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An innovative partial nitrification, in situ fermentation, and Anammox (PNFA) system was developed to achieve mature landfill leachate and waste activated sludge simultaneous treatment. Three separate sequencing batch reactors (SBRs) were used for partial nitrification (PN-SBR), integrated fermentation-denitrification (IFD-SBR), and partial nitrification-Anammox (PNA-SBR). After 200 days of continuous operation, a satisfactory nitrogen removal efficiency (NRE) of 99.2 ± 0.1% was obtained, with an effluent total nitrogen (TN) of 15.2 ± 3.2 mg/L. In IFD-SBR, the volatile fatty acids generated from fermentation drove efficient denitrification, obtaining sludge and nitrogen reduction rates of 4.2 ± 0.7 and 0.61 ± 0.04 kg/m

The eutrophication of a body of water is a globally serious water-polluting problem caused by the excess discharge of nutrients. Nitrogen and phosphorus removal from wastewater is extremely necessary. The biological method is the major technology used for nitrogen removal. With the increasingly strict discharging standard worldwide, the problems of insufficient efficiency, considerable energy consumption, and secondary pollution involved in the traditional nitrogen removal process have become increasingly apparent, resulting in an urgent need for an economic, efficient, and sustainable technology. The widely investigated anaerobic ammonium oxidation (Anammox) process discovered in the 1990s is regarded as the most energy-efficient technology for nitrogen removal without aeration and organic carbon demand. However, it has not been widely adopted because for a long time now, it has faced bottlenecks in stable substrate nitrite (NO2−) acquisition. Recent studies have demonstrated that the NO2−-produced partial denitrification (PD) process provides a completely novel concept for solving this problem. China takes the lead in the investigation and practice of the discovery, development, and microbial mechanism of this process. A novel technology combining PD and Anammox is opening a new window for the low-carbon and high-efficiency treatment of municipal wastewater, which has been regarded as a hot and cutting-edge topic in this field. Compared with the conventional nitrification/denitrification process, it could save aeration energy by 100% and organic carbon source by 80% and reduce the sludge production and greenhouse gas (CO2, N2O) emission in the simultaneous treatment of municipal wastewater and nitrate (NO3−) contained in wastewater. In comparison with the present Anammox process, it is little affected by the substrate concentrate, temperature, and other environmental factors, could break the top limit of nitrogen removal efficiency by Anammox (89%), and reach up to 100% of the total nitrogen removal efficiency, thereby exhibiting great significance and applicable value. In the future, the separation, purification, and molecular metabolic mechanism of the PD and Anammox bacteria should be focused on. Furthermore, the engineering application of the novel technology should also be promoted.

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.

The stable nitrite (NO

This study established the one-stage partial nitrification coupled anammox and partial denitrification coupled anammox process in an anoxic/oxic continuous plug-flow system and operated for 465 days to treat mature landfill leachate. 97.9 %-98.1 % of inorganic nitrogen was removed when the nitrogen loading rate was maintained at 0.33-0.36 kg N/m

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.

Municipal solid waste (MSW) leachate treatment through the anaerobic ammonium oxidation (anammox) process has received increasing attention due to less oxygen consumption, carbon source demand, and sludge production. The recent advances in anammox-mediated MSW leachate treatment are systematically reviewed. During MSW leachate treatment, the anammox technology could be flexibly combined with partial nitrification, partial denitrification, fermentation, and methane oxidation. Additionally, this review comprehensively discussed the specific effects on anammox bacteria (AnAOB) of key stressors in MSW leachate such as dissolved organic matters, salinity, and antibiotics, and introduced the corresponding pretreatment methods. The key control strategies focusing on achieving effective AnAOB retention, amelioration of microenvironments, and stable nitrite source were summarized. Moreover, the potential for nitrous oxide emission mitigation in anammox-based MSW leachate treatment systems was evaluated. Finally, this review highlighted the full-scale applications of anammox-mediated MSW leachate treatment and proposed the prospects as well as research gaps in this field.

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/m

Nutrient removal in carbon limited wastewater with high efficiency and energy saving remains a bottleneck for wastewater treatment plants (WWTPs). This study established a pilot-scale anaerobic/aerobic/anoxic (AOA) system with processing capacity of 100 m

Enhancement of anaerobic ammonium oxidation (anammox) process and enrichment of anammox bacteria in the oxic zone of mainstream sewage treatments are complex. Also, quantification of the anammox contribution for nitrogen removal in the oxic zone is hindered owing to the simultaneous occurrence of anammox and nitrification. An alternating anaerobic/oxic/anoxic/oxic bioreactor whose oxic zone boosted partial nitrification coupling anammox (PN/A) and anoxic zone boosted partial denitrification coupling anammox (PD/A), respectively, was operated to treat real sewage for380 days. Desirable nitrogen removal (effluent total inorganic nitrogen (TIN) of 4.7 ± 1.9 mg N/L) was obtained from low carbon/nitrogen (3.6 ± 0.5) sewage with ammonium concentration of 52.5 ± 2.2 mg N/L in the influent. Under the condition of dissolved oxygen (DO) of 1.5-3 mg/L, anammox bacteria was still enriched within the aerobic biofilms, with the relative abundance increasing to 8.2 % (day 345) from 0 % (no biomass on day 1), which was higher than the value in the flocculent sludge (0.03 %) (P0.001). PN driven by flocculent sludge with high activity of ammonium oxidized bacteria (AOB) ensured sufficient nitrite supply for the anammox process with the existence of continuous DO. During the steady operation period, the maximum anammox contribution in the oxic zone was quantified to be 10.6 % by withdrawing aerobic biofilms from the system.

Until recently, all known nitrite oxidation occurred in oxygen-rich conditions but now the oxidation of nitrite into nitrate within a low oxygen or anoxic environment has been observed in the ocean. However, this phenomenon is rarely reported in wastewater treatments and its mechanism is unknown. In this study, the partial nitrification and nitrite oxidation were conducted in no enough oxygen in order to remove nitrogen from landfill leachate, save energy, and save money. The results show that the NH

Achieving simultaneous semi-partial nitrification and deep phosphorus removal is a preferred process technology for Anammox pretreatment. In this study, semi-partial nitrification combined with in-situ phosphorus recovery (PNPR) was used to treat municipal wastewater. The SRT conflict between the nitrification and phosphorus removal was resolved by in-situ phosphorus recovery every 20 cycles of Anaerobic/Oxid, and a supernatant with more than 10 times the influent phosphorus concentration was obtained, thus achieving bio-enhanced phosphorus removal and recovery with satisfactory semi-partial-nitrification effluent. Interestingly, the results showed that phosphorus removal and recovery process could improve the activity of AOB. The PNPR system's nitrite accumulation rate (NAR) and phosphorus removal rate (PRR) were more than 90% each, whereas the relative abundance of AOB and PAOs increased from 0.04% to 0.74% and from 0.25% to 0.70%, respectively (P 0.01). Furthermore, on average, the NO

A novel process was developed for real domestic wastewater and waste activated sludge (WAS) treatment based on partial denitrification, anammox-biological phosphorus removal, fermentation and partial nitrification (PDA-PFPN). After 246 days of operation, the effluent concentrations of NH

The NH

Anaerobic ammonium oxidation (anammox) has attracted extensive attention as a potentially sustainable and economical municipal wastewater treatment process. However, its large-scale application is limited by unstable nitrite (NO2--N) production and associated excessive nitrate (NO3--N) residue. Thus, our study sought to evaluate an efficient alternative to the current nitritation-based anammox process substituting NO2--N supply by partial-denitrification (PD; NO3--N → NO2--N) under mainstream conditions. Ammonia (NH4+-N) was partly oxidized to NO3--N and removed via a PD coupled anammox (PD/A) process by mixing the nitrifying effluents with raw wastewater (NH4+-N of 57.87 mg L-1, COD of 176.02 mg L-1). Excellent effluent quality was obtained with< 5 mg L-1 of total nitrogen (TN) despite frequent temperature fluctuations (25.7-16.3 °C). The genus Thauera (responsible for PD) was the dominant denitrifiers (36.4%-37.4%) and coexisted with Candidatus Brocadia (anammox bacteria; 0.33%-0.46%). The efficient PD/A allowed up to 50% reduction in aeration energy consumption, 80% decrease in organic resource demand, and lower nitrous oxide (N2O) production compared to conventional nitrification/denitrification process. Our study demonstrates that coupling anammox with flexible NO2--N supply has great potential as a stable and efficient mainstream wastewater treatment.

A continuous plug-flow multistage anoxic/oxic (A/O) system based on one-stage partial nitrification coupled anammox (PNA) process with integrated fixed-film activated sludge (IFAS) was established and operated over 400 days. A step feed strategy effectively controlled free ammonia concentration and alleviated impacts on ammonia oxidizing bacteria (AOB) and anammox bacteria (AnAOB). During day 301-405, 98.1% of total inorganic nitrogen was removed from mature landfill leachate, whereas chemical oxygen demand (COD) removal efficiency was 52.9%. With the enrichment of AnAOB in oxic biofilm, nitrogen removal via the anammox pathway reached 94.3%-95.0%. During system operation, the dominant anammox genus shifted from Candidatus_Brocadia to Candidatus_Kuenenia. Fluorescent in situ hybridization (FISH) indicated AnAOB encapsulated by AOB colonies were mainly distributed inside of the biofilm, which promoted nitrite utilization by the anammox process. This innovative system and the results are of great value to practical applications.

Without additional carbon sources, a low endogenous denitrification rate (EDNR) is the critical factor limiting its application in postdenitrification systems. This study optimized the quantitative distribution of anaerobic carbon source removal pathways based on chemometrics for the first time and explored the effect of anaerobic carbon conversion on anoxic endogenous denitrification. Results showed that enhancing the intracellular carbon storage of glycogen accumulating organisms (GAOs) by optimizing anaerobic duration can effectively improve the EDNR. The anaerobic stage was proposed to end at the peak concentration of polyhydroxyalkanoates (PHAs). A two-stage endogenous denitrification system was established to explore the long-term operating performance before and after optimizing anaerobic duration. Results showed that the average NO

A continuous plug-flow reactor with anaerobic/front-aerobic/anoxic/post-aerobic zones, where partial nitrification occurred in the front-aerobic zone, followed by simultaneous anammox and endogenous denitrification in the anoxic zone (PN-SAED), was built up to treat municipal wastewater. Alternating anoxic/aerobic conditions and longer anoxic duration facilitated stable partial nitrification. The nitrite accumulation ratio (NAR) was maintained at 97.4 ± 1.2%, with temperatures between 13.3℃ to 19.8℃. Candidatus Brocadia were naturally enriched in-situ from the anoxic zone with relative abundances of 31.93% and 6.67% on the agitator blade and carriers, respectively. High removal efficiencies of total inorganic nitrogen (TIN) (95.1 ± 1.9%) and effluent TIN (2.6 ± 1.1 mg N/L) were acquired from low COD/TIN (3.4 ± 0.4) municipal wastewater with anammox contribution of 13.5%±5.8% to TIN removal. The PN-SAED process is a promising mainstream nitrogen removal method.

The N and O isotope fractionation effects in NO

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.

The start-up, efficient, and secure operation of Anammox treating low ammonia sewage, is an important research focus. In this study, a partial nitrification–denitrification coupled with simultaneous Anammox and denitrification (PND-SAD) process was achieved in sequencing batch reactor/up-flow anaerobic sludge bed (SBR-UASB). The key measures to maintain high efficiency PND were: (i) controlling dissolved oxygen in the SBR below 0.5 mg/L, which is not only conducive to PN, but also promotes the contribution of simultaneous nitrification and denitrification to nitrogen removal; (ii) monitoring the nitrate (NO3–-N) of SBR effluent and discharging sludge to wash out nitrate oxidation bacteria when the NO3–-N exceeds 1.0 mg/L. The nitrite accumulation rate reached 97.6%. SBR effluent and domestic sewage entered the UASB. Although Candidatus Brocadia only accounted for 0.8%, its contribution to nitrogen removal reached 76.8%. In PND-SAD system, the aerobic HRT was only 3.8 h, nitrogen removal efficiency up to 97.3%.

Improving contribution and nitrogen removal efficiency (NRE) of partial anammox in municipal wastewater is a researching hotspot. This study developed an innovative PN/A-PD/A process with fixed biocarriers in anaerobic/anoxic chambers for actual sewage treatment in a typical step-feed reactor over 390 days. Two coupled pathways providing continuous NO2– (partial nitrification in oxic chambers and partial denitrification in anaerobic/anoxic chambers) for anammox were introduced to the process, achieving 47% nitrogen loss by anammox in stable phase. The influent and effluent total inorganic nitrogen (TIN) were 51.3 and 11.0 mg/L, respectively, even with chemical oxygen demand (COD)/TIN ratio of 2.9. Anammox activity improved from 6.52 to 9.68 mg NH4+-N/gMLSS/d and abundance on the biocarriers raised to 3.16 × 1010 gene copies/g dry sludge. Overall, this study confirmed partial anammox, spatially coupled with partial nitrification and partial denitrification via oxic/anoxic distribution with step feed mode, as an alternative for application of mainstream anammox.

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.

In this study, an innovative partial nitrification-anammox (PN/A) and endogenous partial denitrification-anammox (EPD/A) process was developed in a single-stage integrated fixed film activated sludge sequencing batch reactor (IFAS-SBR) treating real municipal wastewater with C/N ratio below 3.2. Enhanced efficiency of total nitrogen (TN) removal reached 90.1% with low HRT of 12 h and DO of 0.4 ± 0.1 mg/L. Detailed nitrogen removal mechanism analysis of typical cycle revealed that 89.9% of TN was eliminated through anammox pathway. Anammox bacteria (Candidatus Brocadia) and endogenous denitrifying bacteria (Candidatus Competibacter) were abundant both in biofilms and suspended sludge, meanwhile ammonium-oxidizing bacteria has outcompeted nitrite-oxidizing bacteria, which all favored the synergistic effect of anammox with PN and EPD and contributed to the improvement of nitrogen removal. Overall, the above results confirmed that combined PN/A and EPD/A process is a reliable and efficient alternative for mainstream anammox process.

A modified two-stage partial nitrification/anammox (PN/A) process with short aerobic HRT of 5.7 h was established and realized advanced nitrogen removal from domestic wastewater. The first process was partial nitrification in the PN-SBR, the initiation and stable maintenance of partial nitrification was achieved by pre-anaerobic treatment without inoculation or addition of inhibitor, nitrite accumulation rate was over 90% and maintained over 200 days, meanwhile pre-anaerobic improved the storage of endogenous carbon sources to enhance the nitrogen removal efficiency. The second process was anammox in the AMX-SBR, which was fed with the effluent of PN-SBR, the effluent total inorganic nitrogen was below 5 mgN/L, nitrogen removal efficiency reached 92.06%. Furthermore, Candidatus-Brocadia was enriched after treating domestic sewage with low ammonia from 0.02% to 0.23%.This study demonstrated the feasibility of starting and maintaining partial nitrification by pre-anaerobic treatment and the feasibility of nitrogen removal by coupling simultaneous-nitrification–denitrification with anammox to treat actual municipal sewage.

A novel simultaneous partial nitrification, endogenous denitrification and phosphorus removal (SPNEDpr) system was operated for 213 days in a sequencing batch reactor to treat real domestic sewage. The nutrient removal was achieved under an operation mode of intermittent aeration at unequal intervals with low oxygen concentrations. Through optimizing intermittent aeration conditions, the removal efficiencies of total inorganic nitrogen (TIN), PO43–P and chemical oxygen demand (COD) reached 78.40%, 98.13% and 84%, respectively. Low-oxygen (0.1–0.7 mg/L) and intermittent aeration effectively inhibited nitrite oxidation bacteria (NOB), maintaining stable partial nitrification with nitrite accumulation ratio of 96.45%. Notably, intermittent aeration promoted the formation of aerobic granular sludge, with the sludge particle size increasing from 217.2 ± 5.3 to 351.8 ± 4.8 μm, thereby enhancing the TIN loss efficiency (81.3%). The predominant genus was Candidatus_Competibacter (11.6%), which stored COD as intracellular carbon source and performed the endogenous denitrification. The SPNEDpr process provided a highly efficient and economical method for treating urban sewage.

Biological nitrogen removal from mature landfill leachate is ineffective due to the extremely low carbon/nitrogen (C/N) ratio. Moreover, a large amount of waste activated sludge (WAS) is inevitably generated from WWTPs during the municipal sewage treatment process. In this study, an innovative process was developed to enhance nitrogen removal from low C/N (1:1) mature landfill leachate and to reduce the WAS during a 300-day operation. Two sequencing batch reactors (SBRs) were involved in this process. Firstly, the mature landfill leachate was pumped into an aerobic reactor to undergo partial nitrification (PN-SBR). Then, the PN-SBR effluent and WAS were pumped into an anoxic reactor to undergo integrated fermentation and denitritation (IFD-SBR). The pH profile was treated as a real-time parameter to precisely control the duration of the PN and IFD processes. Partial nitrification and integrated fermentation-denitritation (PNIFD) system achieved a total nitrogen removal efficiency of 95.0% and an average nitrogen removal rate (NRR) of 0.63 kg/m3·d during the last operational phase. Due to a variety of refractory contaminants, the effluent COD concentration was 1865.9 mg/L and a 19.7% COD removal efficiency was obtained under an influent concentration of 2324.5 mg/L. Compared with the traditional nitrogen removal process, PNIFD not only decreased requirements for oxygen by 25% and the external organic carbon by 100%, but also achieved simultaneous reduction of external WAS. More than 53.7% of the external sludge was reduced during the IFD-SBR operational cycle, with an average external sludge reduction rate (SRR) of 5.09 kg/m3·d. Fermentation/denitritation related microorganisms, such as Anaerolineaceae, Acidimicrobiaceae and Thauera, accounted for up to 41.5% of the total abundance in the IFD-SBR. Based on the long and stable operation, this study provides a simple and promising approach for synchronous nitrogen removal and WAS reduction.

The partial nitritation-anammox (PNA) process is the most promising technique to treat municipal sewage; however, nitrite oxidizing bacteria (NOB) are a hindrance to achieve PNA. This study investigated the effects of selectively discharging flocs (200 μm) to washout NOB in a sequencing batch reactor (SBR) over 200 d. The experiment was divided into three phases with different floc sludge retention times (SRTs; 30, 20 and 30 d). When the SRT of the flocs was reduced from 30 to 20 d to washout NOB, a significant reduction of ammonia oxidizing bacteria (AOB) and anaerobic ammonium-oxidizing (anammox) bacteria in the flocs was found. This indicates that a low floc SRT (20 d) leads to the loss of AOB and anammox bacteria in the flocs (200 μm) and destroys PNA. Activity tests and qPCR analysis revealed the variations of functional bacteria in the granules and flocs, indicating that the enrichment of AOB, NOB, anammox bacteria in the granules is caused by the long-term discharging of flocs. High-throughput sequencing analysis revealed that the microbial shift of Tetrasphaera was significant in the flocs and may be connected to the enrichment of anammox bacteria and the stability of the PNA requires further research. All the obtained NOB sequences were affiliated with the genera Nitrospira and could further influence the PNA system. Overall, this study provides an in-depth understanding of the impact of discharging flocs to washout NOB and promotes the application of combing granules/floc PNA in sewage treatment.

The novel partial nitrification endogenous denitrification and phosphorus removal (PNEDPR) process can achieve deep-level nutrient removal from low carbon/nitrogen municipal wastewater without extra carbons. However, its performance is limited by long hydraulic retention time (HRT) and low specific endogenous denitrification rate (rNO2). This study aimed at investigating the effects of two improving strategies on PNEDPR. One was decreasing both anaerobic and anoxic reaction time for shortening HRT from 55 h to 17.5 h. The other was temporarily discharging orthophosphate-rich supernatant for balancing the competition between phosphorus and glycogen accumulating organisms to further raise rNO2 without deterioration of phosphorus removal. Results revealed that, desirable nutrient removal was obtained, as average effluent concentrations of total nitrogen and orthophosphate were 8.4 and 0.5 mg/L with their average removal efficiencies of 86.8% and 90.9%. High-throughput sequencing analysis revealed that, Candidatus_Competibacter conducted nitrogen removal endogenous denitrification and Candidatus_Accumulibacter and Tetrasphaera ensured phosphorus removal.

An up-flow anaerobic sludge blanket reactor (UASB), anoxic/oxic (A/O)–anaerobic ammonia oxidation reactor (ANAOR or anammox reactor), and anaerobic sequencing batch reactor (ASBR) were employed in the treatment of landfill leachate with partial nitrification-anammox and half-denitrification-anammox. The Chemical Oxygen Demand (COD) concentration, ammonium nitrogen (NH4+-N) concentration, and total nitrogen (TN) concentration of the basal leachate was 2200–2500 mg/L, 1200–1300 mg/L, and 1300–1400 mg/L, respectively. After a 1:2 dilution using domestic sewage, the COD, NH4+-N, and TN concentrations in the influent were 800–1000 mg/L, 400–430 mg/L, and 420–440 mg/L, respectively. After treatment, the final COD, NH4+-N, and TN were decreased to 90–100 mg/L, 13–14 mg/L, and 35–38 mg/L, respectively. In the ASBR, organic carbon sources in sewage-diluted landfill leachate were introduced for the conversion of nitrate nitrogen (NO3−-N) into nitrite nitrogen (NO2−-N). This enabled the continued reaction of NO2−-N with NH4+-N from the newly introduced sewage-diluted landfill leachate via anammox. As a result, complete TN removal was achieved in the system. Microbial diversity analysis indicated that the relative abundance of ammonia-oxidizing bacteria (AOB) was four to five times greater than nitrite-oxidizing bacteria (NOB) in the A/O reactor, showing that partial nitrification was prevalent. The relative abundance of the anammox bacterium Candidatus Kuenenia gradually increased in each reactor, reaching a maximum of 1.17%–1.39%. Using this set-up, we achieved advanced, efficient, and economical, COD reduction and nitrogen removal. Taken together, the findings provide important insights into the optimal operation of landfill leachate treatments. Keywords: Landfill leachate, Partial nitrification, Anaerobic ammonia oxidation, Denitrification

This study investigated the nitritation of domestic wastewater through the short-term addition of hydroxylamine (NH2OH). Sequencing batch reactor (SBR) was used and the NH2OH solution with an initial concentration of 5.0 mg/L was injected at each cycle. With NH2OH addition, the nitritation was quickly established in 5 d with nitrite accumulation ratio above 95%. Further, stable nitritation could be maintained without NH2OH addition in the following 53 days, even under unfavorable conditions (DO = 3 mg/L). According to qPCR results, NH2OH significantly reduced and continuously suppressed nitrite oxidizing bacteria (NOB), while the abundance of ammonia oxidizing bacteria was stable, induced the start-up and maintenance of nitritation. Among NOB, NH2OH significantly suppressed Nitrospira, while did not affect Nitrobactor. Nitrobactor gradually increased during the operation, which could induce the final deterioration of nitritation. Overall, this research provided the fundamental knowledge required to optimize the NH2OH addition strategy for operating a stable nitritation in domestic wastewater.

Landfill leachate, generated from the wastes in a landfill, is a type of wastewater with high concentrations of ammonia and organics, causing a serious environmental pollution. Because of its complex and changing characteristics, it is difficult to remove nitrogen from landfill leachate economically and effectively. Hence, nitrogen removal is a significant research priority of landfill leachate treatment in recent years. Biological processes are known to be effective in nitrogen removal. In this work, the biological nitrogen removal treatments were divided into the following processes: conventional nitrification-denitrification process, nitritation-denitritation process, endogenous denitritation process, and anaerobic ammonium oxidation (Anammox) process. This manuscript summarized the theories and applications of these approaches in detail, and concluded that appropriate processes should be selected in accordance with different characteristics of landfill leachate, in order to effectively remove nitrogen from all stages of landfill leachate and reduce disposal costs. Finally, perspective on the challenges and opportunities of biological nitrogen removal from landfill leachate was also presented.

Energy savings via achieving the reduction of aeration and excess sludge is required to realize energy self-sufficiency in wastewater treatment plants. A novel partial nitritation + simultaneous anammox denitrification and sludge fermentation (PN + SADF) process was operated for nearly two years, during which simultaneous energy-efficient nitrogen removal and waste activated sludge (WAS) reduction was achieved, with a stable nitrogen removal efficiency of 80% and external WAS reduction of 40%–50%. In the PN reactor, presence of ammonia oxidizing bacteria and absence of nitrite oxidizing bacteria ensured the stable nitritation. In the SADF reactor, nitrogen was removed via denitrification and anammox by using nutrients and organics released from WAS solubilization. Comparable performance of the SADF reactor at ambient temperature (12–32 °C) to that at 30 °C indicated a practical application potential for the PN + SADF process. An initial estimation of a full-scale PN + SADF process serving a population of 100000 showed that it could save economy and energy in comparison with conventional nitrification-denitrification process. Despite some challenges in implementation, this paper highlights the potential implication for sustaining mainstream nitritation-anammox towards energy-efficient operation with excess sludge reutilization.

In this study, the side-stream heat-shock treatment was used to start up and maintain the nitritation of real sewage. Complete nitrification was obtained when the real sewage was treated in a sequencing batch reactor (SBR). Then, about 50% of the mixed sludge was collected from the SBR and treated with the heat-shock treatment at 60 °C for 40 min in another reactor every 2 weeks. After providing the heat-shock treatment for four times, the effluent nitrate in the SBR gradually decreased from 22.5 to 3.2 mg/L, while the nitrite accumulation rate increased from 4.4% to 81.8%, indicating a successful start-up of nitritation. Further, the sewage nitritation was stable with the regular side-steam heat-shock treatment for 91 days, and the ammonium removal efficiency of 80.6% and nitrite accumulation rate of 91.2% were achieved. This study suggests that the side-stream heat-shock treatment could be used to start up sewage nitritation and maintain stability for a long-term operation.

The breakthrough of this study was the development of plug-flow integrated fixed-film activated sludge (IFAS) reactors operated with alternating anoxic/aerobic (A3) conditions for enhanced anammox processes. The energy-saving capacity and efficiency of this A3-IFAS system was compared with an identical IFAS reactor operated with continuous low oxygen aeration (CLOA) through a test period of 141 days. At the nitritation start-up phase, the nitrite in the effluent of the A3-IFAS system quickly accumulated to 77.9%, which was higher than that of the CLOA-IFAS system (61.7%). The A3-IFAS system reached 94.1 ± 4.0% of total inorganic nitrogen (TIN) removal in the stabilization stage, which was higher than that in the CLOA-IFAS system (66.7 ± 5.3%). A3-IFAS maintained stable partial nitrification and TIN removal, whereas the nitritation in CLOA-IFAS was disturbed on the 96th day and the TIN removal became unstable. Quantitative real-time polymerase chain reaction (qPCR) analysis indicated that ammonia-oxidizing bacteria (AOB) were dominant in both IFAS reactors, while the amount of nitrite-oxidizing bacteria (NOB) in the CLOA-IFAS system was 5.27 times that in the A3-IFAS system. The kinetic analysis revealed that the ratio of the AOB specific growth rate (μAOB) and NOB specific growth rate (μNOB) in A3-IFAS was 4.09, which was higher than that (3.1) in the CLOA-IFAS system, implying that the low growth rate of NOB in anoxic zones is the main reason for the stable nitritation and high TN removal in A3-IFAS.

In this study, intermittent aeration strategy was investigated in a plug-flow reactor on real municipal wastewater. Over 200 days of operation, the total inorganic nitrogen (TIN) removal efficiency of 72.43 ± 7.56% was achieved with a total aerobic hydraulic retention time in the range 2.4-3.0 h under a low C/N ratio of 3.19. The batch tests showed that the activity of nitrite oxidizing bacteria (NOB) was effectively inhibited, and simultaneous nitrification and denitrification via nitrite were observed under double intermittent aeration mode. The Illumina MiSep sequencing revealed that the relative abundance of the Nitrospira as the only detected NOB, decreased from 2.22% (day 0) to 0.91% (day 207) at the genus level. Overall, this study provides a new strategy for NOB suppression to strengthen nitrogen removal from low C/N domestic wastewater through the continuous process.

Anaerobic/aerobic/anoxic (AOA) is suitable for advanced nitrogen removal of low C/N wastewater as an energy-saving process. Investigations of the temperature impact on the AOA process are critical to its application in cold regions or seasons. In this study, the nitrogen removal performance in AOA at low and room temperatures was investigated. The nitrification capacity of the AOA process was recovered at low temperature and the endogenous denitrification performance was enhanced by converting the partial aerobic zone into anoxic. At 15 °C, treating real municipal sewage with a low C/N ratio (3.36), TIN and NH4+-N removal efficiencies of 84.3 ± 6.6% and 97.4 ± 3.3% respectively, were achieved. The oxygen uptake rate test, quantitative PCR, and high-throughput sequencing results indicated that the activity and abundance of ammonia-oxidizing bacteria (AOB) increased at low temperature, which was the key for nitrification capacity recovery. Overall, the recoverability of nitrification capacity in the AOA system made advanced nitrogen removal possible at low temperatures.

A novel process combined simultaneous partial nitrification-anammox (SPNA) with modified denitrifying phosphorus removal (DPR) in two sequencing batch reactors (terms as SPNA-SBR and mDPR-SBR) was proposed for treating ammonia (NH4+-N) and domestic wastewaters. Firstly, NH4+-N contained wastewater was fed into SPNA-SBR for autotrophic nitrogen removal. Meanwhile, domestic wastewater was introduced to mDPR-SBR for anaerobic phosphorus release and intracellular carbon storage. Then, nitrate (NO3–-N) produced in SPNA-SBR was discharged to mDPR-SBR, where NO3–-N was reduced to dinitrogen gas (N2) via anoxic denitrifying phosphorus removal. Finally, the residual NH4+-N and PO43−-P in mDPR-SBR were removed via aerobic simultaneous nitrification, endogenous denitrification and phosphorus removal (SNEDPR). The long-term operation (235-day) suggested that a desirable nutrient removal performance was achieved in the SPNA-mDPR process, and the final effluent NH4+-N, total nitrogen and PO43−-P concentrations were 1.5, 5.0 and 0.2 mg·L−1, respectively. In the SPNA-SBR, SPNA contributed to ∼90% of NH4+-N removal at low aeration intensity (dissolved oxygen (DO): 0.1 mg·L−1); in the mDPR-SBR, ∼70% of PO43−-P and ∼100% of NO3–-N were simultaneously removed via anoxic DPR, and post-aerobic SNEDPR further reduced ∼30% of PO43−-P and ∼70% of NH4+-N. Stoichiometry-based microbial activity analysis demonstrated that, in the mDPR-SBR, high activity of denitrifying phosphorus accumulating organisms (DPAOs) over denitrifying glycogen accumulating organisms (DGAOs) in anoxic NO3–-N removal (71.1% > 28.9%) facilitated DPR, and high percentage of SNEDPR via nitrite (89.6%) over nitrate saved 52% intracellular carbon demand to realize the deep-level nutrient removal without external carbon addition.

In this study, partial nitrification and post endogenous denitrification (PED) were combined with enhancing bacterial phosphorus removal (EBPR) in an anaerobic/aerobic/anoxic operated sequencing batch reactor (SBR) for deep-level nutrient removal from low carbon/nitrogen (C/N, chemical oxygen demand (COD)/total nitrogen (TN)) domestic wastewater. At anaerobic stage, abundant organic matters (96.6% of COD consumption) in raw wastewater were stored as poly-hydroxyalkanoates (PHAs) by phosphorus and glycogen accumulating organisms with enhanced activities, which provided sufficient intracellular carbons for subsequent aerobic phosphorus uptake and anoxic PED. By controlling suitable aeration rate and duration, high nitrite accumulation rate (97.2%) was obtained at aerobic stage, which saved intracellular carbons consumption of PED. Moreover, the subsequent utilization of glycogen after PHAs via PED ensured the deep-level TN removal (94.9%) without external carbon addition. After 160-day operation, the average effluent PO43--P and TN concentrations were 0.4 and 3.0 mg/L, respectively, at C/N of 3.1.

In this study, enhanced removal of nitrogen from real domestic wastewater with low organic carbon to nitrogen ratio of 2.1–3.6 was achieved in a sequencing batch biofilm reactor (SBBR) with enriching anammox bacteria in situ. The SBBR was inoculated with ordinary nitrification sludge and operated under alternative anaerobic, aerobic and anoxic patterns. During the start-up period, influent nitrogen and carbon loading rate gradually increased. According to the quantitative microbial analysis, the growth of ammonium oxidizing bacteria and anammox bacteria were promoted, resulting in an increase from 6.56 × 104 to 1.51 × 107 and from 1.11 × 104 to 1.69 × 106 gene copies·mL−1, respectively. During the steady period (Day 102–205), nitrogen removal efficiency of 88.2% was achieved with an effluent total nitrogen of 6.3 mg N·L−1 for average influent ammonium of 53.9 ± 4.0 mg·L−1. Nitrogen removal was steady when temperature decreased from 27.0 to 12.8 °C. Variations in nitrogen compounds suggested that anammox reaction could be the main pathway for nitrogen removal, according to the results of activity batch and continuous tests. For the high nitrogen removal performance, biofilm and post-anoxic unit could play important roles. Overall, this study provided an alternative method to improve the removal of nitrogen from sewage by directly enriching anammox bacteria.

A partial nitrification sequencing batch reactor was operated to reveal mechanisms behind nitrite oxidation bacteria (NOB) acclimatization in high-ammonia wastewater treatment. The influent NH4+-N increased stepwise from 499.7 ± 4.2 mg/L to 6994.5 ± 7.5 mg/L with initial free ammonia (FA) concentration rising from 37.9 ± 3.2 mg NH3-N/L to 715.3 ± 47.3 mg NH3-N/L, respectively. NOB acclimatized this FA range with NO3–-N production increasing from 29.2 ± 2.6 mg/L to 144.1 ± 31.0 mg/L in a cycle, which was caused by the shift of dominant NOB genus from Nitrospira to Nitrolencea. Nitrosomonas as ammonia oxidation bacteria, could sustain its activity of 62.1 ± 0.1 mg NH4+-N/(gVSS∙L∙h) under the same condition. Hydroxylamine addition could be implemented as an emergency measure to alleviate NOB acclimatization in short-term operation. The findings expanded knowledge about NOB acclimatization types and provided novel insights for addressing this problem in a targeted way.

Introducing fermentation technology into sewage treatment is a sustainable development concept, but future application still faces many challenges. A novel partial nitrification, fermentation-based double denitrification bioprocess (PN-F-Double/DN) was achieved in three separated SBR type reactors, simultaneously treating high ammonia (1766.6 mg/L) mature landfill leachate and external waste activated sludge (WAS, MLSS = 20.6 g/L). Firstly, NH4+-N was oxidized to NO2−-N in partial nitrification reactor (PN-SBR), with nitrite accumulation ratio (NAR) of 96.5%. Next, the PN-SBR effluent (NO2−-N = 1529.8 mg/L) coupled with the WAS were introduced to an anoxic reactor for integrated fermentation-denitrification (IFD-SBR). The occurrence of fermentation was mainly attributed to free nitrous acid (FNA, nitrite protonate form) promoting the splitting decomposition of sludge spatial configuration and interfacial forces. The released volatile fatty acids (VFAs) were utilized in situ during the denitrification process (NO2−-N→N2), obtaining 0.6 kg/m3•d nitrogen removal rate and 3.3 kg/m3•d sludge reduction rate. Finally, undesirable fermentation by-products from IFD-SBR (NH4+-N = 119.2 mg/L) were further removed in the endogenous post-denitrification reactor (EPD-SBR) through operational strategy of anaerobic/aerobic/anoxic by residual VFAs as the carbon source. In the EPD-SBR, Defluviicoccus (0.9%) and Candidatus Competibacter (5.8%) dominated carbon source storage and nitrogen removal, acting as a typical denitrifying glycogen-accumulating organism (DGAO), with an intracellular carbon storage efficiency of 83.1% and nitrogen removal contribution of 93.7%. After 200 days of operation, the PN-F-Double/DN process provided effluent containing, on average, 1.86 mg/L NH4+-N and 5.5 mg/L NOx−-N, with 98.5% TN removal. Compared with traditional bioprocesses, PN-F-Double/DN allowed up to 25% saving in aeration energy consumption, 100% decrease in carbon source demand, and achieve 46.1% external WAS reduction.

As a low consumption and high efficiency process, Partial Nitrification-Anammox/denitratation (PNAD) was applied to co-treat mature landfill leachate with municipal sewage for 300 days. Specifically, ammonia (670.2 ± 63.7 mg N/L) contained in mature landfill leachate was firstly oxidized to nitrite (611.5 ± 28.1 mg N/L) in sequence batch reactor (SBRPN); meanwhile, organic matter in municipal sewage was partially removed in another reactor (SBROMR); finally, nitrite produced (611.5 ± 28.1 mg N/L) in SBRPN and ammonia (53.1 ± 6.4 mg N/L) residing in pretreated municipal sewage were simultaneously degraded through combined Anammox-denitratation process in an up-flow anaerobic sludge bed (UASBAD). A satisfactory effluent quality of 10.3 mg/L TN was obtained after long-term operation, with Anammox and denitrification contributing to 86.2% and 5.8% nitrogen removal efficiency, respectively. Mass balance confirmed 67.2% nitrate generated from Anammox could be reduced to nitrite and in-situ reused. Anammox bacteria genes and nitrate reductase/nitrite reductase ratio were highly detected, accelerating combined Anammox-denitratation. Further, Ca. Brocadia triumph among various Anammox bacteria groups, increasing from 1.2% (day 120) to 3.6% (day 280).

The realization of stable partial nitrification and advanced nitrogen removal are not acquired effectively in conventional pre-denitrification biological nitrogen removal processes treating domestic sewage. Herein, a novel anaerobic/aerobic/anoxic/aerobic (AOAO) continuous plug-flow reactor, characterized with double sludge reflux and a bypass of anaerobic mixed liquor conveyed to anoxic zone, was first constructed to realize stable partial nitrification in treating domestic sewage. The alternating anoxic/aerobic conditions and longer anoxic sludge retention time might be responsible for the partial nitrification. Nitrite accumulation ratio reached 89.3 ± 3.3% with the maximum activity ratio of AOB to NOB increasing from 0.72 to 8.17. A content total inorganic nitrogen (TIN) removal efficiency (93.7 ± 2.2%) and effluent TIN concentration (2.9 ± 0.9 mg N/L) were obtained after 238 days' operation. Specifically, nitrogen balance of the typical cycle showed that about 30.1% of TIN was removed through simultaneous partial nitrification and denitrification (SND) in aerobic zone and 48.2% by endogenous denitrification in anoxic zone. The AOAO process is an economic treatment for domestic sewage with aerobic hydraulic retention time (HRT) of 4 h.

Anaerobic ammonium oxidation (anammox) has drawn increasing attention as a promising option to energy-neutral wastewater treatment. While anammox process still faces challenges in the low-strength and organics-contained municipal wastewater due to its susceptibility and the technical gaps in substrate supply. Effective strategies for extensive implementation of anammox in municipal wastewater treatment plants (WWTPs) remain poorly summarized. In view of the significance and necessity of introducing anammox into mainstream treatment, the growing understanding not only at level of microbial interactions but also on view of upgrading municipal WWTPs with anammox-based processes need to be considered urgently. In this review, the critical view and comprehensive analysis were offered from the perspective of microbial interactions within partial nitrification- and partial denitrification-based anammox processes. To minimize the microbial competition and enhance the cooperation among anammox bacteria and other functional bacteria, targeted control strategies were systematically evaluated. Based on the comprehensive overview of recent advances, the combination of flexible regulation of input organic carbon with anaerobic/oxic/anoxic process and the integration of sludge fermentation with anoxic biofilms in anaerobic/anoxic/oxic process were proposed as promising solutions to upgrade municipal WWTPs with anammox technology. Furthermore, a new perspective of coupling anammox with denitrifying dephosphatation was proposed as a promising method for in-depth nutrients removal from carbon-limit municipal wastewater in this study. This review provides the critical and comprehensive viewpoints on anammox engineering in municipal wastewater and paves the way for the anammox-based upgrading of municipal WWTPs.

In this study, a two-stage combined process of partial nitrification-Anammox (PNA) and partial denitrification-Anammox (PDA) was established achieving advanced nitrogen removal from landfill leachate. The PNA sludge used to treat reject water adapted to the leachate in 37 days, resulting in fast start-up of the PNA process with a nitrogen removal rate (NRR) of 0.22 kgN/(m3·d). Partial denitrification (PD) was induced using sodium acetate and proceeded in a stepwise manner using sludge fermentation liquid (SFL), achieving a NO3−-N to NO2−-N transformation ratio (NTR) of 52.1 ± 1.1% within 16 days. PDA was established via the addition of mature Anammox biofilms. The nitrogen removal efficiency (NRE) of this system was 97.6 ± 1.5%, of which PNA and PDA contributed 74.8 ± 4.0% and 18.7 ± 4.1%, respectively. Nitrosomonas (2.6% in PNA), Thauera (16.0% in PDA) and Candidatus Brocadia (23.0% in PNA, 1.4% in PDA) were dominant in the two-stage system. This study provides valuable and novel insights, supporting the practical application of PNA-PDA processes in landfill sites.

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 O2. Under that condition, advanced nitrogen removal and sludge settleability were achieved and the energy consumption and N2O 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.