Advanced nitrogen removal from wastewater by combining anammox with partial denitrification

Manganese-Catalyzed Ammonia Oxidation into Dinitrogen under Chemical or Electrochemical Conditions*

Earth-abundant metal-catalyzed oxidative conversion of ammonia into dinitrogen is a promising process to utilize ammonia as a transportation fuel. Herein, we report the manganese-catalyzed ammonia oxidation under chemical or electrochemical conditions using a manganese complex bearing (1S,2S)-N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamine. Under chemical conditions using oxidant, up to 17.1 equivalents of N₂ per catalyst are generated. Also, mechanistic studies by stoichiometric reactions reveal that a nucleophilic attack of ammonia on manganese nitrogenous species occurs to form a nitrogen-nitrogen bond leading to dinitrogen. Moreover, we conduct density functional theory (DFT) calculations to confirm the plausible reaction mechanism. In addition, this reaction system is applicable under electrochemical conditions. The catalytic reaction proceeds with 96 % faradaic efficiency (FE) in bulk electrolysis to give up to 6.56 equivalents of N₂ per catalyst.

Primary sludge fermentate as carbon source for mainstream partial denitrification-anammox (PdNA)

Primary sludge fermentate, a concentrated hydrolyzed wastewater carbon, was evaluated for use as an alternative carbon source for mainstream partial denitrification-anammox (PdNA) in a suspended growth activated sludge process in terms of partial denitrification (PdN) efficiency, PdNA nitrogen removal contributions, and final effluent quality. Fermenter operation at a 2-day sludge retention time (SRT) resulted in the maximum achievable yield of 0.14 ± 0.05 g sCOD/g VSS without release of excessive ammonia and phosphorus to the system. Based on the results of batch experiments, fermentate addition led to PdN efficiency of 93 ± 14%, which was similar to acetate at a nitrate residual of 2-3 mg N/L. In the pilot-scale mainstream deammonification reactor, PdN efficiency using fermentate was 49 ± 24%, which was lower than acetate (66 ± 24% during acetate period I and 70 ± 21% during acetate period II), most probably due to lower nitrate and ammonium kinetics in the PdN zone. Methanol cost-saving potential for the application of PdNA as the main short-cut nitrogen pathway was estimated to be 30% to 55% depending on the PdN efficiency achieved. PRACTITIONER POINTS: Primary sludge fermentate was evaluated as an alternative carbon source for mainstream partial denitrification-anammox (PdNA). Fermenter operated at a 1 to 2 day SRT resulted in the maximum achievable yield without the release of excessive ammonia and phosphorus to the system. Although 93% partial denitrification efficiency was achieved with fermentate in batch experiments, around 49% PdN efficiency was achieved in pilot studies. Application of PdNA with fermentate can result in significant methanol cost savings.

Effect of substrate types on contaminant removals, electrochemical characteristics and microbial community in vertical flow constructed wetlands for treatment of urban sewage

The purpose of this study was to investigate the influence of substrates (quartz sand and coke) on the removal of pollutants (COD, NH₄⁺-N and TP), electrochemical characteristics and microbial communities of vertical flow constructed wetlands (VFCW) under high pollutant loads. During operation, the removal rates of COD, NH₄⁺-N and TP by VFCW-C (coke as substrate) were higher than that of VFCW-Q (quartz sand as substrate) by 9.73-19.41%, 5.03%-13.15% and 8.83%-14.58%, respectively. And the resistances of the VFCW-Q and VFCW-C were increased by 1228.9 Ω and 38.3 Ω, while their potentials were dropped from 182.4 mV to 377.9 mV-85.6 mV and 222.0 mV, respectively. The dominant bacteria at the bottoms of VFCW-Q and VFCW-C were individually aerobic denitrifying bacteria (ADNB; 14.98%)/ammonia oxidizing bacteria (AOB; 5.73%) and organics aerobic degrading bacteria (OADB; 12.48%)/ammonia oxidizing bacteria (AOB; 7.24%), while the predominant bacteria at their tops were separately ADNB (11.36%)/OADB (10.52%)/AOB (4.69%) and ADNB (15.09%)/AOB (8.86%) and OADB (3.20%) The removal of pollutants by VFCW-Q and VFCW-C may be mainly attributed to substrate adsorption and microbial degradation.

Deammonification process in municipal wastewater treatment: Challenges and perspectives

The deammonification process has been proved to be an efficient nitrogen removal process in treating high NH₄⁺-N concentration wastewater (sidestream deammonification). It is very hopeful to bring WWTP close to energy autarky. However, the feasibility of applying mainstream deammonification to sewage treatment need to be further explored. Therefore, this review attempts to give an overview of challenges in applying mainstream deammonification and to discuss the impacts of unfavorable conditions on main functional species. In addition, some novel control strategies to maintain the dominant position of desired species were summarized. Efficient solution to the conflict between AnAOB (Anaerobic ammonium-oxidizing bacteria) biomass retention and NOB (Nitrite oxidizing bacteria) wash out was also reviewed. Ultimately, we suggested further studies including effective improved process that achieve combination of autotrophy and organotrophy species based on the metabolic diversity of AnAOB.

Feasibility of partial denitrification and anammox for removing nitrate and ammonia simultaneously in situ through synergetic interactions

In this study, the single-stage partial denitrification-anammox (PD-A) process was started-up in 22 days in a lab-scale up-flow sludge blanket (UASB) reactor to treat wastewater containing NH₄⁺-N and NO₃^--N simultaneously. The TN removal rate reached 97.08% with a low effluent TN of 10 mg/L. High-throughput sequencing results revealed the dominant bacterial strains were related to the genus of Thauera and Candidatus Kuenenia. The PD-A system was started-up based on the optimized PD process via inoculated exogenous anammox sludge attributing to the improvement of bacterial adaptation and co-existence by EPS. The PD process was realized in 18 days with the abundance of PD functional bacterium Thauera through fluctuated C/NO-3-N conditions. Moreover, the detrimental effects of starvation on anammox was weaker than that on PD bacteria. The PD-A process was expected to open a new possible perspective in designing NO₃^--N and NH₄⁺-N wastewater treatment plants.

Sulfate dependent ammonium oxidation: A microbial process linked nitrogen with sulfur cycle and potential application

Sulfate dependent ammonium oxidation (Sulfammox) is a potential microbial process coupling ammonium oxidation with sulfate reduction under anaerobic conditions, which provides a novel link between nitrogen and sulfur cycle. Recently, Sulfammox was detected in wastewater treatments and was confirmed to occur in natural environments, especially in marine sediments. However, knowledge gaps in the mechanism of Sulfammox, functional bacteria, and their metabolic pathway, make it challenging to estimate its environmental significance and potential applications. This review provides an overview of recent advances in Sulfammox, including possible mechanisms, functional bacteria, and main influential factors, and discusses future challenges and opportunities. Future perspectives are outlined and discussed, such as exploration of microbial community structure and metabolic pathways, possible interactions with other microbes, environmental significance, and potential applications for nitrogen and sulfate removal, to inspire more researches on the Sulfammox process.

Enhancement of nitrite production via addition of hydroxylamine to partial denitrification (PD) biomass: Functional genes dynamics and enzymatic activities

This study investigated the activity of partial denitrification (PD) biomass/key enzymes, functional gene expressions in response to 0 ~ 50 mg/L hydroxylamine (NH₂OH) addition. Results indicated that NH₂OH contributed to nitrite (NO₂^--N) production, facilitating the maximum increase of nitrate (NO₃^--N) to NO₂^--N transformation ratio to 80.47 ± 2.82%, leading to 2.56-fold NO₂^--N higher than those of control. The observed transient inhibitory effect on NO₃^--N reduction process was attributed by high-level NH₂OH (35 ~ 50 mg/L). Enzymatic assays revealed the enhanced activity of both NO₃^--N and NO₂^--N reductase while the former showed obvious superiority which led to high NO₂^--N accumulation. These results were further confirmed by the corresponding functional genes (narG, napA, nirS and nirK). Besides, negative influence of NH₂OH addition was limited to PD aggregates, due to the increasing secretion of extracellular polymeric substances (EPS) as well as proteins/polysaccharides ratios in tightly-bound structure of EPS.

Effect of ammonium to nitrite ratio on reactor performance and microbial population structure in anammox reactors

Anaerobic ammonium oxidation (anammox) presents an efficient alternative for conventional nitrogen removal process. In this study, the effect of varying Substrate (ammonium to nitrite) ratios on reactor performance and microbial community structures within three anaerobic sequencing batch reactors (ASBRs) was investigated. Three 1 L ASBRs (Reactors 1, 2 and 3) were operated under similar operational conditions. By varying the ammonium to nitrite ratios, a significant variation in nitrogen removal was observed after 170 days of operation: nitrogen removal efficiencies of 67.17 ± 7.29%, 57.13 ± 11.18% and 56.26 ± 17.05% in Reactors 3, 2 and 1 respectively were achieved. Similarly, using quantitative PCR, an overall variation in the population of anammox bacteria, ammonia oxidizing bacteria (AOB), Nitrospira and copy numbers of nirS, hzo and hzs genes were observed with varying degrees of expression. High throughput sequencing analysis further showed a shift in microbial community structure with an overall increase in population of Planctomycetia from 0.76% to (3%, 25% and 26%) and Betaproteobacteria from 5.38% to (19%, 21% and 43%) within Reactors 1, 2 and 3, respectively. In conclusion, different substrates ratio showed a significant influence on the overall nitrogen removal rate as well as the abundances of the different microbial groups.

Advances and challenges of mainstream nitrogen removal from municipal wastewater with anammox-based processes

Anaerobic ammonium oxidation (anammox) is a novel process of deammonification that exhibits superior ecological and economic potential compared to that of traditional heterotrophic processes. Although this process has been successfully implemented in treating high-strength nitrogen-contaminated wastewater, it still faces many challenges in treating mainstream municipal wastewater. This review aims to provide an overview of the status and challenges of mainstream anammox-based processes. The different configurations and crucial factors are discussed in this review. Finally, the future needs for feasible application are stated. PRACTITIONER POINTS: Factors restricting mainstream application of anammox-based processes are reviewed. Control strategies for selecting and maintaining anammox bacteria are discussed. Recent advances in nitrite production via partial nitrification or denitrification are summarized. Future needs for the feasible application of anammox-based nitrogen removal technology for mainstream municipal wastewater treatment are outlined.

Application of the Anammox Process for Treatment of Liquid Phase Digestate

The liquid phase of the digestate (LPD) contains a relatively high concentration of nitrogen, with total ammonium nitrogen being the dominant form of nitrogen, as well as other essential nutrients such as phosphorus and potassium. Consequently, it must be treated before it is released into the environment. However, there are no reports of co-purification of LPD in the anammox process in sequencing batch reactor with granular sludge, which is a novelty for the presented research. The main objective of this paper is to assess the possibility of nitrogen removal in the anammox process with LPD from biogas plants conducting the co-fermentation process along with the participation of agricultural products (cattle slurry). This publication presents the research results of the efficiency of the anammox process, accounting for the effect of dissolved organic matter. The conducted experiments revealed the potential of LPD purification, which co-ferments waste activated sludge and bovine slurry for the anammox process. In the reactor ammonium utilization rate (AUR) process with LPD addition increased from 2.3 mg N/(g VSS∙h) with 0.5% LPD addition to 8.5 mg N/(g VSS∙h) with 7.5% LPD addition. SAA in the reactor with LPD addition increased from 5.3 mg N/(g VSS∙h) with 0.5% LPD addition to 18.5 mg N/(g VSS∙h) with 4 and 5% LPD addition. With the addition of 7.5% LPD, SAA dropped to a value of 18.1 mg N/(g VSS∙h) in the LPD reactor.

Unravelling nitrogen removal and nitrous oxide emission from mainstream integrated nitrification-partial denitrification-anammox for low carbon/nitrogen domestic wastewater

Stable supply of nitrite is often a major obstacle for achieving mainstream anammox due to washout failure of nitrite oxidizers (NOB) at low influent ammonia of municipal wastewater. In this study, an integrated nitrification, partial denitrification and anammox (INPDA) as a one-stage mainstream nitrogen removal alternative was established in a low-oxygen sequencing batch biofilm reactor treating synthetic sewage. The overall nitrogen removal and nitrous oxide (N₂O) emission were mainly investigated at 50 mg/L NH₄⁺-N influent with a low carbon/nitrogen (C/N) of 2.5. Continuous operation demonstrated that as high as 98.8% NH₄⁺-N and 94.1% TN were removed in SBBR system. Cyclic experiment verified sequential completion of nitrification, partial denitrification and anammox were responsible for high-rate TN removal. During one typical cycle, the trend of N₂O emission was characterized by firstly rapid rise, then fluctuant decrease followed by rapid decrease and finally slow disappearance. The maximum N₂O emission rate reached up to 6.7 μg/(L·min) occurred at 75 min. High-throughput sequencing revealed the co-existence of nitrifying, denitrifying and anammox species and large detection of key functional genes (Hzs, Hdh, Hao, Nor) in an oxygen-limited SBBR, thereby highly correlating nitrogen removal and N₂O emission characteristics. Nitrogen metabolic pathways analysis further suggest denitratation(NO₃^--N to NO₂^--N)-based anammox is a main route for mainstream nitrogen removal. Moreover, N₂O might be generated by both hydroxylamine oxidation step in nitrification and also heterotrophic denitrification pathway. The research findings provide more deep understandings of enhanced nitrogen removal and mitigated N₂O footprint from a single mainstream anammox-based system.

Characteristics and mechanism of anammox granular sludge with different granule size in high load and low rising velocity sewage treatment

An integrated investigation to structural, activity and microbial diversity of anammox granular sludge (AnGS) in a wastewater treatment system with high ammonia nitrogen load was performed and aimed to establish the relationship between granular size and performance. With the increase in granule size, the main component of extracellular polymeric substances (EPS) changed from slime EPS to tightly-bound EPS, while the organic component remained the same, and the specific anammox activity increased. However, the results of qPCR and high-throughput sequencing showed that for granules with sizes inferior than 4.75 mm, the abundance of ammonia-oxidizing bacteria (AnAOB) increased as the size increased, and the copies of AnAOB decreased when the granule size increased above 4.75 mm, and the community complexity increased. According to the correlation analysis results, AnAOB first accumulated and then optimized the flora structure to improve efficiency and 2.8 mm to 4.75 mm was the optimal size of AnGS.

Anammox biofilm system under the stress of Hg(II): Nitrogen removal performance, microbial community dynamic and resistance genes expression

The existence of heavy metals in wastewater has obtained more attention due to its high toxicity and non-degradability. In this study, we investigated the changes of anaerobic ammonium oxidation (Anammox) system under long-term invasion of Hg(Ⅱ). The results indicated that the total nitrogen removal efficiency (TNRE) dropped to around 55 % as Hg(Ⅱ) concentration went up to 20 mg L^-1. But the functional bacteria rapidly developed some resistant abilities and maintained a stable TNRE of 65 % till the end of test. The maximum relative expression fold change of merA, merB, merD and merR were 468.8476, 23.7383, 5.0321 and 15.2514 times, respectively. The high positive correlation between the expression abundance of metal resistance genes and the concentrations of Hg(Ⅱ) revealed the resistant mechanisms of microorganisms to heavy metals. Moreover, the protective strategy based on extracellular polymeric substances also contributed to the stability of Anammox system.

Construction waste as substrate in vertical subsuperficial constructed wetlands treating organic matter, ibuprofenhene, acetaminophen and ethinylestradiol from low-strength synthetic wastewater

This study aimed to evaluate the removal of chemical oxygen demand (COD), total Kjeldahl nitrogen (TKN), total ammonia nitrogen (TAN), total phosphorus (TP), ibuprofen, acetaminophen and ethinylestradiol of synthetic effluent simulating low-strength sewage by sequencing-batch mode constructed wetlands (CWs). To verify the feasibility of using a floating macrophyte in CWs and compare different substrates, three CWs containing light expanded clay aggregates (CWL), expanded clay with porcelain tiles (CWLP) and bricks (CWB) were planted with Pistia stratiotes. The results showed that CWB achieved the highest removals of TKN (78%), TAN (70%) and TP (46%), and CWLP achieved the highest COD removal (75%). LECA favored the removal of ibuprofen (92%, p < 0.05) when compared to bricks (77%), probably by the combination of biodegradation and sorption in the systems. The highest acetaminophen removal (71% to 96%) was observed in CWL, probably via biodegradation, but no significant differences were found between the CWs (p > 0.05). Ethinylestradiol was removed 76% in CWLP and 73% in CWB, both differing statistically from CWL (p < 0.05), demonstrating that brick and the combination of clay with porcelain were better than just clay in this hormone removal. After 188 days of operation, P. stratiotes was able to uptake nitrogen and phosphorus of approximately 0.28 g and 0.25 g in CWL, 0.33 g and 0.21 g CWLP, and 0.22 g and 0.09 g in CWB of, respectively. Adsorption of nitrogen and phosphorus onto the substrates was 0.48 g and 6.84 g in CWL, 0.53 g and 5.69 g in CWLP, and 0.36 g and 10.18 g in CWB, respectively. The findings on this study suggest that adsorption was possible the main process for TP removal onto the evaluated substrates whereas microbial activity was the most probable mechanism for TN removal in the evaluated CW systems.

System Performance Corresponding to Bacterial Community Succession after a Disturbance in an Autotrophic Nitrogen Removal Bioreactor

Performance of a bioreactor is affected by complex microbial consortia that regulate system functional processes. Studies so far, however, have mainly emphasized the selective pressures imposed by operational conditions (i.e., deterministic external physicochemical variables) on the microbial community as well as system performance, but have overlooked direct effects of the microbial community on system functioning. Here, using a bioreactor with ammonium as the sole substrate under controlled operational settings as a model system, we investigated succession of the bacterial community after a disturbance and its impact on nitrification and anammox (anaerobic ammonium oxidation) processes with fine-resolution time series data. System performance was quantified as the ratio of the fed ammonium converted to anammox-derived nitrogen gas (N₂) versus nitrification-derived nitrate (npNO₃ ^-). After the disturbance, the N₂/npNO₃ ^- ratio first decreased, then recovered, and finally stabilized until the end. Importantly, the dynamics of N₂/npNO₃ ^- could not be fully explained by physicochemical variables of the system. In comparison, the proportion of variation that could be explained substantially increased (tripled) when the changes in bacterial composition were taken into account. Specifically, distinct bacterial taxa tended to dominate at different successional stages, and their relative abundances could explain up to 46% of the variation in nitrogen removal efficiency. These findings add baseline knowledge of microbial succession and emphasize the importance of monitoring the dynamics of microbial consortia for understanding the variability of system performance.IMPORTANCE Dynamics of microbial communities are believed to be associated with system functional processes in bioreactors. However, few studies have provided quantitative evidence. The difficulty of evaluating direct microbe-system relationships arises from the fact that system performance is affected by convolved effects of microbiota and bioreactor operational parameters (i.e., deterministic external physicochemical forcing). Here, using fine-resolution time series data (daily sampling for 2 months) under controlled operational settings, we performed an in-depth analysis of system performance as a function of the microbial community in the context of bioreactor physicochemical conditions. We obtained statistically evaluated results supporting the idea that monitoring microbial community dynamics could improve the ability to predict system functioning, beyond what could be explained by operational physicochemical variables. Moreover, our results suggested that considering the succession of multiple bacterial taxa would account for more system variation than focusing on any particular taxon, highlighting the need to integrate microbial community ecology for understanding system functioning.

Effect of Electrostatic Field Strength on Bioelectrochemical Nitrogen Removal from Nitrogen-Rich Wastewater

The effect of electrostatic fields on the bioelectrochemical removal of ammonium and nitrite from nitrogen-rich wastewater was investigated at strengths ranging from 0.2 to 0.67 V/cm in bioelectrochemical anaerobic batch reactors. The electrostatic field enriched the bulk solution with electroactive bacteria, including ammonium oxidizing exoelectrogens (AOE) and denitritating electrotrophs (DNE). The electroactive bacteria removed ammonium and nitrite simultaneously with alkalinity consumption through biological direct interspecies electron transfer (DIET) in the bulk solution. However, the total nitrogen (ammonium and nitrite) removal rate increased from 106.1 to 166.3 mg N/g volatile suspended solids (VSS).d as the electrostatic field strength increased from 0.2 to 0.67 V/cm. In the cyclic voltammogram, the redox peaks corresponding to the activities of AOE and DNE increased as the strength of the electrostatic field increased. Based on the microbial taxonomic profiling, the dominant genera involved in the bioelectrochemical nitrogen removal were identified as Pseudomonas, Petrimonas, DQ677001_g, Thiopseudomonas, Lentimicrobium, and Porphyromonadaceae_uc. This suggests that the electrostatic field of 0.67 V/cm significantly improves the bioelectrochemical nitrogen removal by enriching the bulk solution with AOE and DNE and promoting the biological DIET between them.

Technologies for biological removal and recovery of nitrogen from wastewater

Water contamination is a growing environmental issue. Several harmful effects on human health and the environment are attributed to nitrogen contamination of water sources. Consequently, many countries have strict regulations on nitrogen compound concentrations in wastewater effluents. Wastewater treatment is carried out using energy- and cost-intensive biological processes, which convert nitrogen compounds into innocuous dinitrogen gas. On the other hand, nitrogen is also an essential nutrient. Artificial fertilizers are produced by fixing dinitrogen gas from the atmosphere, in an energy-intensive chemical process. Ideally, we should be able to spend less energy and chemicals to remove nitrogen from wastewater and instead recover a fraction of it for use in fertilizers and similar applications. In this review, we present an overview of various technologies of biological nitrogen removal including nitrification, denitrification, anaerobic ammonium oxidation (anammox), as well as bioelectrochemical systems and microalgal growth for nitrogen recovery. We highlighted the nitrogen removal efficiency of these systems at different temperatures and operating conditions. The advantages, practical challenges, and potential for nitrogen recovery of different treatment methods are discussed.

Enhanced long-term advanced denitrogenation from nitrate wastewater by anammox consortia: Dissimilatory nitrate reduction to ammonium (DNRA) coupling with anammox in an upflow biofilter reactor equipped with EDTA-2Na/Fe(II) ratio and pH control

A long-term experiment in an anaerobic ammonium oxidation (anammox) reactor showed that anammox consortia could perform a stable and efficient Fe(II)-dependent dissimilatory nitrate reduction to ammonium (DNRA) coupled to the anammox (DNRA-anammox) process by controlling the EDTA-2Na/Fe(II) ratio and pH, with a total nitrogen removal rate (TNRR) of 0.23 ± 0.01 kg-N/m3/d. Anammox bacteria (Candidatus Kuenenia) were the dominant and functional microbes in such a nitrate wastewater treatment system. Visual MINTEQ analysis showed that the EDTA-2Na/Fe(II) molar ratio affected the influent composition of Fe and EDTA species and hence nitrate removal, while pH influenced both nitrate removal and the coupling degree of the Fe(II)-dependent DNRA-anammox process due to its own physiology. The kinetic simulation results showed that excess EDTA-2Na imposed a competitive inhibition on the Fe(II)-dependent DNRA-anammox process, and the Bell-shaped (A), (B), (C) and Ratkowsky models could be used to explore the pH dependency of the Fe(II)-dependent DNRA-anammox process.

Flexible Nitrite Supply Alternative for Mainstream Anammox: Advances in Enhancing Process Stability

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 novel anammox reactor with a nitrogen gas circulation: performance, granule size, activity, and microbial community

Anammox process was regarded to be one of the vital links to achieve energy-saving or energy-producing wastewater treatment plant. In the study, an anammox reactor with the nitrogen gas circulation was constructed to culture anammox granules, and the performance, granule size distribution, and microbial community were investigated. Dissolved oxygen loading is found to be an important factor for the start-up of the anammox process, and the nitrogen removal rate of 2.12 kg N m^-3 day^-1 was achieved under the average nitrogen loading rate of 2.6 kg N m^-3 day^-1. The activity test showed that the highest specific anammox activity of 345.9 mg N gVSS^-1 day^-1 was achieved for granules with size of 0.5-1.0 mm. The Illumina high-throughput sequencing analysis revealed the consistent variation of Candidatus Brocadia and Denitratisoma abundance in granues of all sizes, suggesting possible synergistic mechanism between heterotrophic bacteria Denitratisoma and anammox bacteria Ca. Brocadia. Furthermore, the results indicated the reactor with the nitrogen gas circulation is an efficient strategy to start-up anammox.

Evolution of microbial dynamics with the introduction of real seawater portions in a low-strength feeding anammox process

The salinity effect on anammox bacteria has been widely reported; however, rare studies describe the microbial dynamics of anammox-based process response to the introduction of real seawater at mainstream conditions. In this study, an anammox process at mainstream conditions without pre-enriching anammox bacteria was shifted to the feeds of a synthetic wastewater with a portion of seawater mixture. It achieved over 0.180 kg-N/(m³ day) of nitrogen removal rate with an additional seawater proportion of 20% in the influent. The bacterial biodiversity was significantly increased with the increase of seawater proportions. High relative abundance of anammox bacteria (34.24–39.92%) related to Ca. Brocadia was enriched and acclimated to the saline environment. However, the introduction of seawater caused the enrichment of nitrite-oxidizing Ca. Nitrospira, which was responsible for the deterioration of nitrogen removal efficiency. Possible adaptation metabolisms in anammox bacteria and other nitrogen transforming bacteria are discussed. These results highlight the importance of microbial diversity for anammox process under the saline environments of 20% and 40% seawater composition.

The effect of hydraulic retention time on ammonia and nitrate bio-removal over nitrite process

Ammonia and Nitrate Bio-removal Over Nitrite (ANBON) is a new biological process, which couples denitratation with Anaerobic Ammonium Oxidation (ANAMMOX) to carry out simultaneous removal of nitrate nitrogen and ammonia nitrogen. The effect of hydraulic retention time (HRT) on the performance of ANBON process was investigated. The results showed that the optimal HRT was about 0.7 h and the nitrogen removal rate was 26.2 ± 0.7 g N·L^-1·d^-1, which was at top level reported in previous literatures. The change of HRT was found to trigger the change of microbial community in the reactor, which exerted a great effect on the performance of ANBON process. The community analysis based on 16S rRNA gene indicated that Halomonas and Candidatus Kuenenia were the dominant functional bacteria in the denitratation unit and the ANAMMOX unit respectively. These results are helpful for the development and application of ANBON process.

Improving wastewater management using free nitrous acid (FNA)

Free nitrous acid (FNA), the protonated form of nitrite, has historically been an unwanted substance in wastewater systems due to its inhibition on a wide range of microorganisms. However, in recent years, advanced understanding of FNA inhibitory and biocidal effects on microorganisms has led to the development of a series of FNA-based applications that improve wastewater management practices. FNA has been used in sewer systems to control sewer corrosion and odor; in wastewater treatment to achieve carbon and energy efficient nitrogen removal; in sludge management to improve the sludge reduction and energy recovery; in membrane systems to address membrane fouling; and in wastewater algae systems to facilitate algae harvesting. This paper aims to comprehensively and critically review the current status of FNA-based applications in improving wastewater management. The underlying mechanisms of FNA inhibitory and biocidal effects are also reviewed and discussed. Knowledge gaps and current limitations of the FNA-based applications are identified; and perspectives on the development of FNA-based applications are discussed. We conclude that the FNA-based technologies have great potential for enhancing the performance of wastewater systems; however, further development and demonstration at larger scales are still required for their wider applications.

Towards more efficient nitrogen removal and phosphorus recovery from digestion effluent: Latest developments in the anammox-based process from the application perspective

Over the past forty years, anammox-based processes have been extensively researched and applied to some extent. However, some of the long-standing problems present serious impediments to wide application of these processes, and knowledge gap between lab-scale research and full-scale operations is still considerable. In recent years, anammox-based research has developed rapidly and some emerging concepts have been proposed. The focus of this review is on the critical problems facing actual application of anammox processes. The latest developments in anammox-based processes are summarized, and particular consideration is given to the following aspects: (1) the evolution of the chemical stoichiometry of anammox reaction; (2) the status of several main anammox-based processes; (3) the critical problems and countermeasures; (4) the emerging anammox-based processes; and (5) the suggested optimal process integrating partial nitritation, anammox, hydroxyapatite crystallization and denitratation for digestion effluent treatment towards more efficient nitrogen removal and phosphorus recovery in the future.

In situ enrichment of anammox bacteria in anoxic biofilms are possible due to the stable and long-term accumulation of nitrite during denitrification

In situ enrichment of anammox bacteria in anoxic biofilms has been observed, but the specific conditions for anammox competition with denitrification for nitrite are not yet fully understood. Therefore, an anoxic sequencing batch biofilm reactor (SBBR) was used to investigate nitrite production during denitrification. In each SBBR cycle, with nearly 80% of nitrate reduced in 3 h, over 9.7 mg/L nitrite was gradually accumulated and maintained for a long time, despite temperatures gradually decreasing from 32 to 5 ℃. The long-term existence of nitrite was due to the low biofilm nitrite reduction rate (1.2 mgN gVSS^-1 h^-1), which was about 10-fold less than the nitrate reduction rate. Accordingly, nitrite reduction via denitrifiers was continuously suppressed, which was favorable for nitrite reduction through the anammox pathway. Indeed, anammox bacteria were successfully enriched here (Candidatus_Brocadia, 0.1%). This study confirms the potential of anoxic biofilm in enriching anammox bacteria and provides insight into understanding.

An improved start-up strategy for mainstream anammox process through inoculating ordinary nitrification sludge and a small amount of anammox sludge

The difficulties in enriching anammox bacteria and maintaining stable partial nitrification during start-up phase limit the application of mainstream anammox process. In this study, the feasibility of starting up simultaneous partial nitrification, anammox and denitrification (SNAD) reactor treating municipal wastewater by inoculating ordinary nitrification sludge (96.2%) and a small amount of anammox sludge (3.8%) was investigated. A sequencing batch reactor with intermittent aeration was used for the SNAD process. The SNAD reactor was started up in 75 days with a nitrogen removal efficiency of 85.4% at ambient temperature. The nitrogen removal performance maintained stable despite the fluctuating inflow. Anammox bacterial activity exponentially increased although nitrite oxidizing bacteria (NOB) activity in seeding sludge was high. The enhanced ammonium oxidizing bacterial activity and partial denitrification provided sufficient nitrite for anammox bacteria. Moreover, NOB was inhibited by intermittent aeration, anammox bacteria had competitive advantage on nitrite. The improved particle size and settleability of activated sludge also favored the anammox bacterial enrichment. This study provided an improved and easily-implemented start-up strategy for mainstream anammox. The seeding sludge was easily obtained and the operation strategy was simple. These findings were meaningful to the engineering application of mainstream anammox.

Recent advances in partial denitrification in biological nitrogen removal: From enrichment to application

To maximize energy recovery, carbon capture followed by shortcut nitrogen removal is becoming the most promising route in biological wastewater treatment. As the intermediate of microbial denitrification, nitrite could serve as a substrate for anammox bacteria, while N₂O is a combustion promoter that can increase 37% energy release from CH₄ than O₂. Therefore, the important advances in partial denitrification (PD) that produces nitrite or N₂O as the main product using inorganic or organic electron donors were critically reviewed. Specifically, the enrichment strategies of PD microorganisms were obtained by analyzing the selection pressures, metabolism, physiology, and microbiology of these microorganisms. Furthermore, some prospective and promising processes integrating PD microorganisms and the bottlenecks of current applications were discussed. The obtained knowledge would provide new insights into the upgrading of current WWTPs involving commitment to achieve nitrogen removal from wastewaters more economically and environmentally friendly.

Immobilizing partial denitrification biomass and redox mediators to integrate with the anammox process for nitrogen removal

In this study, immobilizing partial denitrification biomass and redox mediators to integrate with the anammox process for nitrogen removal was investigated. Three redox mediators (RMs), namely, 2-methyl-1,4-naphthoquinone (ME), anthraquinone (AQ) and 1-dichloroanthraquinone (1-AQ) were catalyzed to reduce nitrate to only nitrite by denitrification to integrate with the anammox process for nitrogen removal. First, our experimental results showed that there were 35.8, 42.2 and 53.0 mg-N L⁻¹ nitrite accumulation values with the addition of ME, AQ and 1-AQ, respectively, at the dose of 75 µM by the denitrification process at C/N = 2, which were 25.6%, 48.2% and 86.1% higher than that of the control without the addition of any RMs. Nitrate reductase activities were higher than that of nitrite reductase affected by RMs, which was the main reason for nitrite accumulation and further maintenance of the anammox process. Second, owing to the stable nitrite production by the partial denitrifying biomass with the addition of 1-AQ, the nitrogen removal rate of the reactor that integrated the partial denitrification and anammox process reached 1788.36 g-N m⁻³ d⁻¹ only using ammonia and nitrate as the influent nitrogen resource in the long-term operation. Third, the 16S rDNA sequencing results demonstrated that Yersinia frederiksenii and Thauera were the primary groups of the denitrifying biomass, which were considered the dominant partial denitrification species.

In this study, immobilizing partial denitrification biomass and redox mediators to integrate with the anammox process for nitrogen removal was investigated.

Nitrite and nitrate as electron acceptors for bioelectrochemical ammonium oxidation under electrostatic field

Bioelectrochemical ammonium oxidation with nitrite and nitrate as electron acceptors was investigated in bulk solution exposed to electrostatic field. In a bioelectrochemical reactor, electroactive nitrogen removal bacteria including ammonium oxidizing exoelectrogens (AOE) and denitrifying electrotrophs (DNE) were enriched by electrostatic field of 0.2 V/cm in a bulk solution containing nitrite, nitrate, and ammonium. Ammonium was oxidized simultaneously with decreases in nitrite and nitrate as electron acceptors due to direct interspecies electron transfer between AOE and DNE. The specific ammonium oxidation rate was 48 mg NH₄-N/g VSS.d when nitrate fraction was 1/3 in the electron acceptor composed of nitrite and nitrate. The specific ammonium oxidation rate gradually decreased with increasing nitrate fraction. However, it was still 24 mg NH₄-N/g VSS.d when nitrate was the only electron acceptor. This indicates that nitrate can be used as an electron acceptor for bioelectrochemical ammonium oxidation, although it is a less effective than nitrite. This finding provides an advantage that strict nitritation which selectively produces nitrite from ammonium can be avoided when treating ammonia-rich wastewater in a bioelectrochemical reactor.

Keystone taxa regulate microbial assemblage patterns and functional traits of different microbial aggregates in simultaneous anammox and denitrification (SAD) systems

Understanding the impacts of the ecological patterns and niche-based selection on microbial community assembly and nitrogen-cycling network is crucial for achieving energy-neutral wastewater treatment. However, little is known about the niche differentiation and microbial nitrogen-cycling traits of keystone taxa in flocs and granules in anammox-based systems. Herein, the aspects of community assemblage patterns, metabolic functions and nitrogen transformation pathways were explored. The findings discovered that the treatment performance and bacterial community assembly were regulated by core taxa and flocs and granules communities harbored core taxa based on their functional traits. Both niche differentiation and environmental filtering have profound influences on functional bacteria. Furthermore, a combined analysis showed that nitrogen removal in flocs and granules was regulated by different nitrogen transformation pathways. These results suggest that core taxa are the key drivers for the microbial nitrogen-cycling network and improve the understanding of cross-feeding and metabolic pathways between anammox and nitrogen-cycling-related microorganisms.

Partial denitrification providing nitrite: Opportunities of extending application for anammox

Anaerobic ammonium oxidation (anammox) has been extensively investigated for cost-efficient nitrogen removal from wastewater. However, the major issues of nitrate (NO₃^--N) residue and instability in the current combination of nitritation and anammox process necessitates being addressed efficiently. The recently proposed partial-denitrification (PD), terminating NO₃^--N reduction to nitrite (NO₂^--N), has been regarded as a promising alternative of NO₂^--N supplying for anammox bacteria. Given the engineering practices, the steadily high NO₂^--N production, alleviating organic inhibition, and reducing greenhouse gas of PD process offers a viable and efficient approach for anammox implementation. Moreover, it allows for the extending applications of anammox process due to the NO₃^--N removal availability. Here we comprehensively review the important new outcomes and discuss the emerging applications of PD-based anammox including the process development, mechanism understanding, and future trends. Significant greater stability and enhanced nitrogen removal efficiency have been demonstrated in the novel integrations of PD and anammox process, indicating a broad perspective in dealing with the mainstream municipal sewage, ammonia-rich streams, and industrial NO₃^--N contained wastewater. Furthermore, researches are still needed for the predictable and controllable strategies, along with the detailed microbiological information in future study. Overall, the achievement of PD process provides unique opportunity catalyzing the engineering applications of energy-efficient and environmental-friendly wastewater treatment via anammox technology.

Start-up and operational performance of Anammox process in an anaerobic baffled biofilm reactor (ABBR) at a moderate temperature

A lab-scale anaerobic baffled biofilm reactor (ABBR) was used as a novel reactor to start up Anammox process at a moderate temperature around 20 °C and an innovative filling module was adopted as support material. Quick start-up of Anammox process from the aerobic activated sludge was achieved after 47 days operation. The max nitrogen loading rate and nitrogen removing rate attained 1.00 kg N m^-3 d^-1 and 0.90 kg N m^-3 d^-1 after 161 days operation. Scanning electron microscope photographs showed that the structure as well as the states of the micro-aggregates (micro-aggregates sticking on a non-woven fiber, entangling non-woven fibers and enwrapped by non-woven fibers) enhanced biomass retention for Anammox bacteria. Microbial community analysis showed that Anammox bacteria were effectively enriched with Candidatus Brocadia, Candidatus Jettenia and Candidatus Kuenenia being the main Anammox species in the mature biofilms. This contributed to the excellent Anammox operation performance at the moderate temperature.

Combined Partial Denitrification (PD)-Anammox: A method for high nitrate wastewater treatment

Elimination of nitrogen pollution from wastewater containing high-strength nitrate (NO₃^--N) is a significant issue to prevent deterioration of water quality and eutrophication of receiving water body. Traditional denitrification process faces several challenges including the huge organic carbon demand, intermediate products accumulation, and long acclimatization period. In this study, an efficient solution was given by a novel two-stage Partial Denitrification (PD)-Anammox process. High NO₃^--N (1000 mg N/L) wastewater and municipal sewage (COD: 182.5 mg/L, ammonia (NH₄⁺-N): 58.3 mg/L) were simultaneously introduced to the PD reactor for NO₃^--N converting to NO₂^--N. The NH₄⁺-N and NO₂^--N in effluent of PD were removed in subsequent anammox reactor. Results showed that a satisfactory nitrogen removal was achieved by optimizing the volume ratios of influent NO₃^--N and municipal sewage, as well as the external organic matter dosage. The NO₃^--N removal efficiency reached up to 95.8% without accommodation period, along with the NH₄⁺-N removal achieving 92.8%. Anammox contributed to 78.9% of TN removal despite the high COD (76.5-98.6 mg/L) in PD effluent was introduced, indicating the significant stability of the integrated process. The microbial analysis suggested that the Candidatus Brocadia, identified as anammox bacteria, cooperated stable with denitrifying bacteria in 215-day operation. The PD-Anammox process offers an economically and technically attractive approach in the high NO₃^--N wastewater treatment since it has great advantages of much low carbon demand, minimal sludge production, enabling simultaneous treatment of municipal sewage, and avoiding the common issues in traditional denitrification process.

Simultaneous removal of ammonia and nitrate by coupled S0-driven autotrophic denitrification and Anammox process in fluorine-containing semiconductor wastewater

To achieve the simultaneous removal of NH₄⁺-N and NO₃^--N in F^--containing semiconductor wastewater by coupled S⁰-driven autotrophic denitrification and Anammox process, the effect of variable F^- concentration on the Anammox process was investigated by batch experiments. The denitrifying ammonium oxidation (Deamox) reactor was then started-up to explore the feasibility of the coupling of Anammox and sulfur autotrophic denitrification (SADN) for the treatment of semiconductor wastewater. Short-term variation of F^- concentration has an obviously effect on the activity of Anammox sludge, but didn't affect the nitrogen conversion rate. The activity of Anammox obviously decreased after long-term operation of the Deamox reactor when influent F^- concentrations reached 552 mg/L. The sensitivity of Anammox bacteria to F^- concentration is stronger than that of SADN bacteria. Total nitrogen removal efficiency of 98% and total nitrogen removal rate of 4.11 kg/(m³·d) were achieved in the Deamox reactor, when the F^- was pre-treated by calcium ions. Moreover, the high-throughput 16S rRNA gene sequence analysis indicated that variation in F^- concentrations could influence the structure and functional of microbial communities in the Deamox process. Candidatus Kuenenia, Thiobacillus and Sulfurimonas were main functional bacteria that achieved symbiotic.

Electroactive microorganisms enriched from activated sludge remove nitrogen in bioelectrochemical reactor

The bioelectrochemical anaerobic nitrogen removal was demonstrated in an anaerobic batch reactor equipped with a pair of polarized bioelectrodes. The bioelectrochemical reactor was operated in sequential batch mode after inoculating activated sludge and polarizing the electrode to 0.6 V. The medium contains ammonium, nitrite, alkalinity and trace minerals, but no organic carbon source. By the repetitive sequential operation, simultaneous removals of ammonium, nitrite and alkalinity were improved, and the electrochemical activity of the bulk sludge was confirmed from the redox peaks of the cyclic voltammogram. This indicates that ammonia oxidizing exoelectrogens (AOE) and denitritating electrotrophs (DNE) were enriched more in the bulk solution. Biogas production that mainly consisted of nitrogen was observed from the bioelectrochemical reactor, and the minor components in the biogas were methane and carbon dioxide. This demonstrates that AOE use nitrite as an electron acceptor to oxidize ammonia. The requirements of nitrite and alkalinity for the removal of ammonia nitrogen are around 0.72 mg NO₂-N/mg NH₄-N and 1.73 mg as CaCO₃/mg NH₄-N, respectively, and nitrate was not produced as a by-product. The bacterial groups involved in the bioelectrochemical nitrogen removal are electroactive autotrophs and can be enriched from activated sludge by polarized electrode. This bioelectrochemical ammonia oxidation is a novel approach recommended for treatment of nitrogen-rich wastewater.

Efficient Nitrogen Removal of Reject Water Generated from Anaerobic Digester Treating Sewage Sludge and Livestock Manure by Combining Anammox and Autotrophic Sulfur Denitrification Processes

The reject water from anaerobic digestion with high (Total Nitrogen) TN concentration was treated by a demonstration plant combining the anammox process and SOD (SOD®; Sulfur Oxidation Denitrification) process. The anaerobic digestion was a co-digestion of livestock wastewater, food waste water, and sewage sludge so that the TN concentration and conductivity of the reject water were very high. This anammox plant was the first anammox demonstration plant in South Korea. The maximum TN removal efficiency of 80% was achieved for the anammox reactor under nitrogen loading rate (NLR) of 0.45 kg-N/m3·d. As a result of decreasing the dilution of the reject water, the influent conductivity and NLR values were increased to 7.8 mS/cm and 0.7 kg/m3·d, causing a rapid decrease in the TN removal efficiency. The sludge concentration from the hydro-cyclone overflow was about 40 mg-MLVSS/L in which small sized anammox granules were detected. It was proven that the increase in (Mixed Liquor Volatile Suspended Solids) MLVSS concentration in the anammox reactor was not easy under high influent conductivity and NLR. 97% of NO2−-N+NO3−-N generated from the anammox process could be treated successfully by the SOD reactor. A TN removal efficiency of 35% under poor annamox treatment could increase to 67% by applying the SOD reactor post treatment for the removal of NO3−-N. The dominant anammox bacteria in the anammox reactor was identified as Brocadia fulgida and 9.3% (genus level) of the bacteria out of the total bacteria were anammox bacteria.

Achieving advanced nitrogen removal from low C/N wastewater by combining endogenous partial denitrification with anammox in mainstream treatment

Successful application of mainstream anammox would be favorable for energy- and resource-efficient sewage treatment. This study presents a new strategy to achieve mainstream anammox, which combined with endogenous partial denitrification (EPD) for treating sewage wastewater. In this EPD-Anammox system, nitrite was stably produced by EPD with a nitrate-to-nitrite transformation ratio of 80%. Through adjusting the volume exchange ratio of EPD-reactor after anaerobic reaction, a suitable NO₂^--N/NH₄⁺-N ratio of ∼1.20 for anammox reaction was achieved. Further, results showed a stable, high nitrogen removal efficiency (90%) with an effluent total nitrogen of 5.8 mg N/L under low C/N (∼2.9). Anammox contributed 49.8% of the overall nitrogen removal owing to the steady nitrite supply from EPD. Denitrifying glycogen-accumulating organisms (GAOs, 36.6%) having potential for endogenous denitrification and Candidatus Brocadia (34.6%) were respectively dominated in the EPD-SBR and anammox-UASB and responsible for the high nitrite accumulation and anammox reaction.

Fast start-up of the cold-anammox process with different inoculums at low temperature (13 °C) in innovative reactor

Three innovative reactors (CAMBR) through optimally combining with the Anaerobic Baffled reactor and Membrane bioreactor were applied to start up the cold-anammox process at low temperature (13 °C) through inoculating flocculent nitrification sludge (R1), anaerobic granular sludge (R2) and flocculent denitrification sludge (R3), respectively. Results showed that anammox process was started successfully with over 90% total nitrogen removal rate in R1, R2 and R3 after 75d, 45d, and 90d, respectively. Microbial community revealed that Ca. Brocadia and Ca. Jettenia were the dominant anammox bacteria in R1, R2 and R3, accounting for an abundance of 0.08%, 12.18%; 3.17%, 0 and 0.08%, 0.38%, respectively. Three anammox species, Ca. Brocadia caroliniensis, Ca. Brocadia sinica and Ca. Jettenia asiatica were annotated based on the phylogenetic tree, suggesting the anammox species with larger maximum growth rate contributed to the rapid start-up of the cold-anammox process. This study reinforces the potential application of mainstream anammox.

Achieving efficient nitrogen removal and nutrient recovery from wastewater in a combining simultaneous partial nitrification, anammox and denitrification (SNAD) process with a photobioreactor (PBR) for biomass production and generated dissolved oxygen (DO) recycling

This study presents a new way to achieve energy neutral wastewater treatment based on a combined nitrification, anammox, and denitrification (SNAD) process and photobioreactor (PBR) configuration with external recycling instead of aeration, and without an additional carbon source, using fixed-film-activated sludge technology (IFAS). The SNAD-PBR process achieved total nitrogen (TN) and phosphorus removal efficiencies of 90 and 100%, respectively. In addition, dissolved oxygen (DO) was controlled in the range 0.4-1.2 mg/L by the introduction of an external recycling system. The presence of microalgae to serve as a carbon source in the SNAD reactor enabled the denitrifiers to survive. When the reflux ratio was 1:3, the lower COD/N protected the activity of the anammox bacteria, not suppressed by the heterotrophic denitrifiers. Microbial community analysis by Illumina MiSeq sequencing revealed that the new environment was more suitable for Candidatus Brocadia when a reflux system was introduced.

A feasibility study on biological nitrogen removal (BNR) via integrated thiosulfate-driven denitratation with anammox

To exploit the advantages of less electron donor consumptions in partial-denitrification (denitratation, NO₃^- → NO₂^-) as well as less sludge production in autotrophic denitrification (AD) and anammox, a novel biological nitrogen removal (BNR) process through combined anammox and thiosulfate-driven denitratation was proposed here. In this study, the ratio of S₂O₃^2--S/NO₃^--N and pH are confirmed to be two key factors affecting the thiosulfate-driven denitratation activity and nitrite accumulation. Simultaneous high denitratation activity and substantial nitrite accumulation were observed at initial S₂O₃^2--S/NO₃^--N ratio of 1.5:1 and pH of 8.0. The optimal pH for the anammox reaction is determined to be 8.0. A sequential batch reactor (SBR) and an up-flow anaerobic sludge blanket (UASB) reactor were established to proceed the anammox and the high-rate thiosulfate-driven denitratation, respectively. Under the ambient temperature of 35 °C, the total nitrogen removal efficiency and capacity are 73% and 0.35 kg N/day/m³ in the anammox-SBR. At HRT of 30 min, the NO₃^- removal efficiency could achieve above 90% with the nitrate-to-nitrite transformation ratio of 0.8, implying the great potential to apply the thiosulfate-driven denitratation & anammox system for BNR with minimal sludge production. Without the occurrence of denitritation (NO₂^- → N₂O → N₂), theoretically no N₂O could be emitted from this BNR system. This study could shed light on how to operate a high rate BNR system targeting to electron donor and energy savings as well as biowastes minimization and greenhouse gas reductions.

Anammox granular sludge in low-ammonium sewage treatment: Not bigger size driving better performance

An integrated investigation to document high anammox abundance, activity and diversity in upflow anaerobic sludge blanket (UASB) reactor treating low-strength ammonium loading sewage was performed and showed that the optimal anammox granular sludge sizes could mitigate undesirable N₂O emission. The enhanced anammox bacterial abundance, activity and specific anammox rate were achieved with optimal granules sludge sizes of 0.5-0.9 mm with multiple "Jettenia", "Brocadia", and "Anammoxoglobus" species. The tightly-bound extracellular polymeric substance (TB-EPS) was the main EPS layer found in anammox granular sludge, in which polysaccharides play an important structural role. Over this granular sludge sizes, the anammox bacterial abundance and activity did not significantly decrease, but N₂O emission significantly increased. High throughput sequencing and ecological networks demonstrated the patterns of anammox and their co-occurring bacteria, with availability N₂O-producer and N₂O-reducer functional genes. Incomplete denitrification and insufficient carbon source mainly contributed to N₂O production in granular sludge, as supported by results of stratification analysis.

Salinity-Aided Selection of Progressive Onset Denitrifiers as a Means of Providing Nitrite for Anammox

Anaerobic ammonium oxidation (anammox) combined with partial-denitrification (NO₃^- → NO₂^-) is an innovative process for the simultaneous removal of ammonia and nitrate from wastewaters. An efficient method for the selection of partial denitrifying community, which relies on increasing influent salinity, is described. Using this method, a denitratating community was enriched, which showed a nitrite accumulation efficiency higher than 75% as well as a high nitrate conversion efficiency. Community analysis using 16S rDNA indicated that Halomonas became the dominant genus as salinity increased. Metagenomic analysis revealed that there was not a significant difference in reads mapping to downstream denitrification genes in a comparison of samples from cultures with 5% salinity to those without salinity. The majority of the reads mapping to the genes encoding dissimilatory nitrate and nitrite reductases nar and nirS came from Halomonas under high salinity conditions. Two metagenome-assembled genomes taxonomically assigned to Halomonas were obtained, one of which accounted for ∼35% of the reads under high salinity conditions. Both genomes harbored the genes for the complete denitrification pathway. These results indicate progressive onset denitrifiers, a phenotype where nitrite reduction only occurs after nitrate exhaustion, could be successfully enriched with increasing salinity. Progressive onset denitrifiers may be more widespread in natural and artificial habitats than anticipated and are shown here to be valuable for nitrogen mitigating processes.

The role of COD/N ratio on the start-up performance and microbial mechanism of an upflow microaerobic reactor treating piggery wastewater

This study investigated the role of COD/N ratio on the start-up and performance of an upflow microaerobic sludge reactor (UMSR) treating piggery wastewater at 0.5 mgO₂/L. At high COD/N ratio (6.24 and 4.52), results showed that the competition for oxygen between ammonia-oxidizing bacteria, nitrite-oxidizing bacteria and heterotrophic bacteria limited the removal of nitrogen. Nitrogen removal efficiency was below 40% in both scenarios. Decreasing the influent COD/N ratio to 0.88 allowed achieving high removal efficiencies for COD (∼75%) and nitrogen (∼85%) due to the lower oxygen consumption for COD mineralization. Molecular biology techniques showed that nitrogen conversion at a COD/N ratio 0.88 was dominated by the anammox pathway and that Candidatus Brocadia sp. was the most important anammox bacteria in the reactor with a relative abundance of 58.5% among the anammox bacteria. Molecular techniques also showed that Nitrosomonas spp. was the major ammonia-oxidiser bacteria (relative abundance of 86.3%) and that denitrification via NO₃^- and NO₂^- also contributed to remove nitrogen from the system.

Heterotrophic Ammonia and Nitrate Bio-removal Over Nitrite (Hanbon): Performance and microflora

A novel Heterotrophic Ammonia and Nitrate Bio-removal Over Nitrite (Hanbon) process, combining Short Nitrate Reduction (SNR) with Anaerobic Ammonia Oxidation (Anammox), was developed in a lab-scale continuous up-flow reactor. The substrate effects were investigated to characterize the performance of Hanbon process, and the corresponding microflora information was also revealed. Our results showed that the optimal substrate ratio of NH₄⁺-N:NO₃^--N:COD for the Hanbon process was 0.65:1:2.2. The volumetric nitrogen removal rate was up to 9.0 ± 0.1 kgN·m^-3·d^-1 at high influent substrate concentrations of NH₄⁺-N 375 mg L^-1, NO₃^--N 750 mg L^-1 and COD 1875 mg L^-1, which was superior to the reported values of analogous processes. Moreover, the effluent total nitrogen concentration was able to meet the strict discharge standard (less than 10 mg L^-1) at low influent substrate concentration of NH₄⁺-N 26 mg L^-1, NO₃^--N 40 mg·L^-1and COD 88 mg L^-1. Illumina-based 16S rRNA gene sequencing results showed that Halomonas campisalis and Candidatus Kuenenia stuttgartiensis were the dominant bacteria in the SNR section and Anammox section at high substrate concentration condition. However, Halomonas campaniensis and Candidatus Brocadia brasiliensis were raised significantly at low substrate concentration condition. Hanbon process provided in the present work was flexible of treating wastewater with various nitrogen concentrations, deserving further development.

Weak magnetic field: A powerful strategy to enhance partial nitrification

Partial nitrification (PN) combined with anaerobic ammonium oxidation process has been recognized as a promising technology for the removal of nitrogenous contaminants from wastewater. This research aimed to investigate the potential of external magnetic field for enhancing the PN process in short and long term laboratory-scale experiments. Different strength magnetic fields (0, 5, 10, 15, 20 and 25 mT) were evaluated in short-term batch tests and 5 mT magnetic field was found to have better ability to increase the activities of aerobic ammonium oxidizing bacteria (AOB) of PN consortium. Long-term effect of magnetic field on PN consortium was studied with 5 mT magnetic field. The results demonstrated that the positive effect of magnetic field on PN process could also be testified at all of the four stages. Furthermore, a decrease of bacterial diversity was noted with the increase of magnetic field strength. Relative abundance of Nitrosomonadaceae decreased significantly (p < 0.01) from 13.9% in R_CK to 12.9% in R_5mT and 5.5% in R_25mT. Functional genes forecast based on KEGG database indicated that the expressions of functional genes related to signal transduction and cell motility in 5 mT environment were higher expressed compared with no magnetic field addition and high magnetic field addition. The existence of 5 mT magnetic field didn't increase the abundance of AOB but increased the activity of AOB by increasing the rate of free ammonia into the interior of microbial cells. Addition of magnetic field couldn't change the final state of PN process according to the hypothesis proposed in this article. These findings indicated that the weak magnetic field was useful and reliable for the fast start-up of PN process since it was proved as a simple and convenient approach to enhance AOB activity.

Achieving Mainstream Nitrogen Removal through Coupling Anammox with Denitratation

Achieving maintream anammox is critical for energy-neutral sewage treatment. This study presents a new way to achieve mainstream anammox, which couples anammox with denitratation (nitrate reduction to nitrite) instead of nitritation (ammonium oxidation to nitrite). An anoxic/oxic (A/O) biofilm system treating systhetic domestic wastewater was used to demonstrate this concept for over 400 days. This A/O biofilm system achieved a total nitrogen (TN) removal efficiency of 80 ± 4% from the influent with a low C/N ratio of 2.6 and a TN concentration of 60.5 mg/L. Nitrogen removal via anammox was found to account for 70% of dinitrogen production in the anoxic reactor. Batch tests confirmed that the anoxic biofilm could oxidize ammonium using nitrite as electron acceptor, and that it had a higher nitrate reduction rate than the nitrite reduction rate, thus producing nitrite for the anammox reaction. Metagenomic analysis showed that Candidatus Jettenia caeni and Candidatus Kuenenia stuttgartiensis were the top two dominant species in anoxic biofilm. Genes involved in the metabolism of the anammox process were detected in anoxic biofilm. The abundance of nitrate reductase (73360 hits) was much higher than nitrite reductase (13114 hits) in anoxic biofilm. This system can be easily integrated with the high-rate activated sludge technology, which produces an effluent with a low C/N ratio. While this new design consumes 21% more oxygen in comparison to the currently studied nitritation/anammox process, the nitrite-producing process appears to be more stable.

Efficient simultaneous partial nitrification, anammox and denitrification (SNAD) system equipped with a real-time dissolved oxygen (DO) intelligent control system and microbial community shifts of different substrate concentrations

Simultaneous partial nitrification, anammox and denitrification (SNAD) process was studied in a sequencing batch biofilm reactor (SBBR) fed with synthetic wastewater in a range of 2200 mgN/L ∼ 50 mgN/L. Important was an external real-time precision dissolved oxygen (DO) intelligent control system that consisted of feed forward control system and feedback control system. This DO control system permitted close control of oxygen supply according to influent concentration, effluent quality and other environmental factors in the reactor. In this study the operation was divided into six phases according to influent nitrogen applied. SNAD system was successfully set up after adding COD into a CANON system. And the presence of COD enabled the survival of denitrifiers, and made Thauera and Pseudomonas predominant as functional denitrifiers in this system. Denaturing gradient gel electrophoresis (DGGE), fluorescence in situ hybridization (FISH) and 16S rRNA amplicon pyrosequencing were used to analyze the microbial variations of different substrate concentrations. Results indicated that the relative population of ammonia oxidizing bacteria (AOB) members decreased when influent ammonia concentration decreased from 2200 mg/L to 50 mg/L, while no dramatic drop of the percent of anammox bacteria was seen. And Nitrosomonas europaea was the predominant AOB in SNAD system treating sewage, while Candidatus Brocadia was the dominant anammox bacteria.