Simultaneous domestic wastewater and nitrate sewage treatment by DEnitrifying AMmonium OXidation (DEAMOX) in sequencing batch reactor

Anammox-based technologies: A review of recent advances, mechanism, and bottlenecks

The removal of nitrogen via the ANAMMOX process is a promising green wastewater treatment technology, with numerous benefits. The incessant studies on the ANAMMOX process over the years due to its long start-up and high operational cost has positively influenced its technological advancement, even though at a rather slow pace. At the moment, relatively new ANAMMOX technologies are being developed with the goal of treating low carbon wastewater at low temperatures, tackling nitrite and nitrate accumulation and methane utilization from digestates while also recovering resources (phosphorus) in a sustainable manner. This review compares and contrasts the handful of ANAMMOX -based processes developed thus far with plausible solutions for addressing their respective bottlenecks hindering full-scale implementation. Ultimately, future prospects for advancing understanding of mechanisms and engineering application of ANAMMOX process are posited. As a whole, technological advances in process design and patents have greatly contributed to better understanding of the ANAMMOX process, which has greatly aided in the optimization and industrialization of the ANAMMOX process. This review is intended to provide researchers with an overview of the present state of research and technological development of the ANAMMOX process, thus serving as a guide for realizing energy autarkic future practical applications.

Achieving rapid start-up and efficient nitrogen removal of partial-denitrification/anammox process using organic matter in brewery wastewater as carbon source

To find a cost-efficient carbon source for the partial denitrification/anaerobic ammonium oxidation (anammox) (PD/A) process, the practicability of using the organic matter contained in brewery wastewater as carbon source was investigated. Quick self-enrichment of denitrifying bacteria was achieved by supplying brewery wastewater as organic carbon source and using the mature anammox sludge as the seeding sludge. The PD/A process was successfully established after 33-day operation and then the average total nitrogen removal efficiency reached 92.29% when the influent CODCr: NO3--N: NH4+-N ratio was around 2.5: 1.0: 0.67. The relative abundance of Thauera increased from 0.03% in the seeding sludge to 54.29% on day 110, whereas Candidatus brocadia decreased from 30.66% to 2.08%. The metagenomic analysis indicated that the sludge on day 110 contained more nar and napA (total of 41.24%) than nirK and nirS (total of 11.93%). Thus NO2--N was accumulated efficiently in the process of denitrification and sufficient NO2--N was supplied for anammox bacteria in the PD/A process. Using brewery wastewater as carbon source not only saved the cost of nitrogen removal but also converted waste into resource and reduced the treatment expense of brewery wastewater.

Sulfamethoxazole exposure shifts partial denitrification to complete denitrification: Reactor performance and microbial community

This study elucidated the influence on a partial denitrification (PD) system under 0-1 mg/L sulfamethoxazole (SMX) stress in a sequencing batch reactor. The results showed that the nitrite accumulation rate (NAR) significantly (P ≤ 0.01) decreased from 68.68 ± 9.00% to 49.05 ± 11.68%, while the total nitrogen removal efficiency significantly (P ≤ 0.001) increased from 23.19 ± 4.42% to 31.36 ± 2.73% in presence of SMX. The results indicated that SMX exposure switched the PD process to complete denitrification through the deterioration of the nitrite accumulation and the promotion of further nitrite reduction. The SMX removal loading rate increased from 0.21 ± 0.04 to 5.03 ± 0.77 mg-SMX/(g-MLVSS·d) with the extended reactor operation under SMX stress. Low SMX concentration exposure increased extracellular polymeric substances (EPS) content from 107.69 ± 20.78 mg/g-MLVSS (0.05 mg-SMX/L) to 123.64 ± 9.66 mg/g-MLVSS (0.5 mg-SMX/L), while EPS secretion was inhibited under high SMX concentration exposure (i.e., 1 mg-SMX/L). Moreover, SMX exposure promoted the synthesis of aromatic protein-like compounds and changed the functional groups as revealed by EEM and FTIR analysis. Additionally, SMX exposure significantly shifted the microbial community structures at both phylum and genus levels. Particularly, the abundance of Thauera, i.e., functional bacteria related to PD, considerably decreased from 41.69% to 11.62% after SMX exposure, whereas the abundances of Denitratisoma and SM1A02 significantly rose under different SMX concentrations. These outcomes hinted that the addition of SMX resulted in the shifting of partial denitrification to complete denitrification.

Culturing partial-denitrification (PD) granules in continuous flow reactor with waste sludge as inoculum: performance, granular sludge characteristics and microbial community

Partial denitrification granular sludge (PDGS) can provide long-term stable nitrite for anaerobic ammonia oxidation (anammox). The cultivation of ordinary activated sludge from wastewater treatment plants into PDGS can further promote the application of PD in practical engineering. In this study, the feasibility of fast start-up of PDGS was explored by inoculating waste sludge in up-flow anaerobic sludge blanket (UASB) reactor with synergistic control of nitrogen load rate (NLR, 0.05-0.65 kg N/m3/d) and electron donor starvation (EDS) (240-168 mg L-1), and system performance, particle characteristics and microbial structure were studied. The results showed that PD-UASB started successfully within 48 days, the average nitrite accumulation rate (NTR) and nitrate removal ratio (NRR) reached 79.6% and 82.5% after successful initiation, accompanied by high abundance of PD bacteria (Thauera, Pseudomonas, unclassflied commamonadaceae and Limnobacter) (25.3%). The increase of PD activity, and the difference between nitrate reductase (NAR) and nitrite reductase (NIR) contributed to nitrite production. Besides, the sludge shifted from flocculated (≤0.5 mm, 95.37%) to granulated state (0.5-2 mm, 64.74%), which could be due to the increase of extracellular polymers (EPS) (especially T-EPS) and metabolism of specific microorganisms (Bacteroidota and Chloroflexi, 19.92%). Good sludge granulation promoted the settleability of PD (the SVI5 was 47.248 mL/ g. ss after successful start-up). In summary, good PD sludge granulation process could be achieved in a short time by synergistically controlling NLR and EDS.

Tertiary treatment of municipal wastewater in an IBFR dominated by PD/A with unique niche

To explore the feasibility of biofilter reactor to treat municipal secondary effluent deeply without extra carbon source, this paper proposed an integrated biofilter reactor (IBFR) coupling partial denitrification (PD) with anammox (A) to treat the secondary effluent and raw sewage with the flow ratio of 3:1 together. The results show that the effluent concentration of TN and COD in IBFR could be reduced to 10 mg/L and 15 mg/L, respectively, under hydraulic retention time of 1.5 h and nitrogen loading rate of 0.55 kg/(m3·d). The highest specific anammox activity (19.2 mg N/(g TVS·d)) and the maximum extracellular polymeric substance (EPS) content (107.21 mg/g TVS) occurred at the 25-50 cm section of IBFR, where Thauera, Candidatus Anammoximicrobium and Candidatus Brocadia were the dominant denitrifiers and anammox bacteria. Furthermore, the cyclic self-stratification occurred along the reactor height, where the utilization, decomposition, transformation and cross-feeding of EPS enhanced the performance stability of nitrogen and carbon removal, strengthened the niche structure and promoted the synergistic symbiosis. In conclusion, IBFR coupling PD and A demonstrated the possibility to treat secondary effluent without additional carbon sources, which is expected as an alternative approach for tertiary treatment of municipal wastewater.

Ultra-stable mixotrophic denitrification coupled with anammox under organic stress for mainstream municipal wastewater treatment

Sulfur-based autotrophic denitrification (SAD) coupled with anammox is a promising process for autotrophic nitrogen removal in view of the stable nitrite accumulation during SAD. In this study, a mixotrophic nitrogen removal system integrating SAD, anammox and heterotrophic denitrification was established in a single-stage reactor. The long-term nitrogen removal performance was investigated under the intervention of organic carbon sources in real municipal wastewater. With the shortening of hydraulic retention time, the nitrogen removal rate of the mixotrophic system dominated by the autotrophic subsystem reached 0.46 Kg N/m³/d at an organic loading rate of 0.57 Kg COD/m³/d, with COD and total nitrogen removal efficiencies of 82.5 % and 94 %, respectively, realizing an ideal combination of autotrophic and heterotrophic systems. The 15NO3--N isotope labeling experiments indicated that thiosulfate-driven autotrophic denitrification was the main pathway for nitrite supply accounting for 80.6 %, while anammox exhibited strong competitiveness for nitrite under the dual electron supply of sulfur and organic carbon sources and contributed to 65.1 % of nitrogen removal. Sludge granulation created differential functional distributions in different forms of sludge, with SAD showing faster reaction rate as well as higher nitrite accumulation rate in floc sludge, while anammox was more active in granular sludge. Real-time quantitative PCR, RT-PCR and high-throughput sequencing results revealed a dynamically changing community composition at the gene and transcription levels. The decrease in heterotrophic denitrification bacteria abundance indicated the effectiveness of the operational strategy for introduction of thiosulfate and maintaining the dominance of SAD in denitrification process in suppressing the excessive growth of heterotrophic bacteria in the mixotrophic system. The high transcriptional expression of sulfur-oxidizing bacteria (SOB) (Thiobacillus and Sulfurimonas) and anammox bacteria (Candaditus_Brocadia and Candidatus_Kuenenia) played a crucial role in the stable nitrogen removal.

The responding mechanism of indigenous bacteria in municipal wastewater inoculated with different concentrations of exogenous microalgae

Indigenous bacteria popularly exist in real wastewater. Therefore, the potential interaction between bacteria and microalgae is inevitable in microalgae-based wastewater treatment systems. It is likely to affect the performance of systems. Accordingly, the characteristics of indigenous bacteria is worth serious concerning. Here we investigated the response of indigenous bacterial communities to variant inoculum concentrations of Chlorococcum sp. GD in municipal wastewater treatment systems. The removal efficiency of COD, ammonium and total phosphorus were 92.50%-95.55%, 98.00%-98.69%, and 67.80%-84.72%, respectively. The bacterial community responded differently to different microalgal inoculum concentrations, which was mainly affected by microalgal number, ammonium and nitrate. Besides, there were differential co-occurrence patterns and carbon and nitrogen metabolic function of indigenous bacterial communities. All these results indicated that bacterial communities responded significantly to environmental changes caused by the change of microalgal inoculum concentrations. The response of bacterial communities to different microalgal inoculum concentrations was beneficial for forming a stable symbiotic community of both microalgae and bacteria to remove pollutants in wastewater.

Characterization of two keystone taxa, sulfur-oxidizing, and nitrate-reducing bacteria, by tracking their role transitions in the benzo[a]pyrene degradative microbiome

BACKGROUND

Keystone taxa are drivers of microbiome structure and functioning, which may play critical roles in microbiome-level responses to recalcitrant pollution and are a key to bioremediation. However, the characterization and manipulation of such taxa is a major challenge due to the complexity of microbial communities and rapid turnover in both time and space. Here, microcosms were set up with benzo[a]-pyrene (BaP) and/or nitrate based on C-rich, S-rich, and N-limited mangrove sediments as reductive experimental models to trigger and track the turnover of keystone taxa to address this challenge.

RESULTS

Based on microbial co-occurrence network analysis, two keystone taxa, Sulfurovum and Sulfurimonas, were found to exhibit significant role transitions in different microcosms, where these two taxa played nonkeystone roles with neutral relationships in in situ mangrove sediments. However, Sulfurimonas transitioned to be keystone taxa in nitrate-replenished microcosms and formed a keystone guild with Thioalkalispira. Sulfurovum stood out in BaP-added microcosms and mutualized in a densely polycyclic aromatic hydrocarbon (PAH)-degrader-centric keystone guild with Novosphingobium and Robiginitalea, where 63.25% of added BaP was removed. Under the occurrence of nitrate and BaP, they simultaneously played roles as keystone taxa in their respective guilds but exhibited significant competition. Comparative genomics and metagenome-assembled genome (MAG) analysis was then performed to reveal the metabolic potential of those keystone taxa and to empirically deduce their functional role in keystone guilds. Sulfurimonas possesses a better sense system and motility, indicative of its aggressive role in nitrate acquisition and conversion; Sulfurovum exhibited a better ability for oxidation resistance and transporting nutrients and electrons. High-efficiency thermal asymmetric interlaced polymerase reaction (hiTAIL-PCR) and enhanced green fluorescent protein (eGFP)-labeling approaches were employed to capture and label the BaP key degrader to further experimentally verify the roles of keystone taxa Sulfurovum in the keystone guilds. Observations of the enhancement in reactive oxygen species (ROS) removal, cell growth, and degradation efficiency by co-culture of isolated keystone taxa strains experimentally demonstrated that Sulfurovum contributes to the BaP degradative microbiome against BaP toxicity.

CONCLUSIONS

Our findings suggest that the combined use of co-occurrence network analysis, comparative genomics, and co-culture of captured keystone taxa (3C-strategy) in microbial communities whose structure is strongly shaped by changing environmental factors can characterize keystone taxa roles in keystone guilds and may provide targets for manipulation to improve the function of the microbiome. Video Abstract.

Highly efficient and robust treatment of low C/N actual domestic sewage via integrated fermentation, partial-nitrification, partial-denitrification and anammox (IFPNDA)

For achieving efficient and robust treatment of domestic sewage with C/N around 2.8, this study innovatively developed an integrated fermentation, partial-nitrification, partial-denitrification and anammox (IFPNDA) process based on the Anaerobic Baffled Reactor and Continuous-flow Stirred Tank Reactor (ABR-CSTR) bioreactor. Desirable N-removal efficiency of 87.5 ± 2.1% was obtained without external organics, correspondingly effluent total nitrogen (TN) concentration reached 6.1 ± 0.7 mg/L. The N-removal stability was greatly facilitated by the effective linkage between partial nitrification (PN) process and partial denitrification (PD) process in emergency. Highly enriched hydrolytic bacteria (6.9%) and acidogenic bacteria (5.7%) in A1, especially Comamonas (2.8%) and Longilinea (3.5%), induced the significant increase of volatile fatty acids (VFAs) in domestic sewage. Thauera (6.1%) in A2 and Nitrosomonas (5.4%) in A3 acted as the dominant flora of nitrite supplies for anammox in IFPNDA process. Candidatus_Brocadia (2.4%) dominated the advanced nitrogen removal. The IFPNDA process exhibited much potential for achieving energy neutrality during wastewater treatment.

Fast start-up and stable operation of mainstream anammox without inoculation in an A2/O process treating low COD/N real municipal wastewater

It is of great significance to start up the anammox process in the most commonly used anaerobic-anoxic-oxic (A²/O) process in treating mainstream municipal wastewater. Recently, partial-denitrification/anammox (PD/A) has attracted increasing interest as a new avenue in mainstream. This study investigated the in situ start-up of PD/A process in a traditional A²/O process. The PD/A system was rapidly started up within 60 days by adding virgin carriers into the anoxic zone and then run stably for the next 90 days. The in situ anammox activity reached 1.0 ± 0.1 mg NH₄⁺-N/L/h contributing 37.9 ± 6.2% of total nitrogen removal. As a result, the nitrogen removal efficiency of the system increased by 16.9%. The anammox bacteria (AnAOB) on the anoxic biofilms were enriched with a doubling time of 14.53d, and the relative abundance reached 2.49% on day 150. Phylogenetic analysis showed the dominant AnAOB was related to Ca. Brocadia sp. 40, which was the only detected anammox genus in the anoxic biofilm from start-up to stable operation. Batch tests and qPCR results revealed that compared with the floc sludge, the anoxic biofilms exhibited NO₂^- accumulation driven by PD and performed a better coordination between denitrifiers and AnAOB. Overall, this study provides great confidence for the in situ fast start-up of mainstream anammox using conventional activated sludge.

Coupling of Anammox Activity and PAH Biodegradation: Current Insights and Future Directions

Anaerobic ammonium oxidation (anammox) has shown success in past years for the treatment of municipal and industrial wastewater containing inorganic nutrients (i.e., nitrogen). However, the increase in polycyclic aromatic hydrocarbon (PAH)-contaminated matrices calls for new strategies for efficient and environmentally sustainable remediation. Therefore, the present review examined the literature on the connection between the anammox process and PAHs using VOSviewer to shed light on the mechanisms involved during PAH biodegradation and the key factors affecting anammox bacteria. The scientific literature thoroughly discussed here shows that PAHs can be involved in nitrogen removal by acting as electron donors, and their presence does not adversely affect the anammox bacteria. Anammox activity can be improved by regulating the operating parameters (e.g., organic load, dissolved oxygen, carbon-to-nitrogen ratio) and external supplementation (i.e., calcium nitrate) that promote changes in the microbial community (e.g., Candidatus Jettenia), favoring PAH degradation. The onset of a synergistic dissimilatory nitrate reduction to ammonium and partial denitrification can be beneficial for PAH and nitrogen removal.

Strategy of nitrate removal in anaerobic ammonia oxidation-dependent processes

The anaerobic ammonium oxidation (anammox), a microbial process that is considered as a low-cost and high efficient wastewater treatment, has received extensive attention with an attractive application prospect. The anammox process reduces nitrite (NO₂^-) to nitrogen gas (N₂) with ammonium (NH₄⁺) as the electron donor. However, some nitrate (NO₃^-) equivalent to 11% of total nitrogen (TN) is generated in this process, which limits the development of anammox. To overcome this problem, many efforts have been made in this regard, mainly combining with other biological treatment methods (denitrification, denitrifying anaerobic methane oxidation, etc.), introducing the substance into anammox process, etc. Herein, we summarized a detailed review of previous researches on the removal of NO₃^- in the anammox-dependent processes. It is hoped that this review could serve as valuable guidance in future research and practical applications of anammox.

Sulfammox forwarding thiosulfate-driven denitrification and anammox process for nitrogen removal

The coupled process of thiosulfate-driven denitrification (NO₃^-→NO₂^-) and Anammox (TDDA) was a promising process for the treatment of wastewater containing NH₄⁺-N and NO₃^--N. However, the high concentration of SO₄^2- production limited its application, which needs to be alleviated by an economical and effective way to promote the application of TDDA process. In this study, TDDA process was started in a relatively short time by stepwise replacing nitrite with nitrate and operated continuously for 146 days. Results presented that the average total nitrogen removal efficiency of 82.18% can be acquired at a high loading rate of 1.98 kg N/(m³·d) with maximum nitrogen removal efficiency up to 87.04%. It was observed that the increase of S/N ratio improved the denitrification efficiency and slightly inhibit the Anammox process. Batch tests showed that Sulfammox process appeared in TDDA process under certain conditions, further contributing 2.59% nitrogen removal and 10.46% sulfur removal (14.42 mg/L NH₄⁺-N and 37.68 mg/L SO₄^2--S were removed). This finding was mainly attributed to the reduction of sulfate in TDDA system to elemental S⁰ or HS^-, which subsequently was used as an electron donor to realize the recycling of sulfate (SO₄^2--S) pollutants and promote the sulfur-nitrogen (S-N) cycle. High-throughput analysis displayed that Anammox bacteria (Candidatus_Kuenenia), Sulfur-oxidizing bacteria (Thiobacillus) with relatively high abundance of 5.37%, 7.74%, respectively, guaranteeing the excellent nitrogen and sulfate removal performance in the reactor. The enrichment of phyla Chloroflexi (31.79%), Proteobacteria (31.82%), class Ignavibacteriales (10.55%), genus Planctomycetes (13.57%) further verified the exitence of Sulfammox process in the TDDA reactor. This study provides a new perspective for the practical application of TDDA in terms of reducing the production of high concentration SO₄^2- and saving operational cost and strengthening deeply nitrogen removal.

The mixed/mixotrophic nitrogen removal for the effective and sustainable treatment of wastewater: From treatment process to microbial mechanism

Biological nitrogen removal (BNR) is one of the most important environmental concerns in the field of wastewater treatment. The conventional BNR process based on heterotrophic nitrogen removal (HeNR) is suffering from several limitations, including external carbon source dependence, excessive sludge production, and greenhouse gas emissions. Through the mediation of autotrophic nitrogen removal (AuNR), mixed/mixotrophic nitrogen removal (MixNR) offers a viable solution to the optimization of the BNR process. Here, the recent advance and characteristics of MixNR process guided by sulfur-driven autotrophic denitrification (SDAD) and anammox are summarized in this review. Additionally, we discuss the functional microorganisms in different MixNR systems, shedding light on metabolic mechanisms and microbial interactions. The significance of MixNR for carbon reduction in the BNR process has also been noted. The knowledge gaps and the future research directions that may facilitate the practical application of the MixNR process are highlighted. Overall, the prospect of the MixNR process is attractive, and this review will provide guidance for the future implementation of MixNR process as well as deciphering the microbially metabolic mechanisms.

A review on upgrading of the anammox-based nitrogen removal processes: Performance, stability, and control strategies

The anaerobic ammonia oxidation (anammox) process is a promising biological nitrogen removal technology. However, owing to the sensitivity and slow cell growth of anammox bacteria, long startup time and initially low nitrogen removal rate (NRR) are still limiting factors of practical applications of anammox process. Moreover, nitrogen removal efficiency (NRE) is often lower than 88 %. This review summarizes the most common methods for improving NRR by increasing microorganism concentration, and modifying reactor configuration. Recent integrated anammox-based systems were evaluated, including hydroxyapatite (HAP)-enhanced one-stage partial nitritation/anammox (PNA) process for a high NRR of over 2 kg N/m3/d at 25 °C, partial denitrification/anammox (PDA) process, and simultaneous partial nitrification, anammox, and denitrification process for a high NRE of up to 100 %. After discussing the challenges for the application of these systems critically, a combined system of anaerobic digestion, HAP-enhanced one-stage PNA and PDA is proposed in order to achieve a high NRR, high NRE, and phosphorus removal simultaneously.

Sulfur containing mixed wastes in anaerobic processing by new immobilized synthetic consortia

Methanogenic biotransformation of unusual substrates (sulfur (S)-containing wastes: non-purified vacuum gas oil, straight-run gasoline fraction (Naphtha), gas condensate, and straight-run diesel fraction) coming from oil industry after their oxidative desulfurization was investigated. Nitrogen-containing wastes (hydrolysates of chicken manure and Chlorella vulgaris biomass) were added as co-substrates to mixture with oil industry wastes. The 100 % conversion of S-organic compounds to inorganic sulfide accumulated in the reaction liquid medium was achieved with simultaneous production of biogas containing high methane percent (greater than 70 %). Polishing of effluents from methane tank was carried out by denitrifying oxidation of ammonium (DEAMOX). The high process efficiency was due to use of original immobilized artificial consortia at the stage of methanogenesis and DEAMOX. This study reveals the real potential in the processing of very complex mixtures of large-scale wastes, usually inhibiting methanogenesis, by developing biocatalysts based on synthetic biology approaches.

Coupling sulfur-based denitrification with anammox for effective and stable nitrogen removal: A review

Anoxic ammonium oxidation (anammox) is an energy-efficient nitrogen removal process for wastewater treatment. However, the unstable nitrite supply and residual nitrate in the anammox process have limited its wide application. Recent studies have proven coupling of sulfur-based denitrification with anammox (SDA) can achieve an effective nitrogen removal, owing to stable provision of substrate nitrite from the sulfur-based denitrification, thus making its process control more efficient in comparison with that of partial nitrification and anammox process. Meanwhile, the anammox-produced nitrate can be eliminated through sulfur-based denitrification, thereby enhancing SDA's overall nitrogen removal efficiency. Nonetheless, this process is governed by a complex microbial system that involves both complicated sulfur and nitrogen metabolisms as well as multiple interactions among sulfur-oxidising bacteria and anammox bacteria. A comprehensive understanding of the principles of the SDA process is the key to facilitating the development and application of this novel process. Hence, this review is conducted to systematically summarise various findings on the SDA process, including its associated biochemistry, biokinetic reactions, reactor performance, and application. The dominant functional bacteria and microbial interactions in the SDA process are further discussed. Finally, the advantages, challenges, and future research perspectives of SDA are outlined. Overall, this work gives an in-depth insight into the coupling mechanism of SDA and its potential application in biological nitrogen removal.

Partnering of anammox and denitrifying bacteria benefits anammox's recovery from starvation and complete nitrogen removal

The cooperative metabolic activity of anammox and denitrifying bacteria could speed up anammox's recovery and reduce nitrate generated from the anammox reaction. In this study, a laboratory-scale model system containing a defined anammox culture AMX and a simultaneous nitrification and denitrification (SND) bacterium - Thauera sp. strain SND5 was established and investigated. Several lines of evidence revealed that strain SND5 consumed soluble microbial products (SMPs) generated by culture AMX (as high as 1.5 mg/L), stimulating anammox activity after long-term starvation. At low C/N ratios with an optimal C/N of 1, SND5 completely consumed organic carbon first at anoxic condition, storing carbon intracellularly as poly-β-hydroxybutyrate (PHB) (as high as 0.6 mg/L biomass), thereby creating a favorable environment for the growth of anammox bacteria. The anammox reaction and nitrate reduction supported by PHB catabolism could then proceed simultaneously, resulting in enhanced nitrogen removal. Cooperative interactions between anammox and denitrifying bacteria involving SMPs consumption and PHB synthesis may play a significant role in nitrogen cycling at nitrite- and carbon-limited environments.

Sulfidation forwarding high-strength Anammox process using nitrate as electron acceptor via thiosulfate-driven nitrate denitratation

A single up-flow thiosulfate-driven nitrate denitratation coupled with the sulfurized Anammox (TDSA) with the core-shell structure (S⁰@ Anammox granules) provided a chemical/energy-saving way for the removal of high-content ammonium with nitrate as electron acceptor. Approximately 83.66% total nitrogen removal efficiency (TNRE) could be achieved by the sulfurized Anammox encrusted by S⁰/S_n^2- at a high loading rate (2.6 kg-N/(m³·d)) via resisting high concentration of free ammonia (FA) (22.35 mg/L), mainly through S₂O₃^2-, S⁰/S_n^2- -driven partial denitrification-Anammox (PDN-Anammox) process. Moreover, S⁰/S_n^2--PDN-Anammox was largely restricted when intermittently aerated, but still resulting in 74.47% TNRE due to the partial nitrification-Anammox (PN-Anammox). The sequencing analysis revealed that Anammox bacterium (Candidatus_Kuenenia) and sulfur-oxidizing bacterium (Thiobacillus) coexisted, in which Anammox process occurred mainly via NO instead of NH₂OH. This study provided a new perspective for high concentration nitrogen wastewater removal in engineering applications.

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.

Synergistic partial denitrification, anammox and in-situ fermentation (SPDAF) process for treating domestic and nitrate wastewater: Response of nitrogen removal performance to decreasing temperature

A synergistic partial denitrification, anaerobic ammonium oxidation (Anammox), and in-situ fermentation (SPDAF) system was established to solve problems of wastewater treatment plants (WWTPs) in combined treatment of domestic sewage, and nitrate wastewater discharged from industrial areas. The SPDAF system was started up at decreasing temperatures (26.8-18.9 ℃), and remained robust at abrupt temperature drop and drastic temperature fluctuations (20.7-14.1 ℃). The influent and effluent total inorganic nitrogen (TIN) were 97.0 ± 3.7 mg/L and 10.3 ± 4.0 mg/L, respectively. In-situ fermentation supplemented electron donors for NO₃^--N reduction. A high TIN removal efficiency, of 89.5 ± 3.9% was obtained. Specifically, Anammox contributed 90.9 ± 5.2% to TIN removal. Furthermore, the abundances of hydrolysis and acidogenesis bacteria were 14.02% and 29.47% in the low and high zones, respectively, which promoted fermentation and the use of complex organics. This study provided novel insights for actual operation of WWTPs.

Novel biotechnologies for nitrogen removal and their coupling with gas emissions abatement in wastewater treatment facilities

Wastewaters contaminated with nitrogenous pollutants, derived from anthropogenic activities, have exacerbated our ecosystems sparking environmental problems, such as eutrophication and acidification of water reservoirs, emission of greenhouse gases, death of aquatic organisms, among others. Wastewater treatment facilities (WWTF) combining nitrification and denitrification, and lately partial nitrification coupled to anaerobic ammonium oxidation (anammox), have traditionally been applied for the removal of nitrogen from wastewaters. The present work provides a comprehensive review of the recent biotechnologies developed in which nitrogen-removing processes are relevant for the treatment of both wastewaters and gas emissions. These novel processes include the anammox process with alternative electron acceptors, such as sulfate (sulfammox), ferric iron (feammox), and anodes in microbial electrolysis cells (anodic anammox). New technologies that couple nitrate/nitrite reduction with the oxidation of methane, H₂S, volatile methyl siloxanes, and other volatile organic compounds are also described. The potential of these processes for (i) minimizing greenhouse gas emissions from WWTF, (ii) biogas purification, and (iii) air pollution control is critically discussed considering the factors that might trigger N₂O release during nitrate/nitrite reduction. Moreover, this review provides a discussion on the main challenges to tackle towards the consolidation of these novel biotechnologies.

Successful application of municipal domestic wastewater as a co-substrate in 2,4,6-trichlorophenol degradation

Wastewater containing 2,4,6-trichlorophenol (2,4,6-TCP) is highly toxic and causes harmful effects on aquatic ecosystems and human health. In this study, wastewater containing high levels of 2,4,6-TCP was successfully co-metabolized by introducing municipal domestic wastewater (MDW) as the co-catabolic carbon source. The concentration of degraded 2,4,6-TCP increased from 0 to 208.71 mg/L by adjusting the influent MDW volume during a 150-day-long operation. An MDW dose of 500 mL was found optimal, with an average concentration of 250 mgCOD/L. Unlike the long-term experiment, changing the MDW adding mode in a typical cycle further increased the concentration of 2,4,6-TCP removed to 317 mg/L. The main MDW components, such as the sugars, VFAs, and slowly biodegradable organic substances, improved 2,4,6-TCP degradation, achieving a TOC removal efficiency of 90.98% and a dechlorination efficiency of 100%. The MDW level did not change the 2,4,6-TCP degradation rate (μ_TCP) in a typical cycle compared to the single carbon source, and the μ_TCP remained at a high level of 50 mg 2,4,6-TCP/h. Macrogenetic analysis demonstrated that MDW addition promoted the growth of 43 bacterial genera (41.49%) responsible for 2,4,6-TCP degradation and intermediates' metabolism. The key genes for 2,4,6-TCP metabolism (pcpA, chqB, mal-r, pcaI, pcaF, and fadA) were detected in the activated sludge, which were distributed among the 43 genera. To conclude, this study proposes a new carbon source for co-metabolism to treat 2,4,6-TCP-polluted wastewater.

Electrode-dependent ammonium oxidation with different low C/N ratios in single-chambered microbial electrolysis cells

Alternative method should be found to solve the ammonia accumulation in anaerobic digestion. Herein, electrode-dependent ammonium oxidation was successfully achieved in anaerobic single-chambered microbial electrolysis cells (MECs)under different low C/N ratios (0, 1, and 1.5), with an applied voltage of 0.6 V as well as an initial NH₄⁺-N and NO₃^--N concentration of 500 and 300 mg/L. The nitrogen removal performance of MECs and the controls indicated that applying a voltage stimulated nitrogen removal under low C/N ratios of 0, 1, and 1.5. However, the remaining organic carbon in MEC with a relatively higher C/N ratio of 3 inhibited the ammonium oxidation. Current changes and cyclic voltammetry demonstrated that the bioanode with several bioelectrochemical activities could promote ammonium oxidation. The dominant genera Truepera, Aquamicrobium, Nitrosomonas, Arenimonas, Comamonas, and Cryobacterium enriched on both electrodes could be the key functional taxa in MECs with C/N ratios of 0, 1, and 1.5. The remaining sodium acetate in MEC with C/N ratio of 3 inhibits microbial community structure and relative abundance, which may adversely affected nitrogen removal. Further caculation showed that nitrogen balance was essentially achieved, while electron balance was disrupted since electrons may be consumed through NO₃^--N recycle and cell synthesis, and finally caused low coulombic efficiency.

Development of a kinetic model to evaluate thiosulfate-driven denitrification and anammox (TDDA) process

Recently, the integration of sulfur-driven denitrification and anammox process has been extensively studied as a promising alternative nitrogen removal technology. Most of these studies investigated the process feasibility and monitored the community dynamics. However, an in-depth understanding of this new sulfur-nitrogen cycle bioprocess based on mathematical modeling and elucidation of complex interactions among different microorganisms has not yet been achieved. To fill this gap, we developed a kinetic model (with 7 bioprocesses, 12 variables, and 19 parameters) to assess the sulfur(thiosulfate)-driven denitrification and anammox (TDDA) process in a single reactor. The parameters used in this process were separately estimated by fitting the data obtained from the experiments. Then, the model was further validated under different conditions, and the results demonstrated that the developed model could describe the dynamic behaviors of nitrogen and sulfur conversions in the TDDA system. The newly developed branched thiosulfate oxidation model was also verified by conducting a metagenomics analysis. Using the developed model, we i) examined the interactions between sulfur-oxidizing bacteria and anammox bacteria at steady-state conditions with varying substrates to demonstrate the reliability of TDDA, and ii) evaluated the feasibility and operation of the TDDA process in terms of practical implementation. Our results will benefit further exploration of the significance of this novel S-N cycle bioprocess and guide its future applications.

Development of a novel denitrifying phosphorus removal and partial denitrification anammox (DPR + PDA) process for advanced nitrogen and phosphorus removal from domestic and nitrate wastewaters

A novel energy-efficient DPR + PDA (denitrifying phosphorus removal and partial denitrification anammox) process for enhanced nitrogen and phosphorus removal was developed in the combined ABR-CSTR reactor. After 220 days operation, excellent total inorganic nitrogen (TIN) and phosphorus removal (97.57% and 95.66%, respectively) were obtained under external C/NO₃^--N of 0.7, with the effluent TIN and PO₄^3--P concentrations of 3.51 mg/L and 0.28 mg/L, respectively. At the steady period, DPR contributed major TN removal (58.65%), while PDA mediated an increasingly considerable impact and finally achieved 37.07%, in which anammox accounted for a significant percentage. Batch tests demonstrated that efficient PD with nitrate-to-nitrite transformation ratio of 97.67% supplying stable nitrite for anammox, and phosphorus was mainly removed using nitrate as electron acceptor via DPR with the ideal phosphorus release/uptake rate (7.73/22.17 mgP/gVSS/h). Accumulibacter (6.24%) dominated high phosphorus removal performance, while Thauera (8.26%) and Candidatus Brocadia (2.57%) represented the superior nitrogen removal performance.

Highly enriched anammox within anoxic biofilms by reducing suspended sludge biomass in a real-sewage A2/O process

This study proposes a novel strategy of stably enriching anammox in mainstream, based on the competitive difference to NO₂^- between anoxic biofilms and suspended sludge. A modified anaerobic-anoxic-oxic (A²/O) process run for 500 days with actual municipal wastewater. Microbial analysis revealed that anoxic-carrier biofilms had a significantly higher abundance of anammox (qPCR: 0.74% - 4.34%) than suspended sludge (P< 0.001). Batch tests showed that anammox within anoxic-carrier biofilms contributed to significant nitrogen removal, coupled with partial-denitrification (NO₃^- → NO₂^-). The anammox genus, Ca. Brocadia, was highly enriched when suspended sludge was accidentally lost. Further batch tests found that reducing suspended biomass helped anammox enrichment in anoxic-carrier biofilms, because the suspended sludge had strong NO₂^- competition (NO₂^- → N₂) with anammox (increased nirK). Metagenomic sequencing revealed that Ca. Brocadia dominates in the anoxic-carrier biofilms, and is the most important narG contributor to NO₃^- → NO₂^-, which could have promoted the competition of NO₂^- with heterotrophic bacteria. For this A²/O process, the low effluent total nitrogen (8.9 mg ± 1.0 mg N/L) was attributed to partial-denitrification coupling with anammox, demonstrating that this process is applicable to the general influent N-concentration range (30 mg - 50 mg NH₄⁺-N/L) of municipal wastewater treatment plants (WWTPs). Based on the special competitive preference of anammox for NO₂^-, this study provides a promising and practical alternative for enriching anammox bacteria in municipal WWTPs.

Exploration and verification of the feasibility of sulfide-driven partial denitrification coupled with anammox for wastewater treatment

Anaerobic ammonia oxidation (anammox) is a well-developed biotechnology for treating high-strength ammonium wastewaters. Recently, partial denitrification has been considered as an alternative to supply anammox with the required nitrite. In this study, a process of sulfide-driven partial denitrification and anammox (SPDA) was developed and operated continuously in an upflow anaerobic sludge blanket (UASB) reactor for 392 days. This reactor was fed with synthetic wastewater containing 100 mgN/L nitrate, 80 mgN/L ammonium and 20-80 mgS/L sulfide. After 160 days of operation, the reactor reached stable performance, and the nitrogen removal efficiency and rate were maintained at 80% and 0.29 kgN/(m³•d), respectively. The estimated nitrogen removal via anammox and sulfide-driven denitrification were 87.2% and 12.8%. Additional batch experiments were conducted to investigate the effects of sulfide on anammox and the mechanisms of nitrogen removal in the SPDA system. The following results were obtained: (1) sulfide had an inhibitory effect on the specific anammox activity with IC₅₀ of 9.7 mgS-H₂S/L. (2) The rapid oxidation of sulfide by sulfur-oxidizing bacteria (SOB) could relieve the toxic effects of sulfide on the anammox in the SPDA system. (3) Sulfide bio-oxidation was a two-step reaction with biologically produced elemental sulfur (BPS⁰) as the intermediate, and the second step using BPS⁰ as the electron donor, can efficiently produce nitrite via partial denitrification (NO₃^- → NO₂^-) as a supply for anammox. Finally, a high-throughput sequencing analysis identified Thiobacillus and Sulfurimonas as the dominant genera of SOB in the SPDA system, and Candidatus Kuenenia as the dominant anammox bacteria. Overall, this research gives the foundation for the practical application of sulfide-driven partial denitrification and anammox process in the future.

Sequencing batch reactor technology for landfill leachate treatment: A state-of-the-art review

Landfill has become an underlying source of surface and groundwater pollution if not efficiently managed, due to the risk of leachate infiltration into to land and aquifers. The generated leachate is considered a serious environmental threat for the public health, because of the toxic and recalcitrant nature of its constituents. Thus, it must be collected and appropriately treated before being discharged into the environment. At present, there is no single unit process available for proper leachate treatment as conventional wastewater treatment processes cannot achieve a satisfactory level for degrading toxic substances present. Therefore, there is a growing interest in examination of different leachate treatment processes for maximum operational flexibility. Based on leachate characteristics, discharge requirements, technical possibilities, regulatory requirements and financial considerations, several techniques have been applied for its degradation, presenting varying degrees of efficiency. Therefore, this article presents a comprehensive review of existing research articles on the pros and cons of various leachate degradation methods. In line with environmental sustainability, the article stressed on the application and efficiency of sequencing batch reactor (SBR) system treating landfill leachate due to its operational flexibility, resistance to shock loads and high biomass retention. Contributions of integrated leachate treatment technologies with SBR were also discussed. The article further analyzed the effect of different adopted materials, processes, strategies and configurations on leachate treatment. Environmental and operational parameters that affect SBR system were critically discussed. It is believed that information contained in this review will increase readers fundamental knowledge, guide future researchers and be incorporated into future works on experimentally-based SBR studies for leachate treatment.

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.

Development of simultaneous nitrification-denitrification and anammox and in-situ analysis of microbial structure in a novel plug-flow membrane-aerated sludge blanket

This study proposed a novel one-stage plug-flow microaerobic sludge blanket (PMSB) with membrane aerated for treating low carbon to nitrogen (C/N) ratio municipal sewage. The performance of simultaneous nitrification, denitrification, and anammox in the reactor was investigated. The results illustrated that the removal efficiencies of ammonium and total nitrogen (TN) were 93.2% and 87.1% with a C/N ratio of 4. High throughput sequencing revealed that aerobic bacteria, anaerobic bacteria and facultative anaerobe could co-exist at the same time in the sludge blanket. Meanwhile, a notable correlation between the oxygen concentration and the distance of the membrane module was analyzed. It was shown that the microbial community of functional bacteria developed in different aeration sites due to the oxygen concentration gradient. Microbial community structure was analyzed depending on the sludge stratification in the sludge blanket.

Effects of loading rates and N/S ratios in the sulfide-dependent autotrophic denitrification (SDAD) and Anammox coupling system

This study investigated the shock resistance and the stability of a novel sulfide-dependent autotrophic denitrification (SDAD) and anaerobic ammonium oxidation (Anammox) coupling process for simultaneous removal of sulfide and nitrogen-containing wastewater in a single reactor. Results show that the total nitrogen (TN) removal efficiency reached 86.7% at a nitrogen loading rate (NLR) of 1.52 kgN m^-3 d^-1. Sulfide was fully oxidized, achieving the removal efficiency of 100% throughout the whole process. Batch tests suggest that Anammox remained dominant with the cooperation of partial SDAD (PSDAD) and could always compete over short-cut SDAD (SSDAD) for nitrite. High-throughput sequencing analysis revealed that Anammox bacteria remained active despite a relatively lower abundance and diversity than denitrifying bacteria. Candidatus Kuenenia might be the main contributor to Anammox, while Thiobacillus and Sulfurimonas were closely related to SDAD.

Achieving efficient nitrite accumulation in glycerol-driven partial denitrification system: Insights of influencing factors, shift of microbial community and metabolic function

Partial denitrification (PD), which could provide sufficient nitrite for subsequent anaerobic ammonium oxidation, is a novel strategy for mainstream nitrogen removal. In this study, the performance of using glycerol as electron donor for nitrite accumulation in PD process was evaluated. Results showed that a C/N of 4.5 was effective for nitrite production (average nitrite accumulation rate: 34.32 mg N h^-1 gMLVSS^-1; average nitrate-to-nitrite transformation ratio (NTR): 91.1%) with pH ranging from 6.0 to 9.0. Also, a stable nitrite accumulation was achieved in long-term operation with the average NTR of 80.1%. Mechanism investigation found that the denitrifying bacteria Saccharibacteria (77.9%) was enriched in glycerol-driven reactors. Moreover, the enzymatic activity as well as the encoding genes (i.e. narG, narH and napA) involved in nitrate reduction were much higher than that for nitrite reduction (i.e. nirK), and this disparity was responsible for the efficient nitrite accumulation in glycerol-driven PD system.

Newly designed high-coverage degenerate primers for nitrogen removal mechanism analysis in a partial nitrification-anammox (PN/A) process

Reliable tools for quantification of different functional populations are required to achieve stable, effective nutrients removal in partial nitrification and anammox (PN/A) processes. Here we report the design and validation of degenerate PCR primer pairs targeting anammox bacteria, aerobic ammonium-oxidizing bacteria (AeAOB) and nitrite-oxidizing bacteria (NOB) with high coverage but without sacrificing specificity. The new primer pairs are able to cover a broader range of the targeted populations (58.4 vs 21.7%, 49.5 vs 47.6%, 80.7 vs 57.2% and 70.5 vs 42.3% of anammox bacteria, AeAOB, Nitrobacter and Nitrospina, respectively) than previously published primers. Particularly, the Amx719F/875R primer can retrieve a larger number of 16S rRNA genes from different types of samples with amplicons covering all known anammox bacteria genera (100% coverage) including the recently found novel genus, Asahi BRW. These newly desinged primers will provide a more reliable molecular tool to investigate the mechanisms of nitrogen removal in PN/A processes, which can provide clearer links between reactor performance, the metabolic activities and abundances of functional populations, shedding light on conditions that are favorable to the establishment of stable PN/A.

New insights on biological nutrient removal by coupling biofilm-based CANON and denitrifying phosphorus removal (CANDPR) process: Long-term stability assessment and microbial community evolution

It was difficult to obtain a stable and efficient biological nutrient removal for high-strength wastewater treatment, the possibility of exploiting innovative CANDPR process, integrating biofilm-based completely autotrophic nitrogen removal over nitrite (CANON) with denitrifying phosphorus removal (DPR) was evaluated to resolve the difficulty. Results revealed that the excellent NH₄⁺-N, PO₄^3--P and COD removal efficiencies of 96%, 96% and 91%, were achieved respectively under a high nitrogen loading rate (0.79 kg·m^-3·d^-1) without adding organic matters during 320 days operation. Promoting NO_x^--N recirculation demonstrated as an efficient strategy for further nutrient depletion, facilitating the enhanced NO₃^--N removal to 100% with the considerably high P-uptake performance. Batch tests confirmed that denitrifying phosphorus accumulating organisms (DPAOs) using NO₃^--N as electron acceptors accounting for 68% in total PAOs. Dechloromonas was identified as dominating genus in DPR, while Nitrosomonas (1.31%), Candidatus_Kuenenia (5.53%) and Candidatus_Brocadia (1.77%) contributed to the desirable nitrogen removal, indicating that cooperative consortia of DPAOs, AOB and AnAOB were harvested during long-term operation. The CANDPR process was verified to be energy-saving and treatment-reliable for renovating of existing plants.

Feasibility of Adjusting the S2O32-/NO3- Ratio to Adapt to Dynamic Influents in Coupled Anammox and Denitrification Systems

In this study, anammox, sulfur-based autotrophic denitrification, and heterotrophic denitrification (A/SAD/HD) were coupled in an expanded granular sludge bed (EGSB) reactor to explore the feasibility of enhancing denitrification performance by adjusting the S₂O₃^2-/NO₃^- (S/N) ratio to accommodate dynamic influents. The results indicated that the optimal influent conditions occurred when the conversion efficiency of ammonium (CEA) was 55%, the S/N ratio was 1.24, and the chemical oxygen demand (COD) was 50 mg/L, which resulted in a total nitrogen removal efficiency (NRE) of 95.0% ± 0.5%. The S/N ratio regulation strategy was feasible when the influent COD concentration was less than 100 mg/L and the CEA was between 57% and 63%. Characterization by 16S rRNA sequencing showed that Candidatus Jettenia might have contributed the most to anammox, while Thiobacillus and Denitratisoma were the dominant taxa related to denitrification. The findings of this study provide insights into the effects of CEA and COD on the performance of the A/SAD/HD system and the feasibility of the S/N ratio regulation strategy.

Recent advances in controlling denitritation for achieving denitratation/anammox in mainstream wastewater treatment plants

Denitratation (NO₃^-→NO₂^-)/anammox is a promising method for anammox application in mainstream wastewater treatment plants (WWTPs) to reduce oxygen and organic matter consumption. Achieving nitrite production via denitratation and controlling denitritation (NO₂^-→N₂) is the basis of the denitratation/anammox process. To control denitritation, the denitrifying biocommunity and growth rate are critically reviewed for biocommunity optimization. Then, the short-term and long-term effects of pH on denitritation were summarized and the possible mechanism was discussed, along with the effect of C/N ratio and organic matter type on denitritation. Meanwhile, the strategies for producing nitrite via controlling denitritation are discussed, as well as the processes for achieving nitrogen removal via denitratation/anammox in WWTPs. Finally, the practical application of denitratation/anammox in a full-scale mainstream WWTP is documented.

Formation of partial-denitrification (PD) granular sludge from low-strength nitrate wastewater: The influence of loading rates

Granular sludge has been believed to be a promising technology in wastewater biological treatment. However, the formation of granules at low substrate concentration is a difficult task that has seldom been achieved. This study aimed at forming the granules in the recently developed partial-denitrification (PD, NO₃^--N→NO₂^--N) for nitrite production. Two sequencing batch reactors (SBRs) were operated at a low nitrate of 30 mg N/L with nitrate loading rate (NLR) of 0.12 (R1) and 0.24 kg N/m³/d (R2). Results showed that the granulation of PD sludge experienced a developing and matured process with the progressive increase in size followed by maintaining a stable value. Higher NLR resulted in a more rapid granulation with the larger and looser structure. While the granules under lower NLR appeared to be denser and more compact with better settling ability. Microbial communities of two SBRs were revealed to show little difference, with the PD functional bacteria of Thauera (50.7% in R1 and 55.4% in R2) dominated during the granulation process. The Flavobacterium, likely to be closely related with sludge granulation, accounted for a higher proportion in R2 (10.16%) than R1 (5.91%), which might result in a larger granule formed in R2. This study clearly confirmed the feasibility of granulation of PD sludge under low nitrogen loads, shedding new light on the low-strength nitrate wastewater treatment with an efficient and economical way.

Simultaneous Ammonium oxidation denitrifying (SAD) in an innovative three-stage process for energy-efficient mature landfill leachate treatment with external sludge reduction

High-loaded ammonia and low-strength organics mature landfill leachate is not effectively treated by conventional biological processes. Herein, an innovative solution was proposed using a three-stage Simultaneous Ammonium oxidation Denitrifying (SAD) process. Firstly, ammonia (1760 ± 126 mg N/L) in wastewater was oxidized to nitrite in a partial nitrification sequencing batch reactor (PN-SBR). Next, 93% PN-SBR effluent and concentrated external waste activated sludge (WAS; MLSS = 23057 ± 6014 mg/L) were introduced to an anoxic reactor for integrated fermentation and denitrification (IFD-SBR). Finally, ammonia (101.4 ± 13.8 mg N/L) released by fermentation in the IFD-SBR and residual 7% nitrite in the PN-SBR were removed through the anaerobic ammonium oxidation (anammox) process in the SAD up-flow anaerobic sludge bed (SAD-UASB). In addition, NO₃^--N generation during the anammox process could be reduced to nitrite by partial denitrification (PD) and reused as substrate for anammox. A satisfactory total nitrogen (TN) removal efficiency (98.3%), external sludge reduction rate (2.5 kg/m³ d) and effluent TN concentration (16.7 mg/L) were achieved after long-term operation (280 days). The IFD-SBR and SAD-UASB contributed to 81.9% and 12.3% nitrogen removal, respectively. Microbial analysis showed that anammox bacteria (1.5% Candidatus Brocadia) cooperated well with partial denitrifying bacteria (4.3% Thauera) in SAD-UASB, and average nitrogen removal contribution were 83.1% during significant stability of anammox and 9.2% during the denitrification process, respectively. The three-stage SAD process provides an environmental and economic approach for landfill leachate treatment since it has the advantage of 25.4% less oxygen, 100% organic matter savings and 47.9% less external sludge than traditional biological processes.

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.

Effective nitrogen removal in a granule-based partial-denitrification/anammox reactor treating low C/N sewage

The partial-denitrification/anammox (PDA) process is a promising method to achieve mainstream anammox in wastewater treatment plants (WWTPs). To investigate the feasibility of developing a granule-based process for effective nitrogen removal via PDA, an upflow anaerobic sludge bed reactor was used as a PDA reactor treating low C/N sewage for over 200 days. Granules were formed with an average particle size of 1.92 mm. Metagenomic analysis revealed that the two most abundant genera in granules were Thauera (17.46%) and Candidatus Brocadia (6.24%) which played important roles in achieving partial-denitrification and anammox, respectively. Effective nitrogen removal was achieved with an average effluent TN concentration of 8.74 mg/L when influent TN concentration and COD/TN ratio were 42.56 mg/L and 1.52, respectively. Nitrogen removal via anammox accounted for 90% of dinitrogen production in the PDA reactor. Finally, a granule-based combined process of PDA with nitrification was proposed for achieving anammox in mainstream WWTPs.

Anammox and partial denitrification coupling: a review

As a new wastewater biological nitrogen removal process, anammox and partial denitrification coupling not only plays a significant role in the nitrogen cycle, but also holds high engineering application value. Because anammox and some denitrifying bacteria are coupled under harsh living conditions, certain operating conditions and mechanisms of the coupling process are not clear; thus, it is more difficult to control the process, which is why the process has not been widely applied. This paper analyzes the research focusing on the coupling process in recent years, including anammox and partial denitrification coupling process inhibitors such as nitrogen (NH₄⁺, NO₂⁻), organics (toxic and non-toxic organics), and salts. The mechanism of substrate removal in anammox and partial denitrification coupling nitrogen removal is described in detail. Due to the differences in process methods, experimental conditions, and sludge choices between the rapid start-up and stable operation stages of the reactor, there are significant differences in substrate inhibition. Multiple process parameters (such as pH, temperature, dissolved oxygen, redox potential, carbon-to-nitrogen ratio, and sludge) can be adjusted to improve the coupling of anammox and partial denitrification to modify nitrogen removal performance.

As a new wastewater biological nitrogen removal process, anammox and partial denitrification coupling not only plays a significant role in the nitrogen cycle, but also holds high engineering application value.

Coupling of sulfur(thiosulfate)-driven denitratation and anammox process to treat nitrate and ammonium contained wastewater

This study investigated the feasibility of a new biological nitrogen removal process that integrates sulfur-driven autotrophic denitratation (NO₃^-→NO₂^-) and anaerobic ammonium oxidation (Anammox) for simultaneous removal of nitrate and ammonium from industrial wastewater. The proposed sulfur(thiosulfate)-driven denitratation and Anammox process was developed in two phases: First, the thiosulfate-driven denitratation was established in the UASB inoculated with activated sludge and fed with ammonium, nitrate and thiosulfate for 52 days until the nitrite level in the effluent reached 32.1 mg N/L. Second, enriched Anammox biomass was introduced to the UASB to develop the integrated thiosulfate-driven denitratation and Anammox (TDDA) bioprocess (53-212 d). Results showed that nitrate and ammonium could be efficiently removed from synthetic wastewater by the integrated TDDA system at a total nitrogen (TN) removal efficiency of 82.5 ± 1.8% with an influent NH₄⁺-N of 101.2 ± 2.2 mgN/L, NO₃^--N of 101.1 ± 1.5 mgN/L and thiosulfate of 202.5 ± 3.2 mg S/L. It was estimated that Anammox and autotrophic denitritation (NO₂^-→N₂) contributed to about 90% and 10% of the TN removal respectively at stable operation. The established TDDA system was further supported by high-throughput sequencing analysis that sulfur-oxidizing bacteria (e.g., Thiobacillus and Sulfurimonas) coexisted with Anammox bacteria (e.g., Ca. Kuenenia and Ca. Anammoxoglobus) in this syntrophic biocenosis. Additionally, batch experiments were conducted to reveal the kinetic rates and to reconcile the stoichiometry of the electron donor/acceptor couples of the TDDA process. The results unraveled the mechanisms in the new bioprocess: i) sulfite and elemental sulfur (S⁰) were initially generated from branched thiosulfate; ii) oxidation of sulfite and elemental sulfur coupled with fast and slow denitratation; iii) nitrite produced from denitratation together with ammonium were effectively converted to dinitrogen gas via Anammox.

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.

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

Anaerobic ammonium oxidation (anammox) is an important scientific discovery in the field of wastewater treatment. This process is a sustainable option in nitrogen removal due to its energy-efficient and cost-effective advantage. Great effort has been made recently to remove ammonium from industrial and municipal wastewater via the anammox process with a preceding partial nitrification (PN) converting part of NH₄⁺ to NO₂^-. Anammox process is seldom involved in the nitrate removal. Nitrate (NO₃^-), one of the main nitrogen compounds produced from various industries, is typically converted to nitrogen gas via denitrification process where a large amount of carbon source is consumed. Within this context, we reviewed the current technologies for high-strength nitrate wastewater treatment. It is found that nitrite accumulation often occurs during nitrate reduction, and its accumulating level would be increased at certain conditions (i.e., low C/N ratio and high pH). Hence, this provides a great opportunity to employ the anammox process to further convert nitrite in a more sustainable way. In this review, we highlight a new approach for industrial nitrate wastewater treatment via partial denitrification coupled with anammox process (PD-A). We also discuss the conditions to achieve successful PD-A process, economic and environmental benefits, and potential challenges as well as the future perspectives in practical application.

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.

Synergy of partial-denitrification and anammox in continuously fed upflow sludge blanket reactor for simultaneous nitrate and ammonia removal at room temperature

In this study, the synergy of high nitrite (NO₂^--N) accumulated partial-denitrification (PD) and anammox in a continuously fed upflow anaerobic sludge blanket (UASB) reactor was verified for simultaneous nitrate (NO₃^--N) and ammonia (NH₄⁺-N) removal. A 225-days operation demonstrated that the relatively low total nitrogen (TN) concentration of 6.56 mg/L in effluent could be achieved with influent NH₄⁺-N and NO₃^--N both of 30 mg/L, resulting in a high TN removal of 89.1% at 17.5 °C. Batch tests revealed that the NO₃^--N-to-NO₂^--N transformation ratio (NTR) of PD stabilized at 90% during the whole operation, which played a crucial role in the desirable performance. However, the PD and anammox activity was negatively impacted by the limited mass transfer with serious sludge flotation. Significantly, hydrodynamic mixing optimization by adjusting liquid recirculation ratio effectively enhanced the nitrogen removal. Moreover, protein composition and tightly-bound structure of EPS played an important role in the sludge stability.

Analysis of enhanced nitrogen removal mechanisms in a validation wastewater treatment plant containing anammox bacteria

Anammox bacteria have attracted attention due to their apparent importance in saving energy and reducing organic chemical demands. Here, we report the detection and quantification of anammox bacteria with an improved primer set in a validation wastewater treatment plant. The improved primer set was shown to detect a broad range of anammox bacteria (47.3%) facilitating more accurate analyses of nitrogen removal mechanisms. Nitrogen removal efficiency and dominant nitrogen removal mechanisms were compared in the modification-Johannesburg (Mod-JHB), modified Ludzack-Ettinger (MLE) single-feed, and anoxic-oxic-anoxic-oxic (AOAO) step-feed modes. In the Mod-JHB configuration, simultaneous nitrification and denitrification (SND) and anammox were found to be responsible for more than 80% of total inorganic nitrogen (TIN) removal (98.5 ± 0.8% of TIN removal). Decrease of anoxic SRT from 5 to 2.5 days did not have any obvious effect on nitrogen removal or the abundance of functional microorganisms. Microbial batch tests demonstrated that both partial nitrification and dissimilatory nitrate reduction to ammonium (DNRA) were responsible for maintaining the anammox process. Short SRT (2 days) in the aerobic zone may explain the presence of partial nitrification. This study provides insights to the analysis of nitrogen removal mechanisms in validation wastewater treatment plants (WWTPs) aiming for high nitrogen removal efficiency.

Treated and untreated wastewater effluents alter river sediment bacterial communities involved in nitrogen and sulphur cycling

Studying the dynamics of nitrogen and sulphur cycling bacteria in river surface sediments is essential to better understand their contribution to global biogeochemical cycles. Evaporitic rocks settled at the headwater of the Deba River catchment (northern Spain) lead to high values of sulphate concentration in its waters. Besides, the discharge of effluents from untreated and treated residual (urban and industrial) wastewaters increases the concentration of metals, nutrients and organic compounds in its mid- and low-water courses. The aim of this study was to assess the impact of anthropogenic contamination from untreated and treated residual and industrial wastewaters on the structure and function of bacterial communities present in surface sediments of the Deba River catchment. The application of a quantitative functional approach (qPCR) based on denitrification genes (nir: nirS+nirK; and nosZ), together with a 16S rRNA gene metabarcoding structural analysis, revealed (i) the high relevance of the sulphur cycle at headwater surface sediments (as reflected by the abundance of members of the Syntrophobacterales order, and the Sulfuricurvum and Thiobacillus genera) and (ii) the predominance of sulphide-driven autotrophic denitrification over heterotrophic denitrification. Incomplete heterotrophic denitrification appeared to be predominant in surface sediments strongly impacted by treated and untreated effluents, as reflected by the lower values of the nosZ/nir ratio, thus favouring N₂O emissions. Understanding nitrogen and sulphur cycling pathways has profound implications for the management of river ecosystems, since this knowledge can help us determine whether a specific river is acting or not as a source of greenhouse gases (i.e., N₂O).

Achieving enhanced denitrification via hydrocyclone treatment on mixed liquor recirculation in the anoxic/aerobic process

This work presents the novel application of hydrocyclones for mixed liquor recirculation (MLR) treatment in the anoxic/aerobic (A/O) process to enhance the denitrification process. An exhaustive investigation on treated activated sludge and A/O effluents was conducted in batch and continuous operation tests. The median diameter of the sludge flocs was decreased from 78.82 μm to 15.77-23.31 μm, and the extracellular polymeric substances (EPS) desorption was observed, thus leading to the release of the soluble chemical oxygen demand (SCOD). A marked increase in the BOD₅/TN ratio was consequently achieved, which supplied the carbon source and improved the biodegradability of the MLR. The hydrocyclone treatment also enabled a 7.17% ± 0.93% specific oxygen utilization rate (SOUR) increase at the optimal hydrocyclone intensity of 0.13 MPa, owing to the desorption of positioned microbial secretion from the microorganism cells. The nitrate reductase and nitrite reductase were also improved by 15.13% ± 1.16% and 17.61% ± 1.55%, respectively. The nitrate removal efficiency was enhanced by 13.6%, and the nitrogen oxide gases varied slightly; this behavior was consistent with the variations in the key enzymes involved in denitrification. The A/O process operated in the mode of hydrocyclone-treated MLR, compared with in the conventional mode, resulted in a 15.56% higher TN removal, and the other effluent parameters remained stable. Hydrocyclone disruption is thus a convenient and energy-efficient process with broad implications in denitrification development.