Partial Nitrification-anammox Research Articles

Mathematical simulation and experimental verification of a novel control strategy for a single stage PN/A (partial nitrification/Anammox) SBR (sequencing batch reactor)

The single reactor PN-A process offers a cost-effective solution for nitrogen removal. The successful startup and operation of the PN/A reactor requires the enrichment and balanced growth of AOB (Ammonium oxidizing bacteria) and AMX (AMAMMOX bacteria). The precise aeration supply or DO (dissolved oxygen) control was regarded to be the key for successful operation of the PN/A reactor. However, due to the low DO concentration requirement for the PN/A reactor, it is not practical to apply the precise aeration or DO concentration control. In this study, a new control strategy for the rapid startup and operation of the single stage PN/A (partial nitrification) SBR (sequencing batch reactor) was put forward. The control strategy relies on the periodic measurement of effluent NH4+, NO2-, and NO3- concentration to determine the nitrogen loading rate and aeration pump speed (i.e., not exact air supply volume). It does not require precise oxygen supply or DO (dissolved oxygen) control. This feature makes the control strategy easily applicable for practical application. A calibrated mathematical model was used to evaluate the control strategy, which was shown to be effective in achieving satisfactory nitrogen removal and NOB suppression. The control strategy was tested in a lab PN/A batch reactor, in which TN removal efficiency of 80% was achieved under low influent NH4+ concentration of 100 mg N/L and nitrogen loading rate of 0.12 Kg N/(m3•d) within 60 days of the experiment.

Robust nitrogen removal from municipal wastewater by partial nitrification anammox at ultra-low dissolved oxygen in a pure biofilm system

Efficient nitrogen removal from municipal wastewater applying a pure biofilm system has promise. In this study, a partial nitrification anammox (PNA) pure biofilm system was established in a sequencing batch reactor with anaerobic/oxic/anoxic operation; using this reactor, robust nitrogen removal from municipal wastewater at ambient temperature was achieved with a nitrogen removal efficiency (NRE) of 93.3 %. Partial nitrification with anammox could be coupled at dominant nitrite-oxidizing bacteria (NOB) abundance by controlling ultra-low dissolved oxygen (<0.1 mg/L) in the aerobic section where the contribution to nitrogen removal was 79.4 %. Microorganisms with different oxygen affinity spatially distributed on the carrier. Ammonia-oxidizing bacteria (AOB) dominated on the surface of the carrier, while anammox bacteria dominated on the interior of the carrier, with their relative abundance increasing from 0.26 % to 1.78 %. The intercalary NOB were inhibited by the restricted oxygen transfer. Overall, this study provides a new approach to realize PNA in biofilm system.

Start-up and optimization of a one-stage partial nitrification-anammox (PN-A) process treating low ammonium concentration wastewater: experimental results and modeling investigation.

Although the partial nitrification-anammox process (PN-A) has achieved great success in nitrogen removal for the high ammonium concentration wastewater, its application is still limited in low ammonium concentration wastewater treatment due to its instability and low nitrogen removal efficiency. In this study, a sequencing batch reactor (SBR) with continuous aeration was employed to enrich ammonia oxidation bacteria (AOB) and suppress nitrite oxidation bacteria (NOB) first; then, the SBR was operated intermittently aerated SBR (IASBR), to which the anammox granular sludge (AMX) was added to achieve complete autotrophic nitrogen removal under low influent ammonium concentration of 100mg/L. A mathematical model was used to optimize the IASBR aeration strategy to achieve sub-optimal nitrogen removal. The experimental results showed that high nitrite accumulation efficiency (above 80%) in the SBR and a fast start-up within 100days and a stable TN (total nitrogen) removal efficiency of 70% were achieved in the IASBR. Meanwhile, the simulation results indicated that keeping aeration duration at 4h, kLa (oxygen transfer coefficient) at 50day-1, or aeration duration at 2.5h, kLa at 80day-1 could obtain a higher total nitrogen removal efficiency (TNR) (TNR > 80%), and the TN removal could also be improved by increasing hydraulic retention time (HRT) under the optimal oxygen supply rate.

Achieving mainstream anammox in biological aerated filter by regulating bacteria community structure

Although mainstream partial nitrification-anammox (PN-A) is a highly efficient and sustainable wastewater treatment process, it is difficult to achieve and stabilize due to the competition among functional bacteria. In this study, achieving one-stage mainstream anammox via regulating bacteria community structure was studied in a lab-scale biological aerated filter (BAF). The results showed that high free ammonia with 89.57 mg/L, nitrite nitrogen (NO2−-N) competition between anammox bacteria (AnAOB) and nitrite oxidizing bacteria (NOB), and backwash regulated the bacteria community structure. After backwash, Candidatus Kuenenia became the dominant bacteria and the relative abundance increased to 5.56 %. In BAF, one-stage mainstream anammox with total nitrogen (TN) being lower than 15 mg/L in the effluent was achieved using lag-time of bacteria activity recovery caused by alternating operation of high and low ammonia nitrogen (NH4+-N), which have great potential applied in municipal wastewater treatment plants (MWWTPs).

Effects of hydroxylamine on treatment of anaerobic digestate of pig manure in partial nitrification-anaerobic ammonium oxidation

Partial nitrification-anaerobic ammonium oxidation (PN-anammox) was started up within 40 days by bioaugmentation and aeration control, and its performance in the treatment of anaerobic digestate of pig manure (ADPM) was evaluated. Inhibitors in ADPM decreased the nitrogen removal rate (NRR) by 0.24 g N/L/d. The effect and mechanism of hydroxylamine (NH2OH) alleviation of PN-anammox inhibition during ADPM treatment were investigated. As an intermediate product of anammox and ammonia-oxidizing bacteria, NH2OH strengthened energy metabolism, improved the activity and abundance of functional bacteria, and eliminated miscellaneous bacteria, increasing the average NRR by 31%. However, the average nitrous oxide emission was increased by 10.1% via hydroxylamine oxidation. The results showed that synergy and competition among nitrogen-transforming microorganisms were crucial for NRR and that NH2OH played an essential role in maintaining efficient operation. This study lays a foundation for restoring PN-anammox for treating livestock wastewater.

Enhanced nitrogen removal in partial nitrification-anammox (PNA) suspended sludge system for real municipal wastewater treatment at extremely low carbon to nitrogen ratio

The chemical oxygen demand (COD) to total inorganic nitrogen ratios (C/N) fluctuations under mainstream conditions has a crucial influence on the performance of partial nitrification-anammox (PNA) systems. In this study, a PNA sequencing batch reactor was applied for real municipal wastewater treatment with a continuous decrease in C/N (4.3 to 1.4). The results showed that with the decrease of C/N, the nitrite accumulation ratio (NAR) decreased from 94.1% to 60.7%, and nitrogen removal by endogenous denitrification was attenuated. However, the nitrogen removal efficiency (NRE) significantly increased from 43.8% to 91.5%, which was attributed to the enhanced performance of PNA under aerobic conditions and additional nitrite provided by endogenous partial denitrification (EPD) for anammox under anoxic conditions. Glycogen accumulating organisms (GAOs) that dominated the EPD process increased remarkably from 0.42% to 2.78% exhibiting a competitive advantage over phosphorus accumulating organisms (PAOs) at low organic loads. Co-existence of highly enriched anammox bacteria (0.91%) and GAOs (2.78%) ensured the enhanced nitrogen removal during the aerobic and anoxic phases. This study provides in-depth perspectives on the PNA combing with EPD under varied C/N and broadening the flexibility of PNA applications.

Enhanced nitrogen removal from municipal wastewater via a novel combined process driven by partial nitrification/anammox (PN/A) and partial denitrification/anammox (PD/A) with an ultra-low hydraulic retention time (HRT)

Anaerobic ammonia oxidation (Anammox) is a highly productive research area in municipal wastewater treatment. A novel combined process driven by partial nitrification/anammox (PN/A) and partial denitrification/anammox (PD/A) was established in this paper using a sequencing batch reactor (SBR) and two up-flow sludge beds (USBs). Municipal wastewater after carbon removal pretreatment in SBR entered PN/A-USB. PN/A process was initiated and enhanced by optimizing the intermittent aeration mode under low dissolved oxygen (DO). After enhancing and stabilizing the PD/A process, PN/A effluent entered the PD/A-USB along with raw municipal wastewater at a ratio of 4:1 and the combined system was established. Through this, this study achieved a nitrogen removal efficiency (NRE) of 84.9 % from municipal wastewater at an ultra-low total hydraulic residence time (HRT) of 3.9 h. Candidatus Brocadia (1.8 % in PN/A, 1.0 % in PD/A) was the only functional anammox bacterium in the combined process.

Stable enhanced nitrogen removal from low COD/N municipal wastewater via partial nitrification-anammox in the traditional continuous anoxic/oxic process through bio-augmentation of partial nitrification sludge under decreasing temperatures

The application of partial nitrification-anammox (PNA) in continuous flow processes for treating low COD/N (C/N) sewage remains a critical challenge. Here, a traditional continuous anoxic/oxic (A/O) process was operated to investigate nitrogen removal from municipal wastewater by the bio-augmentation of partial nitrification sludge combined with the inoculation of biocarriers under decreasing temperatures. Stable enhanced nitrogen removal via PNA was achieved. The average total inorganic nitrogen in influent and effluent was 44.3 and 7.1 mg N/L under a low C/N ratio (3.4) and a short hydraulic retention time (8.2 h). The bio-augmentation of partial nitrification sludge enhanced the PNA process under low temperatures (16.9 ± 0.6 °C). The nitrogen removal efficiency remained stable at 83.3 ± 5.7 % as the temperature decreased from 29.1 to 16.3 °C, and the relative abundance of Ca. Brocadia in carrier biofilms increased from 2.22 % to 4.31 % and 3.27 % in two aerobic chambers after 70 days of operation.

Advanced nitrogen removal from municipal sewage via partial nitrification-anammox process under two typical operation modes and seasonal ambient temperatures

A novel two-stage partial nitrification-anammox (PN-A) process was developed, achieving nitrogen removal from low carbon/nitrogen ratio municipal sewage under two typical operational modes and seasonal ambient temperatures. When complete nitritation-anammox was performed at temperatures greater than 19.4 °C, the effluent concentration of total inorganic nitrogen (TIN) was 4.1 mg/L, corresponding to a nitrogen removal efficiency (NRE) of 94.3 %. In contrast, when partial nitritation-anammox was performed at temperatures below 19.4 °C, the effluent TIN was 12.3 mg/L, corresponding to a NRE of 83.6 %. The relative abundance of Nitrosomonas and Nitrosomonadaceae increased from 0.02 % to 0.28 %, while Ca. Brocadia decreased from 1.85 % to 1.30 %, with the contribution of anammox to nitrogen removal being highest under low temperatures (19.4℃ to 13.8℃), at 59.0 %. This novel two-stage PN-A process provides a new approach for the stable operation of wastewater treatment plants (WWTPs) under low ambient temperatures.

Microalgae-bacterial biomass outperforms PN-anammox biomass for oxygen saving in continuous-flow granular reactors facing extremely low-strength freshwater aquaculture streams

The dissolved oxygen (DO) concentration in water streams is one of the most important and critical quality parameters in aquaculture farms. The main objective of this study was to evaluate the potential of two Continuous Flow Granular Reactors, one based on Partial Nitrification-Anammox biomass (Aquammox CFGR) and the other on Microalgae-Bacteria biomass (AquaMab CFGR), for improving dissolved oxygen availability in the recirculation aquaculture systems (RAS). Both reactors treated the extremely low-strength effluents from a freshwater trout farm (1.39 mg NH4+-N/L and 7.7 mg TOC/L). The Aquammox CFGR, removed up to 68% and 100% of ammonium and nitrite, respectively, but the DO concentration in the effluent was below 1 mg O2/L while the anammox activity was not maintained. In the AquaMab CFGR, bioaugmentation of aerobic granules with microalgae was attained, producing an effluent with DO concentrations up to 9 mg O2/L and removed up to 77% and 80% of ammonium and nitrite, respectively, which is expected to reduce the aeration costs in fish farms.

Improving CH4/O2 energy ratio of meat processing wastewater treatment systems through micro-sieving

ABSTRACT Micro-sieving is an effective way to prevent organic pollutants enter the main biological process and reduce aeration intensity and sludge production. However, few researchers quantified the effect of micro-sieving on meat processing wastewater treatment. The current study developed Excel-based models to compare the energy consumption and production of historical, conventional and innovative systems. Historical and conventional systems employ activated sludge (AS), nitrification, and denitrification as the main biological processes. Innovative systems are designed by using up-flow anaerobic sludge blanket (UASB) and partial nitrification/Anammox (PN/A) as the main treatment processes. Results show that the CH4/O2 energy ratios of the innovative treatment system are as high as 10 times of the historical and conventional systems. Therefore, innovative system can achieve electrical self-sufficiency. Micro-sieving reduces 5% of aeration demand. Furthermore, the impact of micro-sieving on the innovative treatment systemis assessed by cost-benefit analysis. System with micro-sieving has the shortest payback time of 2.1 years and reduces cost of the innovative treatment system. An excel-based model was developed to quantify the impact of micro-sieving on energy and cost of innovative treatment system, thereby providing a valuable reference for future sustainable engineering design of meat processing wastewater treatment.

Analyzing the roles of cyclic dimeric guanosine monophosphate (c-di-GMP) on the formation of autotrophic granules and autotrophic biofilm in integrated fixed-film activated sludge (IFAS) reactor

This study is the first to reveal the important roles of c-di-GMP on the formation of autotrophic granular sludge (AUGS) and autotrophic biofilm (AUB) in integrated fixed-film activated sludge (IFAS) system. The results revealed that the partial nitrification/anammox (PN/A) performance was successfully developed in the IFAS system and high nitrogen removal efficiency was obtained. Notably, it was clearly indicated that the intracellular c-di-GMP content was highly linearly correlated with the concentration of protein (PN) in tightly bound extracellular polymeric substances (TB-EPS) during AUGS formation. It was also found that the c-di-GMP content was highly linearly correlated with the PN concentration during AUB formation, and also had a good linear correlation with the polysaccharide (PS) content in TB-EPS. This proves that intracellular c-di-GMP played a vital role in regulating the TB-EPS secretion, thereby further affecting the aggregation ability of AUGS and AUB. In addition, when the c-di-GMP regulates the content of PN in TB-EPS, the secondary structure of the PN also changes significantly correspondingly, affecting the hydrophobicity of AUGS and AUB. In long-term operation, Candidatus_Kuenenia, Nitrosomonas and Flavobacterium gradually became dominant species, which were considered to favor the formation of AUGS and AUB by promoting the production of c-di-GMP and TB-EPS.

Individual and combined effect of humic acid and fulvic acid on distinct anammox-based systems: Inhibition and resistance

Anammox-based technologies hold great advantages in treating ammonia-rich wastewater where humic substances (HSs), mainly including humic acid (HA) and fulvic acid (FA), are ubiquitous and threaten the anammox activity. Here, the individual and combined effects as well as the mechanism of HA and FA on two typical anammox-based systems, partial nitrification/anammox (PN/A) and partial denitrification/anammox (PD/A), were investigated. Anammox activity was inhibited by 50 % with 1811.0 mg/L HA and 769.1 mg/L FA in PN/A system, but much higher level of 11706.0 mg/L HA and 5016.0 mg/L FA in PD/A system. The combined inhibition of HA and FA on anammox activity was additive in PN/A but antagonistic in PD/A. Transcription responses of functional genes (hzsA, hzsB, hdh, nirK, and nirS) suggested HA mainly inhibited the transmembrane transport of NH4+ and NO2–, while FA mostly inhibited the activity of intracellular hydrazine synthase (HZS). Interestingly, the strengthened denitrification alleviated the HSs stress on anammox in PD/A. Moreover, the protective microstructure with anammox bacteria encapsulated within abundant Thauera in PD/A granules further prevented PD/A from HSs shock. This study enables a better understanding of the HSs impacts on anammox-based systems, which is critical to integrating anammox technology into actual wastewater treatment.

Fates of quaternary ammonium compound resistance genes and the corresponding resistant strain in partial nitrification/anammox system under pressure of hexadecyl trimethyl ammonium chloride

Hexadecyl trimethyl ammonium chloride (ATMAC-C16) is a kind of quaternary ammonium compound (QACs) which is extensively consumed as disinfectants, antimicrobials and surfactants. Here, the partial nitrification/anammox (PN/A) system was exposed to different levels of ATMAC-C16 (0–10 mg/L) and the main objective was to reveal the long-term microbiological responses of PN/A system to ATMAC-C16, importantly, explore the tolerance of PN/A to ATMAC-C16 and the key resistant strain. Nitrogen removal efficiency was influenced by environmental and extreme levels of ATMAC-C16 through mainly affecting the anammox (hzsB) gene. Two types of anammox, Candidatus Jettenia and Candidatus Kuenenia, were enriched under the pressure of ATMAC-C16, which allowed PN/A system to maintain good nitrogen removal performance. ATMAC-C16 might cause the hormesis of entire microbial population in PN/A system, leading to the enhancement of cell viability. ATMAC-C16 decreased the relative abundances of most antibiotics resistance genes (ARGs) but significantly enriched QACs resistance genes (QRGs). The tolerance of PN/A system to ATMAC-C16 might be strengthened by inducing the efflux pumps encoding genes (qacH-01/02). Microbial hosts dynamic and co-selection mechanism among ARGs and QRGs resulted in the opposite trends of qacEdeltal-01/02 and qacH-01/02. Pseudoxanthomonas mexicana was identified as the ATMAC-C16 resistant strain, and its resistance to 10 mg/L ATMAC-C16 might not only obtain by capturing the qacH gene, but also benefit from its own efflux pump system. Therefore, from the perspective of the transmission of resistance genes, especially for QRGs, the spread risk of QRGs and ATMAC-C16 resistant strain in PN/A technique should be taken seriously.

Multiple roles of complex organics in polishing THP-AD filtrate with double-line anammox: Inhibitory relief and bacterial selection

Anammox process has been widely regarded as an energy-efficient method for sludge digestion filtrate treatment. However, the complex high-strength organics in the filtrate, especially of Anaerobic Digestion after Thermal Hydrolysis Pretreatment (THP-AD), brings serious threat to anammox bacteria, and the high nitrate residue in effluent remains another significant barrier in operation. In this study, a novel double-line anammox-mediated system, integrating the Partial Nitrification/Anammox (PNA) with Partial Denitrification/Anammox (PDA) processes in separately sequencing batch reactors (SBRs), was developed to polish the THP-AD filtrate. When the real THP-AD filtrate (1946.5 mg NH4+-N/L, 2076.0 mg COD/L) was fed to the front PNA reactor (SBRPNA) with 5-fold dilution, effluent total nitrogen (TN) remained at 93.0 mg/L. Notably, the final effluent TN was effectively polished to as low as 8.8 mg/L by the following PDA reactor (SBRPDA), which was fed with the SBRPNA effluent and real domestic wastewater (71.0 mg NH4+-N/L, 209.1 mg COD/L). More severe inhibition on anammox activity was observed in SBRPNA rather than SBRPDA by refractory organics in filtrate. Fortunately, it could be alleviated with the enhanced degradability of particulate organics and aromatic protein-like compounds, attributed to the enrichment of class Anaerolineae in both SBRPNA and SBRPDA. This further stimulated the electron donor supply for PDA process with much lower external carbon source demand. 16S rRNA sequencing analysis revealed that Candidatus Brocadia as dominant anammox bacteria were efficiently enriched in both SBRPNA and SBRPDA, indicating its unexpected toughness and adaptability to the complex organic compounds in THP-AD filtrate. Overall, this study suggested that the novel double-line anammox would be a promising alternative for cost-efficient nitrogen removal from high-strength wastewater containing complex organic matter.

Partition of Anammox and Nitrifiers Through Bio-Carriers for Full-Scale Sidestream Partial Nitrification-Anammox Plant.

This study assessed the activity and community structure in different types of sludge to reveal the partition mechanism of anammox and nitrifiers in a full-scale partial nitrification–anammox plant. Batch experiments confirmed that suspended sludge had higher partial nitrification capacity, and biofilm sludge had higher anammox activity, 16.9 times higher than suspended sludge. qPCR analysis confirmed that the amoA gene was mainly present in suspended sludge, and the highest abundance of the Amx gene was observed in biofilm sludge, reaching 1.01 × 107 copies/ng DNA. High-throughput results revealed that Nitrosomonas was the main ammonia-oxidizing bacteria with high activity in suspended sludge, and Candidatus Brocadia had the highest abundance of 13.4% in biofilm sludge. This is the exploration of the microbial community of three different sludge types in the full-scale sidestream PN/A system for the first time, which can guide the construction and replication of full-scale PN/A plants.

Research on Ammonia Removal from Reject Water Produced from Anaerobic Digestion of Thermally Hydrolyzed Sludge Through Partial Nitrification—Anammox

Research on Ammonia Removal from Reject Water Produced from Anaerobic Digestion of Thermally Hydrolyzed Sludge Through Partial Nitrification—Anammox

Advanced treatment of landfill leachate through combined Anammox-based biotreatment, O3/H2O2 oxidation, and activated carbon adsorption: technical performance, surrogate-based control strategy, and operational cost analysis

The complexity of landfill leachate makes it difficult to treat it with a single biological/ physical/chemical process. Moreover, the dynamic leachate characteristics pose a challenge for effective process control. Therefore, a combined treatment, consisting of a one-stage partial nitrification-Anammox process, an O3/H2O2 process, and a granular activated carbon filtration (GAC) process, was investigated. Meanwhile, a novel surrogate-based ozone dose control strategy for O3/H2O2 process was evaluated. Results show that this three-stage process offers high removal of total nitrogen (> 90%), COD (chemical oxygen demand, 60–82%), and micropollutants (atrazine, alachlor, carbamazepine, and bisphenol A, > 96%), satisfying discharge requirements. In the combined post-treatment, ozone dosing for COD removal can be real-time controlled by UVA254 reduction monitoring, based on a specific correlation between COD and UVA254 changes. On the other hand, O3/H2O2 pre-treatment controlled at a 50% UVA254 reduction shows to be the optimal point, when adsorption is designed as the main step for COD removal. Cost analysis shows that post-treatment with low (high) organic load i.e., COD ≤ (≥)540 mg/L, a combination with O3/H2O2 (GAC) as the main step appears to be more cost-effective. Therefore, a dynamic operation strategy in response to the leachate change is recommended.

Analyzing the sludge characteristics and microbial communities of biofilm and activated sludge in the partial nitrification/anammox process

The implementation of anammox based process remains a challenge in treating low ammonium wastewater. In present research, partial nitrification/anammox (PN/A) performance was investigated in the integrated fixed-film activated sludge (IFAS) system. The influent of the IFAS reactor was simulated residential domestic sewage. The nitrite oxidizing bacteria (NOB)'s proliferation was observed when dissolved oxygen (DO) concentrations were 0.65 and 0.35 mg/L. By conducting intermittent aeration (aerobic 1 min/anoxic 5 min) and low DO condition (0.20 mg/L) strategy, the IFAS reactor performed good nitrogen removal performance. The average total nitrogen (TN) removing efficiency and TN removing rate reached 80.6% and 0.13 kg N/(m3·d), respectively. In long term operation, extracellular polymeric substance (EPS) concentration of activated sludge was largely increased, which favored the formation of granules. The maximum frequency diameter of sludge flocs was increased to 1000 μm. The anammox bacteria and ammonium oxidizing bacteria (AOB) were both enriched within activated sludge and biofilm. High throughput analyzing result implied that anammox bacteria and AOB's abundances were 2.01% and 5.04% in activated sludge, while 2.43% and 2.45% in biofilm, respectively.

Application of intermittent aeration in nitrogen removal process: development, advantages and mechanisms

• Applications of intermittent aeration in N removal are critically reviewed. • Strategies and mechanisms of enhanced N removal efficiency are presented. • Variations in N transformation and microbial interactions are discussed. • Advantages of intermittent aeration in the integrated anammox process are outlined. • Directions for future work in N removal with intermittent aeration are suggested. Intermittent aeration has been shown to be one of the most effective and cost-effective strategies for biological wastewater treatment. This review presents an overview of the application and advantages of intermittent aeration, with the mechanisms of improvement in process stability discussed. Intermittent aeration was applied to maximize the utilization of organic carbon for denitrification rather than oxidized by O 2 . Under that condition, advanced nitrogen removal and sludge settleability were achieved and the energy consumption and N 2 O emissions were reduced. With the discovery that nitrite oxidation bacteria are selectively inhibited while ammonium oxidation bacteria are less affected under intermittent aeration conditions, intermittent aeration has been applied to the partial nitrification process. Furthermore, the non-aerated phases of intermittent aeration have been found to facilitate the synergism of anaerobic ammonium oxidizing bacteria (AnAOB) and denitrifying bacteria, providing a suitable environment for AnAOB growth. Therefore, intermittent aeration is considered to be an effective operational strategy for anammox-based processes, such as single-stage partial nitrification-anammox. In addition, simultaneous nutrient removal and sludge reduction could also be achieved. In order to regulate the synergism of microbial species, dissolved oxygen (DO) concentrations and the durations of aerated and non-aerated periods can be adjusted based on the monitored parameters such as nitrogen concentrations, pH and oxidation reduction potential profiles. Finally, the current limitations, gaps in knowledge and the areas requiring further research are proposed.