The correct table is shown below. The authors would like to apologise for any inconvenience caused.

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 °C on aerobically pre-treated municipal wastewater (23 mg _NH4-N 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 mg _O2 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 NO ₃ ⁻ concentrations (0.5 mg _N L ⁻¹ ) were consistently achieved at aerobic nitrogen removal rates (80 mg _N L ⁻¹ 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 NO ₂ ⁻ between NOB and AMX - the latter retained in the biofilm and acting as a “NO ₂ -sink” - was identified by the model as key mechanism leading to a difference in the actual growth rates of AOB and NOB (i.e., μ _NOB < μ _AOB in flocs) and allowing for the selective NOB washout over a broad range of simulated sludge retention times (SRT = 6.8–24.5 d). Experimental results and model predictions demonstrate the increased operational flexibility, in terms of variables that can be easily controlled by operators, offered by hybrid systems as compared to solely biofilm systems for the control of NOB in mainstream PN/A applications.

The implementation of autotrophic anaerobic ammonium oxidation processes for the removal of nitrogen from municipal wastewater (known as "mainstream anammox") bears the potential to bring wastewater treatment plants close to energy autarky. The aim of the present work was to assess the long-term stability of partial nitritation/anammox (PN/A) processes operating at low temperatures and their reliability in meeting nitrogen concentrations in the range of typical discharge limits below 2 mgNH4-N·L-1 and 10 mg_Ntot·L^-1. Two main 12-L sequencing batch reactors were operated in parallel for PN/A on aerobically pre-treated municipal wastewater (21 ± 5 mgNH4-N·L-1 and residual 69 ± 19 mg_CODtot·L^-1) for more than one year, including over 5 months at 15 °C. The two systems consisted of a moving bed biofilm reactor (MBBR) and a hybrid MBBR (H-MBBR) with flocculent biomass. Operation at limiting oxygen concentrations (0.15-0.18 mgO2·L-1) allowed stable suppression of the activity of nitrite-oxidizing bacteria at 15 °C with a production of nitrate over ammonium consumed as low as 16% in the MBBR. Promising nitrogen removal rates of 20-40 mg_N·L^-1·d^-1 were maintained at hydraulic retention times of 14 h. Stable ammonium and total nitrogen removal efficiencies over 90% and 70% respectively were achieved. Both reactors reached average concentrations of total nitrogen below 10 mg_N·L^-1 in their effluents, even down to 6 mg_N·L^-1 for the MBBR, with an ammonium concentration of 2 mg_N·L^-1 (set as operational threshold to stop aeration). Furthermore, the two PN/A systems performed almost identically with respect to the biological removal of organic micropollutants and, importantly, to a similar extent as conventional treatments. A sudden temperature drop to 11 °C resulted in significant suppression of anammox activity, although this was rapidly recovered after the temperature was increased back to 15 °C. Analyses of 16S rRNA gene-targeted amplicon sequencing revealed that the anammox guild of the bacterial communities of the two systems was composed of the genus "Candidatus Brocadia". The potential of PN/A systems to compete with conventional treatments for biological nutrients removal both in terms of removal rates and overall effluent quality was proven.

Direct treatment of municipal wastewater (MWW) based on anaerobic ammonium oxidizing (anammox) bacteria holds promise to turn the energy balance of wastewater treatment neutral or even positive. Currently, anammox processes are successfully implemented at full scale for the treatment of high-strength wastewaters, whereas the possibility of their mainstream application still needs to be confirmed. In this study, the growth of anammox organisms on aerobically pre-treated municipal wastewater (MWW_pre-treated), amended with nitrite, was proven in three parallel reactors. The reactors were operated at total N concentrations in the range 5-20mg_N{bullet operator}L^-1, as expected for MWW. Anammox activities up to 465mg_N{bullet operator}L^-1{bullet operator}d^-1 were reached at 29°C, with minimum doubling times of 18d. Lowering the temperature to 12.5°C resulted in a marked decrease in activity to 46mg_N{bullet operator}L^-1{bullet operator}d^-1 (79 days doubling time), still in a reasonable range for autotrophic nitrogen removal from MWW. During the experiment, the biomass evolved from a suspended growth inoculum to a hybrid system with suspended flocs and wall-attached biofilm. At the same time, MWW_pre-treated had a direct impact on process performance. Changing the influent from synthetic medium to MWW_pre-treated resulted in a two-month delay in net anammox growth and a two to three-fold increase in the estimated doubling times of the anammox organisms. Interestingly, anammox remained the primary nitrogen consumption route, and high-throughput 16S rRNA gene-targeted amplicon sequencing analyses revealed that the shift in performance was not associated with a shift in dominant anammox bacteria ("Candidatus Brocadia fulgida"). Furthermore, only limited heterotrophic denitrification was observed in the presence of easily biodegradable organics (acetate, glucose). The observed delays in net anammox growth were thus ascribed to the acclimatization of the initial anammox population or/and the development of a side population beneficial for them. Additionally, by combining microautoradiography and fluorescence in situ hybridization it was confirmed that the anammox organisms involved in the process did not directly incorporate or store the amended acetate and glucose. In conclusion, these investigations strongly support the feasibility of MWW treatment via anammox.