New direction in biological nitrogen removal from industrial nitrate wastewater via anammox

Stable continuous flow CANDAN process transitioning from anammox UASB reactor by facilitating indigenous nitrite-producing denitrification community

The emerging nitrogen removal process known as CANDAN (Complete Ammonium and Nitrate removal via Denitratation-Anammox over Nitrite) has been developed in Sequencing Batch Reactors (SBRs). Yet, starting up and maintaining stability in continuous-flow reactors remain challenging. This study explores the feasibility of transitioning the CANDAN process from an anammox-dominated process by introducing appropriate external organics to facilitate indigenous nitrite-producing denitrification community in an Upflow Anaerobic Sludge Blanket (UASB) reactor. 150-day operation results indicate that under feeding rates of domestic wastewater at 0.54 L/h and nitrate-containing wastewater at 1.08 L/h, excellent N removal was achieved, with effluent TN below 10.0 mg N/L. Adding external sodium acetate at a COD/NO3--N = 2.0 triggered denitratation, ex-situ denitrification activity tests showed increased nitrite production rates, maintaining the nitrate-to-nitrite transformation ratio (NTR) above 90 %. Consequently, anammox activity was consistently maintained, dominating Total Nitrogen (TN) removal with a contribution as high as 78.3 ± 8.0 %. Anammox functional bacteria, Brocadia and Kuenenia were identified and showed no decrease throughout the operation, indicating the robustness of the anammox process. Notably, the troublesome of sludge flotation, did not occur, also contributing to sustained outstanding performance. In conclusion, this study advances our understanding of the synergistic interplay between anammox and denitrifying bacteria in the Anammox-UASB system, offering technical insights for establishing a stable continuous-flow CANDAN process for simultaneous ammonium and nitrate removal.

Reactivation performance and sludge transformation after long-term storage of Partial denitrification/Anammox (PD/A) process

This study explores the startup of innovative Partial denitrification/Anammox (PD/A) process using long-term stored sludge (>2 years at 4 °C). Results indicate a swift recovery performance, characterized by a progressive increase in the activity of functional microorganisms with improved nitrogen volumetric loading rate during operation. Stable nitrogen removal efficiency of 99.6 % was attained at 14.2 °C under influent nitrate and ammonium of 120 and 100 mg/L, respectively. A distinctive transformation was observed as the initially black seeding sludge transitioned to brownish-red, accompanied by rapid sludge granulation with size notably increased from 263.1 μm (day 4) to 1255.0 μm (day 128), significantly contributing to the rapid PD/A performance recovery. Microbial community analysis revealed substantial increases in functional bacteria, Thauera (0.09 %-10.4 %) and Candidatus Brocadia (0.003 %-1.98 %), coinciding with enhanced nitrogen removal performance. Overall, this study underscores the viability of long-term stored PD/A sludge as a seed for rapid reactor startup, offering useful technical support to advance practical PD/A process implementation.

Nitrogen Loss and Migration in Rice Fields under Different Water and Fertilizer Modes

Irrigating aquaculture wastewater in appropriate irrigation and drainage modes in paddy fields could reduce water and fertilizer loss. However, the precise mechanisms involved in the degradation and movement of nitrogen in various water and fertilizer modes are still not fully understood. This study involves conducting a controlled experiment using barrels to examine the effects of various water quality, irrigation and drainage methods, and fertilization levels. The aim is to analyze the patterns of nitrogen degradation, loss, migration, and absorption in surface water, underground drainage, and soil leakage at different depths. The results showed the following: (1) The paddy field has a significant purification effect on aquaculture wastewater after one day of irrigation, reached at 78.55-96.06%. (2) Aquaculture wastewater irrigation increased nitrogen concentration in the plough layer, which helps rice roots absorb nitrogen and boosts plant TN. (3) In special dry years, underground seepage is the predominant method of nitrogen loss, and underground drainage nitrogen concentration peaks 2-6 days after fertilization. (4) Under aquaculture wastewater irrigation, the TN loss load of II decreased by 27.65-42.45% than FSI. Compared with IA-80, the TN degradation rate of IA in surface water increased by 18.51%, TN loss load decreased by 5.48%, TN absorption rate significantly increased by 14.61%, and yield increased by 31.14% significantly. IA is recommended in special dry years, which can improve the TN absorption rate and ensure high yield while significantly reducing the loss load of nitrogen. The findings can provide a basis for the purification of aquaculture wastewater through paddy field ecosystems in response to fertilizer supply levels.

Optimizing waste molasses utilization to enhance electron transfer via micromagnetic carriers: Mechanisms and high-nitrate wastewater denitrification performance

The biological denitrification of high-nitrate wastewater (HNW) is primarily hindered by insufficient carbon sources and excessive nitrite accumulation. In this study, micromagnetic carriers with varying micromagnetic field (MMF) strengths (0.0, 0.3, 0.6, 0.9 mT) were employed to enhance the denitrification of HNW using waste molasses (WMs) as a carbon source. The results revealed that 0.6 mT MMF significantly improved the total nitrogen removal (TN) efficiency at 96.3%. A high nitrate (NO3--N) removal efficiency at 99.3% with a low nitrite (NO2--N) accumulation at 25.5 mg/L was achieved at 0.6 mT MMF. The application of MMF facilitated the synthesis of adenosine triphosphate (ATP) and stimulated denitrifying enzymes (e.g., nitrate reductase (NAR), nitrite reductase (NIR), and nitric oxide reductase (NOR)), which thereby promoting denitrification. Moreover, the effluent chemical oxygen demand (COD), tryptophan and fulvic-like substances exhibited their lowest levels at 0.6 mT MMF. Analysis through 16S ribosomal ribonucleic acid gene sequencing indicated a significant enrichment of denitrifying bacteria including Castellaniella Klebsiella under the influence of MMF. Besides, the proliferation of Acholeplasma, Klebsiella and Proteiniphilum at 0.6 mT MMF promoted the hydrolysis and acidification of WMs. This study offers new insights into the enhanced utilization of WMs and the denitrification of HNW through the application of MMF.

Isolation of a marine-derived yeast with potential applications in industrial nitrite utilizing

The nitrite efficient utilization microorganism Wickerhamomyces anomalus RZWP01 was identified. Using nitrite and ammonium as the sole nitrogen source, the nitrogen removal rate of W. anomalus RZWP01 was 97.4% and 87.1%, respectively. W. anomalus RZWP01 grew well in the nitrite medium with glucose or xylose as the only carbon source. However, the W. anomalus RZWP01 cannot live on the nitrite medium with lactose, citric acid, and methanol as the only carbon source. The maximal cell concentration occurred in the nitrite medium with glucose as the only carbon source at a C/N ratio of 20 for 48 h, reaching 8.92 × 108 cell mL-1. W. anomalus RZWP01 was the first reported yeast that can efficiently utilize nitrite. The isolation and identification of W. anomalus RZWP01 enriched the microbial resources of nitrite-degrading microorganisms and provided functional microorganisms for the water treatment of sustainable aquaculture.

Hydroxyapatite (HAP) formation in acetate-driven partial denitrification process: Enhancing sludge granulation and phosphorus removal

Partial denitrification/anammox (PD/A) processes have emerged as a promising technology for efficient nitrogen removal from wastewater. However, these processes fail to remove phosphorus (P), a key pollutant that contributes to water eutrophication. To address this issue, the potential of inducing hydroxyapatite (HAP) precipitation in PD processes to achieve simultaneous P removal was investigated for the first time. Specifically, three SBRs (R1-R3) for PD were operated with adding varying concentrations of external Ca (30, 60, and 120 mg/L, respectively). Results demonstrated significant P reduction in all three SBRs, particularly in R3 with high Ca, which achieved an 80 % removal efficiency. Notably, sludge granulation was observed during operation, with the granule size in R3 with high Ca reaching 906.1 μm during the stable period, exceeding those in R2 (788.7 μm) and R1 (707.1 μm). This led to good settle ability of the PD sludge, as demonstrated by the lowest SVI5 (20 mL/g MLSS). Moreover, the decrease in the MLVSS/MLSS ratio suggested that the inorganic content accumulated, as observed by confocal laser scanning microscopy in the interior of the granules. Elemental composition analysis suggested that PD granules contained high P and Ca, while the X-ray diffraction (XRD) results confirmed the formation of HAP. Overall, this study demonstrated that PD-HAP coupled granular sludge process has potential as a robust and efficient method for nitrite production, as well as effective P removal and recovery, thereby advancing the application of anammox processes in wastewater treatment.

Adaptation of anammox process for nitrogen removal from acidic nitritation effluent in a low pH moving bed biofilm reactor

Acidic partial nitritation (PN) has emerged to be a promisingly stable process in wastewater treatment, which can simultaneously achieve nitrite accumulation and about half of ammonium reduction. However, directly applying anaerobic ammonium oxidation (anammox) process to treat the acidic PN effluent (pH 4-5) is susceptible to the inhibition of anammox bacteria. Here, this study demonstrated the adaptation of anammox process to acidic pH in a moving bed biofilm reactor (MBBR). By feeding the laboratory-scale MBBR with acidic PN effluent (pH = 4.6 ± 0.2), the pH of an anammox reactor was self-sustained in the range of pH 5 - 6. Yet, a high total nitrogen removal efficiency of over 80% at a practical loading rate of up to 149.7 ± 3.9 mg N/L/d was achieved. Comprehensive microbial assessment, including amplicon sequencing, metagenomics, cryosection-FISH, and qPCR, identified that Candidatus Brocadia, close to known neutrophilic members, was the dominant anammox bacteria. Anammox bacteria were found present in the inner layer of thick biofilms but barely present in the surface layer of thick biofilms and in thin biofilms. Results from batch tests also showed that the activity of anammox biofilms could be maintained when subjected to pH 5 at a nitrite concentration of 10 mg N/L, whereas the activity was completely inhibited after disturbing the biofilm structure. These results collectively indicate that the anammox bacteria enriched in the present acidic MBBR could not be inherently acid-tolerant. Instead, the achieved stable anammox performance under the acidic condition is likely due to biofilm stratification and protection. This result highlights the biofilm configuration as a useful solution to address nitrogen removal from acidic PN effluent, and also suggests that biofilm may play a critical role in protecting anammox bacteria found in many acidic nature environments.

Sustainable nitrogen removal in anammox-mediated systems: Microbial metabolic pathways, operational conditions and mathematical modelling

Anammox-mediated systems have attracted considerable attention as alternative cost-effective technologies for sustainable nitrogen (N) removal from wastewater. This review comprehensively highlights the importance of understanding microbial metabolism in anammox-mediated systems under crucial operation parameters, indicating the potentially wide applications for the sustainable treatment of N-containing wastewater. The partial nitrification-anammox (PN-A), simultaneous PN-A and denitrification (SNAD) processes have demonstrated sustainable N removal from sidestream wastewater. The partial denitrification-anammox (PD-A) and denitrifying anaerobic methane oxidation-anammox (DAMO-A) processes have advanced sustainable N removal efficiency in mainstream wastewater treatment. Moreover, N2O production/emission hotspots are extensively discussed in anammox-based processes and are related to the dominant ammonia-oxidizing bacteria (AOB) and denitrifying heterotrophs. In contrast, N2O is not produced in the metabolism pathways of AnAOB and DAMO-archaea; Moreover, the actual contribution of N2O production by dissimilatory nitrate reduction to ammonium (DNRA) and DAMO-bacteria in their species remains uncertain. Thus, PD-A and DAMO-A processes would achieve reduction in greenhouse gas production, as well as energy consumption for the reliability of N removal efficiencies. In addition to reaction mechanisms, this review covers the mathematical models for simultaneous anammox, partial nitrification and/or denitrification (i.e., PN-A, PD-A, and SNAD). Promising NO3- reduction technologies by endogenous PD, sulfur-driven autotrophic denitrification, and DNRA by anammox are also discussed. In summary, this review provides a better understanding of sustainable N removal in anammox-mediated systems, thereby encouraging future investigation and exploration of the sustainable N bio-treatment from wastewater.

Potential causes of partial-denitrification (PD) granular sludge breakdown under high nitrate loading rates

The granule instability has been frequently reported during the operation of high loading rates. While, there no research was performed on the recently developed anoxic partial-denitrification (PD) granules, a novel pathway in producing nitrite from nitrate for anammox process. Herein, this work, for the first time, investigated the influence of nitrate loading rates on the instability of PD granules and identified the key causes. Two lab-scale sequencing batch reactors (SBRs) were operated with nitrate loading rates (NLR) increased from 0.48 to 3.84 kg N/m³/d (R1, 8 cycles/d), and 0.96 to 7.68 kg N/m³/d (R2, 16 cycles/d) by gradually elevating the influent nitrate concentration. Results showed that nitrite production rates increased with the NLRs, with a maximal value of 5.26 kg N/m³/d obtained. However, the compact regular PD granules were not stable and broke down when NLR was above 3.84 kg N/m³/d, which resulted in serious sludge washing out from SBR. The high NLRs led to the extracellular polymeric substances (EPS) transformation in terms of its composition and structure, which the protein content in the EPS and the tightly bound EPS (T-EPS) fraction was significantly decreased, this was supposed to be the major reason causing the breakdown of PD granules. Besides, it was found the PD granule in R2 was more deteriorated than that in R1 under the same high NLR, suggesting the short starvation (idle) times in SBR cycle was likely another reason impairing the stability of PD granules. Overall, this research provides useful information in development of granule-based PD systems and sheds light on achieving high-rate nitrite production in SBR with great stability.

Integrating conventional nitrogen removal with anammox in wastewater treatment systems: Microbial metabolism, sustainability and challenges

The various forms of nitrogen (N), including ammonium (NH₄⁺), nitrite (NO₂^-), and nitrate (NO₃^-), present in wastewaters can create critical biotic stress and can lead to hazardous phenomena that cause imbalances in biological diversity. Thus, biological nitrogen removal (BNR) from wastewaters is considered to be imperatively urgent. Therefore, anammox-based systems, i.e. partial nitrification and anaerobic ammonium oxidation (PN/anammox) and partial denitrification and anammox (PD/anammox) have been universally acknowledged to consider as alternatives, promising and cost-effective technologies for sustainable N removal from wastewaters compared to nitrification-denitrification processes. This review comprehensively presents and discusses the latest advances in BNR technologies, including traditional nitrification-denitrification and anammox-based systems. To a deep understanding of a better-controlled combining anammox with traditional processes, the microbial community diversity and metabolism, as well as, biomass morphological characteristics were clearly reviewed in the anammox-based systems. Explaining simultaneous microbial competition and control of crucial operation parameters in single-stage anammox-based processes in terms of optimization and economic benefits makes this contribution a different vision from available review papers. The most important sustainability indicators, including global warming potential (GWP), carbon footprint (CF) and energy behaviours were explored to evaluate the sustainability of BNR processes in wastewater treatment. Additionally, the challenges and solutions for BNR processes are extensively discussed. In summary, this review helps facilitate a critical understanding of N removal technologies. It is confirmed that sustainability and saving energy would be achieved by anammox-based systems, thereby could be encouraged future outcomes for a sustainable N removal economy.

Biological nitrogen removal from low carbon wastewater

Nitrogen has traditionally been removed from wastewater by nitrification and denitrification processes, in which organic carbon has been used as an electron donor during denitrification. However, some wastewaters contain low concentrations of organic carbon, which may require external organic carbon supply, increasing treatment costs. As a result, processes such as partial nitrification/anammox (anaerobic ammonium oxidation) (PN/A), autotrophic denitrification, nitritation-denitritation and bioelectrochemical processes have been studied as possible alternatives, and are thus evaluated in this study based on process kinetics, applicability at large-scale and process configuration. Oxygen demand for nitritation-denitritation and PN/A is 25% and 60% lower than for nitrification/denitrification, respectively. In addition, PN/A process does not require organic carbon supply, while its supply for nitritation-denitritation is 40% less than for nitrification/denitrification. Both PN/A and nitritation-denitritation produce less sludge compared to nitrification/denitrification, which saves on sludge handling costs. Similarly, autotrophic denitrification generates less sludge compared to heterotrophic denitrification and could save on sludge handling costs. However, autotrophic denitrification driven by metallic ions, elemental sulfur (S) and its compounds could generate harmful chemicals. On the other hand, hydrogenotrophic denitrification can remove nitrogen completely without generation of harmful chemicals, but requires specialized equipment for generation and handling of hydrogen gas (H2), which complicates process configuration. Bioelectrochemical processes are limited by low kinetics and complicated process configuration. In sum, anammox-mediated processes represent the best alternative to nitrification/denitrification for nitrogen removal in low- and high-strength wastewaters.

Beyond an Applicable Rate in Low-Strength Wastewater Treatment by Anammox: Motivated Labor at an Extremely Short Hydraulic Retention Time

The application of anammox technology in low-strength wastewater treatment is still challenging due to unstable nitrite (NO₂^--N) generation. Partial denitrification (PD) of nitrate (NO₃^--N) reduction ending with NO₂^--N provides a promising solution. However, little is known about the feasibility of accelerating nitrogen removal toward the practical application of anammox combined with heterotrophic denitrification. In this work, an ultrafast, highly stable, and impressive nitrogen removal performance was demonstrated in the PD coupling with an anammox (PD/A) system. With a low-strength influent [50 mg/L each of ammonia (NH₄⁺-N) and NO₃^--N] at a low chemical oxygen demand/NO₃^--N ratio of 2.2, the hydraulic retention time could be shortened from 16.0 to 1.0 h. Remarkable nitrogen removal rates of 1.28 kg N/(m³ d) and excellent total nitrogen removal efficiency of 94.1% were achieved, far exceeding the applicable capacity for mainstream treatment. Stimulated enzymatic reaction activity of anammox was obtained due to the fast NO₂^--N jump followed by a famine condition with limited organic carbon utilization. This high-rate PD/A system exhibited efficient renewal of bacteria with a short sludge retention time. The 16S rRNA sequencing unraveled the rapid growth of the genus Thauera, possibly responsible for the incomplete reduction of NO₃^--N to NO₂^--N and a decreasing abundance of anammox bacteria. This provides new insights into the practical application of the PD/A process in the energy-efficient treatment of low-strength wastewater with less land occupancy and desirable effluent quality.

Achieving partial denitrification using organic matter in brewery wastewater as carbon source

To find a cost-effective carbon source for partial denitrification (PD), brewery wastewater was utilized to test the viability of initiating PD. The S_bre (sludge from the biological treatment tank of Tsingtao Brewery Plant sewage treatment station) and S_lab (sludge from laboratory) were fed with brewery wastewater at COD_Cr/NO₃^--N (C/N) ratios of 8.0-10.0 and 5.0 for 95 days at 25 ± 1 °C, respectively. The mean NO₃^--N to NO₂^--N transformation ratio (NTR) in long-term operation was 40.0% in the S_bre system and 83.2% in the S_lab system. Batch tests with C/N ratio of 2.2-4.4 were  conducted after 95 days incubation and the result suggested that C/N ratio of 4.3 ± 0.1 contributed more to NO₂^--N accumulation in both systems. Thauera bacteria, known to be beneficial for NO₂^--N accumulation, became the dominant community. The relative abundances of Thauera on day 95 in the S_bre and S_lab system were 83.36% and 79.11%, respectively.

Enhanced Nitrogen Removal from Domestic Wastewater by Partial-Denitrification/Anammox in an Anoxic/Oxic Biofilm Reactor

A partial-denitrification coupling with anaerobic ammonium oxidation (anammox) process (PD/A) in a continuous-flow anoxic/oxic (A/O) biofilm reactor was developed to treat carbon-limited domestic wastewater (ammonia (NH4+-N) of 55 mg/L and chemical oxygen demand (COD) of 148 mg/L in average) for about 200 days operation. Satisfactory NH4+-N oxidation efficiency above 95% was achieved with rapid biofilm formation in the aerobic zone. Notably, nitrite (NO2−-N) accumulation was observed in the anoxic zone, mainly due to the insufficient electron donor for complete nitrate (NO3−-N) reduction. The nitrate-to-nitrite transformation ratio (NTR) achieved was as high as 64.4%. After the inoculation of anammox-enriched sludge to anoxic zones, total nitrogen (TN) removal was significantly improved from 37.3% to 78.0%. Anammox bacteria were effectively retained in anoxic biofilm utilizing NO2−-N produced via the PD approach and NH4+-N in domestic wastewater, with the relative abundance of 5.83% for stable operation. Anammox pathway contributed to TN removal by a high level of 38%. Overall, this study provided a promising method for mainstream nitrogen removal with low energy consumption and organic carbon demand.

A novel simultaneous partial nitritation, denitratation and anammox (SPNDA) process in sequencing batch reactor for advanced nitrogen removal from ammonium and nitrate wastewater

This study presented a novel simultaneous partial nitritation (PN), denitratation and anammox (SPNDA) process for treating ammonium and nitrate wastewater. Results indicated that SPNDA could achieve a great total nitrogen (TN) removal of 97.6 ± 0.5%, leading to effluent TN concentration of only 3.4 mg/L. Mass balance indicated that nitrogen removal rates via anammox, simultaneous nitrification and denitrification were 96.7% and 3.3%, respectively. Extended aerobic duration (12 h) and low dissolved oxygen (DO) concentration (0.15 mg/L) could improve ammonia-oxidizing bacteria (AOB) activity and maintain PN stability. The stable suppression of nitrite-oxidizing bacteria activity was attributed to the low DO (0.15 mg/L) and high free ammonia (3.63 mg/L) in SPND. Besides, the nitrogen conversion mechanisms for SPNDA were revealed based on a typical operational cycle. Microbial analysis showed that AOB (Nitrosomonas) and partial denitrifying bacteria (Thauera and Denitratisoma) coexisted with anammox bacteria (Candidatus Brocadia and Candidatus Anammoxoglobus) in the mixotrophic bio-community.

Nitrogen Removal by an Anaerobic Iron-Dependent Ammonium Oxidation (Feammox) Enrichment: Potential for Wastewater Treatment

Nitrogen pollution in water is a growing concern. Anthropogenic activities have increased the amount of nitrogen released into watercourses, which harms human health and the environment, and causes serious problems, such as eutrophication. Feammox is a recently discovered biological pathway associated with the nitrogen cycle that has gained scientific interest. This process couples anaerobic ammonium oxidation with iron reduction. This work presents a study on the Feammox mechanism from the enrichment of an activated sludge obtained from a sewage treatment plant. The enrichment was carried out at neutral pH to study the N2 pathway, that is, the Feammox process with the oxidation of ammonium (NH4+) directly to N2. In addition, different sources of iron were studied: iron chloride (FeCl3); ferrihydrite; and goethite. The characterization of the sludge showed the genes associated with ammonia monooxygenase, nitrate and nitrite reductases processes, along with relevant microbial species. The enrichment, carried out for 42 days and monitored every 14 days, showed that FeCl3 as a source of Fe was more effective for the coupled process of oxidation of NH4+ and the reduction of Fe(III) to Fe(II). At the end of the enrichment period, a removal of 31% and 32.2% of NH4+, and an increase in Fe(II) concentration by 52.4 and 63.9 times regarding the initial value were achieved in aerobic and anaerobic sludge, respectively. This study provides information on the potential of Feammox in the removal of N from wastewater, and the oxidation/reduction yields in the initial enrichment phase.

Hot spots and hot moments of nitrogen removal from hyporheic and riparian zones: A review

The Earth is experiencing excessive nitrogen (N) input to its various ecosystems due to human activities. How to effectively and efficiently remove N from ecosystems has been, is and will be at the center of attention in N research. Hyporheic and riparian zones are widely acknowledged for their buffering capacity to reduce contaminants (especially N) transport downstream. However, these zones are usually misunderstood that they can remove N at all spots and at any moments. Here pathways of N removal from hyporheic and riparian zones are reviewed and summarized with an emphasize on their hot spots and hot moments. N is biogeochemically removed by denitrification, anammox, nitrifier denitrification, denitrifying anaerobic methane oxidation, Feammox and Sulfammox. Hot moments of N removal are mainly triggered by precipitation, fire and snowmelt. Finally, some research needs are outlined and discussed, such as developing approaches for multiscale sampling and monitoring, quantifying the effects of hot spots and hot moments at hyporheic and riparian zones and evaluating the impacts of human activities on hot spots and hot moments, to inspire more research on hot spots and hot moments of N removal. By this review, we hope to bring awareness of the heterogeneity of hyporheic and riparian zones to catchment managers and policy makers when tackling N pollution problems.

Development of a denitrification system using primary sludge as solid carbon source - Potential to couple with anammox process

Heterotrophic denitrification is a robust and reliable process for nitrogen removal from wastewater. However, wastewater often faces the issue of lacking carbon source. In this study, the feasibility of using primary sludge, a by-product of wastewater treatment plants, to support denitrification of high-strength nitrite wastewater was investigated. Results suggest the desired performance can be achieved with the influent nitrite concentration of 400 to 1200 mg N/L, and the optimal primary sludge dosage for the complete nitrite removal was 3.6 g VSS/g N. Ammonium removal was also detected along with nitrite removal. Microbial analysis reveals various types of denitrifying bacteria and a large number of macromolecular organics degrading bacteria existed in the microbial community. Notably, anammox bacteria, Candidatus Brocadia, was also identified with an abundance of 0.1%. The slow kinetics of carbon source release from primary sludge was likely the reason for the existence of anammox process. This study developed a promising nitrogen removal process using an alternative carbon source for denitrification, and it shows great potential to couple denitrification with anammox to reduce ammonium residue.

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.

Achieving partial denitrification-anammox in biofilter for advanced wastewater treatment

Recently, partial denitrification (PDN) - anaerobic ammonium oxidation (anammox) process has been widely studied in activated sludge for nitrate wastewater treatment. However, achieving PDN-Anammox in biofilter for domestic wastewater treatment was never reported. In this study, two lab-scale PDN biofilter and Anammox biofilter were built up to treat simulated domestic wastewater. The results showed that stable nitrogen removal performance was kept with averaged effluent nitrogen of 10.2 mg/L. Stable nitrite accumulation performance was achieved with low abundance of nitrite reductase gene, while influent composition influenced nitrogen transformation pathway in PDN biofilter. When treating domestic wastewater, nitrification and partial denitrification led to the higher nitrite accumulation ratio of 75.4%. The percentage contribution of anammox biofilter was 74.6% for nitrogen removal, and Candidatus Brocadia was dominant genus. After long-term operation, limited substrate concentration caused interspecific competition among various anammox bacteria, leading to an increasing proportion of Candidatus Brocadia fulgida. PDN-Anammox biofilter is a feasible process to advanced wastewater treatment, which could save aeration consumption and carbon source addition, and reduce sludge production.

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.