Advanced nitrogen removal in a single return anaerobic/aerobic/anoxic/aerobic (AnOAO) bioreactor treating municipal wastewater through hydroxylamine addition: Performance and microbial community

Intermittent hydroxylamine dosing to strengthen stability of partial nitrification and nitrogen removal efficiency through continuous-flow anaerobic-aerobic-anoxic reactor treating municipal wastewater

Intermittent hydroxylamine (NH2OH) dosing strategy was applied to enhance the stability of partial nitrification and total nitrogen (N) removal efficiency (TNRE) in a continuous-flow process. The results showed 2 mg/L of NH2OH dosing (once every 6 h) could maintain stably partial nitrification with nitrite accumulation rate (NAR) of 91.6 % and TNRE of 92.6 %. The typical cycle suggested NH2OH dosing could promote simultaneous nitrification-denitrification (SND) and endogenous denitrification (END) while inhibit exogenous denitrification (EXD). Nitrification characteristics indicated the NH2OH dosing enhanced stability of partial nitrification by suppressing specific nitrite oxidation rate (SNOR), Nitrospira and nitrite oxidoreductase enzyme (Nxr). The microbial community suggested the aerobic denitrfiers, denitrifying glycogen accumulating organisms (DGAOs) and traditional denitrfiers were the potential contributor for advanced N removal. Moreover, NH2OH dosage was positively associated with NAR, SND and END. Overall, this study offers a feasible strategy to maintain sustainably partial nitrification that has great application potential.

An overlooked effect of hydroxylamine on anammox granular sludge: Promoting granulation and boosting activity

The exogenous hydroxylamine dosing has been proven to enhance nitrite supply for anammox bacteria. In this study, exogenous hydroxylamine was fed into a sequencing batch reactor to investigate its long-term effect on anammox granular sludge. The results showed that hydroxylamine enhanced the reactor's performance with an increase in total nitrogen removal rate from 0.23 to 0.52 kg N/m3/d and an increase in bacterial activity from 11.65 to 78.24 mg N/g VSS/h. Meanwhile, hydroxylamine promoted granulation by eluting flocs. And higher anammox activity and granulation were supported by extracellular polymeric substances (EPS) characteristics. Moreover, Candidatus Brocadia's abundance increased from 1.10 % to 3.03 %, and its symbiosis with heterotrophic bacteria was intensified. Additionally, molecular docking detailed the mechanism of the hydroxylamine effect. Overall, this study would provide new insights into the hydroxylamine dosing strategy application.

Enhancing nitrogen removal performance through intermittent aeration in continuous plug-flow anaerobic/aerobic/anoxic process treating low-strength municipal sewage

Advanced nitrogen removal cannot be achieved through the conventional biological nitrogen removal process, which requires higher carbon sources and aeration energy. The proposal of intermittent aeration in the aerobic chambers offered an innovative approach to enhance nitrogen removal in low carbon-to-nitrogen ratio (C/N) municipal sewage, using a plug-flow reactor with anaerobic/aerobic/anoxic (AOA) process. Due to the effective utilization of internal carbon sources through the intermittent aeration, the total inorganic nitrogen removal efficiency (NRE) increased to 77.9 ± 3.2 % with the mean aerobic hydraulic retention time of only 3.2 h and a low C/N of 3.3 during the operation of 210 days. Polyhydroxyalkanoates dominated the nitrogen removal in this AOA system, accounting for 48.0 %, primarily occurring in the alternant aerobic/anoxic chambers. Moreover, the microbial community structure remained unchanged while the NRE increased to 77.9 %. This study provided an efficient and economic strategy for the continuous plug-flow AOA process.

Pilot-scale demonstration of self-enrichment of anammox bacteria in a two-stage nitrification-denitrification suspended sludge system treating municipal wastewater under extremely low nitrogen loading rate

In suspended sludge system, efficient enrichment and retention of anammox bacteria are crucial obstacles in mainstream wastewater treatment by anammox process. In this study, anammox bacteria was self-enriched in a pilot-scale suspended sludge system of two-stage nitrification-denitrification process serving municipal wastewater treatment. With the low ammonia (NH4+-N) of 9.3 mg/L, nitrate (NO3--N) of 15.6 mg/L and COD/NO3--N of 2.2 under extremely low nitrogen loading rate of 0.012 kg N/m3/d, anammox activity bloomed after its abundance increasing from 5.9 × 107 to 4.6 × 109 copies/g dry sludge. Significant NH4+-N removal was occurred and maintained stably in the denitrification reactor with anammox bacteria accounting for 1.13%, even under temperature decreasing to 20.0℃. The adequately anoxic environment, efficient retention with the static settlement, and NO2- production via NO3- reduction provided favorable environment for anammox bacteria. This study demonstrated the feasibility and great potential in mainstream anammox application without seeding specific sludge.

Multi-chambers of pilot-scale reactor enhanced partial nitritation performance

Nowadays, applying anammox to treat high nitrogenous side-stream wastewater has taken a step forward. However, the partial nitritation process is sensitive to the ammonium concentration and the nitrogen loading rate, which significantly influences the nitrogen removal performance. This study investigated the performance of a novel nitritation pilot-scale reactor which was divided into four chambers. The nitrite accumulation efficiency reached more than 90 % in the rural wastewater treatment process. As the reactor was divided into four chambers, the comprehensive statistical results showed that the concentration of free ammonium in the front chambers had been effectively improved. The proportion of free ammonium concentration (>0.1 mg NH₃·L^-1), which could inhibit the activity of nitrite oxidizing bacteria, in first chamber (PN1) was 2 times higher than in the last chamber (PN4). Meanwhile, Nitrosomonas, responsible for ammonium oxidation to nitrite, was highly enriched in the first two chambers even though the dissolved oxygen was maintained at 1.5 ± 0.3 mg·L^-1. Compare to conventional reactor, the resistance of the novel reactor to volumetric shock loading has been enhanced. Even though the ammonium loading rate fluctuated greatly, the effluent was still stable and could meet the demand following the anammox process. This study demonstrated that the reactor with multi-chambers could effectively improve the nitrite accumulation efficiency in the partial nitritation process and thus provide a new perspective on the partial nitritation process in a single reactor and further promote the anammox performance in the wastewater treatment process.

Stable nitrogen removal in the novel continuous flow anammox system under deteriorated partial nitrification: Significance and superiority of the anaerobic-oxic-anoxic-oxic operation mode

The collapse of mainstream anammox system caused by deterioration of partial nitrification (PN) is easy to occur and it is vital to quickly restore the stable nitrogen elimination performance. Herein, a novel continuous push-flow anaerobic-oxic-anoxic-oxic (AOAO) process treating sewage was used to restore the nitrogen elimination performance rapidly under deteriorated PN. The increased abundances of Nitrospira and Candidatus Nitrotoga was responsible for the deterioration of PN. Effluent total inorganic nitrogen of 8.7 mg N/L and a stable nitrogen removal rate of 0.083 kg N/m³·d were obtained with the aerobic hydraulic retention time (HRT) of 3.75 h even PN deteriorated. Endogenous partial denitrification coupled anammox in the anoxic zone was essential to maintain stable nitrogen removal under the deterioration of PN and the anammox contribution increased from 17.2 % to 23.6 %. The AOAO system shows robustness on nitrogen removal even PN deteriorated under the decrease of HRT from 16 to 12 h.

Rapidly recovering and maintaining simultaneous partial nitrification, denitrification and anammox process through hydroxylamine addition to advance nitrogen removal from domestic sewage

The collapse of simultaneous partial nitrification, denitrification and anammox (SPNDA) system, caused by the destruction of partial nitrification (PN), is the most likely phenomenon to occur. Therefore, recovering the process quickly and maintaining efficient nitrogen removal is a valuable topic for research. In the anaerobic/aerobic/anoxic operation mode, SPNDA process was used to treat domestic sewage in a sequencing batch biofilm reactor. After the deterioration of PN effect, with the addition of hydroxylamine, the activity of ammonia-oxidizing bacteria in the nitrobacteria increased (61.0-91.3 %), whereas the accumulation of nitrite quickly recovered to 90.4 % within 5 days. Meanwhile, the nitrogen removal efficiency improved (61.8-95.6 %) and the effluent TN was 2.1 mg/L. Furthermore, Candidatus Brocadia was enriched (0.50-1.82 %) in the system. The results indicated that the addition of hydroxylamine was an effective strategy to recover and economically maintain the SPNDA process for advanced nitrogen removal from domestic sewage.