Simultaneous partial nitrification, anaerobic ammonium oxidation and denitrification (SNAD) in a full-scale landfill-leachate treatment plant

Phenacetin promoted the rapid start-up and stable maintenance of partial nitrification: Responses of nitrifiers and antibiotic resistance genes

Phenacetin (PNCT) belongs to one of the earliest synthetic antipyretics. However, impact of PNCT on nitrifying microorganisms in wastewater treatment plants and its potential microbial mechanism was still unclear. In this study, PN could be initiated within six days by PNCT anaerobic soaking treatment (8 mg/L). In order to improve the stable performance of PN, 21 times of PNCT aerobic soaking treatment every three days was conducted and PN was stabilized for 191 days. After PN was damaged, ten times of PNCT aerobic soaking treatment every three days was conducted and PN was recovered after once soaking, maintained over 88 days. Ammonia oxidizing bacteria might change the dominant oligotype to gradually adjust to PNCT, and the increase of abundance and activity of Nitrosomonas promoted the initiation of PN. For nitrite-oxidizing bacteria (NOB), the increase of Candidatus Nitrotoga and Nitrospira destroyed PN, but PN could be recovered after once aerobic soaking illustrating NOB was not resistant to PNCT. KEGG and COG analysis suggested PNCT might disrupt rTCA cycle of Nitrospira, resulting in the decrease of relative abundance of Nitrospira. Moreover, PNCT did not lead to the sharp increase of absolute abundances of antibiotic resistance genes (ARGs), and the risk of ARGs transmission was negligible.

Nitrogen removal and carbon reduction of mature landfill leachate under extremely low dissolved oxygen conditions by simultaneous partial nitrification anammox and denitrification

In this study, a SNAD-SBBR process was implemented to achieve ammonia removal and carbon reduction of mature landfill leachate under extremely low dissolved oxygen conditions (0.051 mg/L) for a continuous operation of 266 days. The process demonstrated excellent removal performance, with ammonia nitrogen removal efficiency reaching 100 %, total nitrogen removal efficiency reaching 87.56 %, and an average removal rate of 0.180 kg/(m3·d). The recalcitrant organic compound removal efficiency reached 34.96 %. Nitrogen mass balance analysis revealed that the Anammox process contributed to approximately 98.1 % of the nitrogen removal. Candidatus Kuenenia achieved a relative abundance of 1.49 % in the inner layer of the carrier. In the SNAD-SBBR system, the extremely low DO environment created by the highly efficient partial nitrification stage enabled the coexistence of AnAOB, denitrifying bacteria, and Nitrosomonas, synergistically achieving ammonia removal and carbon reduction. Overall, the SNAD-SBBR process exhibits low-cost and high-efficiency characteristics, holding tremendous potential for landfill leachate treatment.

Metagenomics and proteomics profiling of extracellular polymeric substances from municipal waste sludge and their application for soil and water bioremediation

This study assessed the components of anaerobically digested sludge, activated sludge, and microbial and extracellular polymeric substance (EPS) enzymes to identify the mechanisms underlying nitrogen removal and soil regeneration. 16S rRNA gene amplicon-based sequencing was used to determine the microbial community composition and the related National Center for Biotechnology Information (NCBI) protein database was used to construct a conventional library from the observed community. EPS components were identified using gel-free proteomic (Liquid Chromatography with tandem mass spectrometry-LC/MS/MS) methods. Alginate-like EPS from aerobically activated sludge have strong potential for soil aggregation and water-holding capacity, whereas total EPS from anaerobic sludge have significant potential for ammonia removal under salt stress. Fourier transform infrared spectroscopy (FTIR) revealed that both EPS may contain proteins, carbohydrates, humic compounds, uronic acid, and DNA and determined the presence of O-H, N-H, C-N, CO, and C-H functional groups. These results demonstrate that the overall enzyme activity may be inactivated at 30 g L-1 of salinity. An annotation found in Kyoto Encyclopedia of Genes and Genomes (KEGG)- KEGG Automatic Annotation Server (KAAS) revealed that the top two metabolic activities in the EPS generated from the anaerobic sludge were methane and nitrogen metabolism. Therefore, we focused on the nitrogen metabolism reference map 00910. EPS from the anaerobically digested sludge exhibited nitrate reductase, nitrite reductase, and dehydrogenase activities. Assimilatory nitrate reduction, denitrification, nitrification, and anammox removed ammonia biochemically. The influence of microbial extracellular metabolites on water-holding capacity and soil aggregation was also investigated. The KAAS-KEGG annotation server was used to identify the main enzymes in the activated sludge-derived alginate-like extracellular EPS (ALE-EPS) samples. These include hydrolases, oxidoreductases, lyases, ligases, and transporters, which contribute to soil fertility and stability. This study improves our understanding of the overall microbial community structure and the associated biochemical processes, which are related to distinct functional genes or enzymes involved in nitrogen removal and soil aggregation. In contrast to conventional methods, microbial association with proteomics can be used to investigate ecological relationships, establishments, key player species, and microbial responses to environmental changes. Linking the metagenome to off-gel proteomics and bioinformatics solves the problem of analyzing metabolic pathways in complex environmental samples in a cost-effective manner.

Elucidating prioritized factor for mainstream partial nitritation between C/N ratio and dissolved oxygen: Response surface methodology and microbial community shifts

Recently, C/N ratio is suggested as a promising control factor with dissolved oxygen (DO) achieving mainstream partial nitritation (PN); however, their combined effects on mainstream PN are still limited. This study evaluated the mainstream PN with respect to the combined factors, and investigated the prioritized factor affecting the community of aerobic functional microbes competing with NOB. Response surface methodology was performed to assess the combined effects of C/N ratio and DO on the activity of functional microbes. Aerobic heterotrophic bacteria (AHB) played the greatest role in oxygen competition among functional microbes, which resulted in relative inhibition of nitrite-oxidizing bacteria (NOB). The combination of high C/N ratio and low DO had a positive role in the relative inhibition of NOB. In bioreactor operation, the PN was successfully achieved at ≥ 1.5 of C/N ratio for 0.5-2.0 mg/L DO conditions. Interestingly, aerobic functional microbes outcompeting NOB were shifted with C/N ratio rather than DO, suggesting C/N ratio is more prioritized factor achieving mainstream PN. These findings will provide insights into how combined aerobic conditions contribute to achieve mainstream PN.

Resilience of anammox application from sidestream to mainstream: A combined system coupling denitrification, partial nitritation and partial denitrification with anammox

Anaerobic ammonium oxidation (anammox) is a potential process to achieve the neutralization of energy and carbon. Due to the low temperature and variation of municipal sewage, the application of mainstream anammox is hard to be implemented. For spreading mainstream anammox in practice, several key issues and bottlenecks including the start-up, stable NO₂^--N supply, maintenance and dominance of AnAOB with high activity, prevention of NO₃^--N buildup, reduction of sludge loss, adaption to the seasonal temperature and alleviation of COD impacts on AnAOB are discussed and summarized in this review in order to improve its startup, stable operation and resilience of mainstream anammox. Hence a combined biological nitrogen removal (CBNR) system based on conventional denitrification, shortcut nitrification-denitrification, Partial Nitritation and partial Denitrification combined Anammox (PANDA) process through the management of organic matter and nitrate is proposed correspondingly aiming at adaptation to the variations of seasonal temperature and pollutants in influent.

Mechanism of suspended sludge impact on anammox enrichment in anoxic biofilm through long term operation and microbial analysis

The basic premise of anammox-technical application reliability in municipal wastewater treatment is substantially enriched anammox bacteria. To enrich the anammox, the special interaction mechanism between the suspended sludge (SS) and anoxic biofilm was investigated over three months in a partial denitrification/anammox biosystem subjected to dynamic changes in SS (absence→ presence→ absence). Results show that the introduction of SS significantly decreased the anammox nitrogen removal efficiency (83.8 ± 6.5%→ 48.7 ± 17.0%). With the presence or absence of SS, the spatial distribution of anammox bacteria within the anoxic biofilm gradually changed between the inner and outer layers, as detected by CLSM-FISH. qPCR and metagenomic sequencing show that changes in the presence and absence status of SS significantly reduced the abundance of the NO reducing functional gene, while the NO supply capacity (NO₃^-→NO) was improved, further favoring the anammox process. Batch tests and typical cycles further demonstrated that the anammox bacteria can stably acquire NO₂^-, and anammox bacteria in the anoxic biofilm competed far more NO₂^- than denitrifying bacteria according to the typical pH curve. Accordingly, the abundance of Candidatus Brocadia, as detected by high throughput sequencing, decreased in the anoxic biofilms with the introduction of SS, but greatly increased (0.82%→2.22%) after SS discharge. This study sheds new light on the high in-situ enrichment of anammox in mainstream.

Advanced nitrogen removal from mature landfill leachate based on novel step-draining partial nitrification-denitrification and Anammox process: Significance of low volume exchange ratio

To save external carbon source dosage and simplify NH4+ to NO2- ratio control strategy, this study established a novel step-draining based partial nitrification-denitrification and Anammox (PND-AMX) system for advanced nitrogen removal from mature landfill leachate. Separation of partial nitrification and denitrification was realized based on step-draining, achieving 74.8 % nitrogen removal. 25 % was the optimal volume exchange ratio for synergistic removal of organics and nitrogen, allowing full use of carbon source. NH4+ to NO2- ratio was easily controlled by varying the volume ratio of the first and second effluent of PND reactor. Brocadia, Kuenenia and Jettenia collectively accounted for 13.61 % in AMX reactor, contributing 21.0 % of nitrogen removal. Nitrogen removal efficiency and nitrogen removal rate reached 98.3 ± 1.2 % and 3.07 ± 0.09 kgN/(m3∙d), respectively. Partial Anammox process based on step-draining was easier to realize and of practical significance for application in treatment of landfill leachate.

Future trends and patterns in leachate biological treatment research from a bibliometric perspective

Leachate has become a great deal of concern due to its complex properties which are primarily caused by the high concentrations of organics and ammonia. Thus, proper leachate treatment is required prior to its discharge. Leachate can be treated in various ways, and biological treatment is one of the approaches. This treatment has been shown to be both effective and cost-efficient while offering the possibility of resource recovery in the form of bioenergy. In this study, the underlying patterns in publications related to leachate biological treatment were uncovered through bibliometric analysis. This study also lays the groundwork for a deeper understanding of the past, current, and future trends of the leachate biological treatment. Research publications from 1974 to 2021 were retrieved from the Scopus database, and it was identified that 2013 articles were published in the span of 47 years. From the analyzed publications, China played a leading role in publishing leachate biological treatment research articles as well as having the most productive institutions and authors. Meanwhile, the USA was found to be the most active country in initiating international collaborations with 33 countries. The research hotspots were also successfully identified using keyword co-occurrences analysis. Anaerobic digestion and constructed wetland were revealed to be the research hotspots. The critical role of biological treatment in removing nitrogen from leachate was also highlighted. Besides, numerous research gaps were identified in the application of aerobic granular sludge (AGS) for leachate treatment. This can be a potential area for research in the future. Finally, future research should be encouraged to focus on the use of sustainable treatment systems in which energy recovery in the form of biogases is promoted.

Effects of Caffeine and COD from Coffee Wastewater on Anaerobic Ammonium Oxidation (Anammox) Activities

An anaerobic ammonium oxidation (anammox) process was employed to remove nitrogen from wastewater generated from a coffee brewing facility. The effects of caffeine and chemical oxygen demand (COD) in coffee wastewater on anammox activity were investigated. The anammox activity was inhibited in synthetic wastewater with a caffeine concentration greater than 350 mg/L. Daily additions of caffeine at 2.5 mg/L for 28 days to the same substrate did not inhibit anammox activity. However, daily additions of coffee wastewater with COD of ≥387 mg/L and caffeine at 2.5 mg/L significantly inhibited anammox activity. Because the pH was increased in the system, resulting in an increase in free ammonia (FA) concentration, one could postulate that FA is an inhibitor of anammox activity. Quantitative polymerase chain reaction (qPCR) analysis was employed to determine the populations of anammox and denitrifying bacteria. Coffee wastewater with bacterial COD to total nitrogen (bCOD:TN) ratios of 0.3–0.6:1 did not have any effect on the abundances of anammox and denitrifying bacteria. The results from this work suggest that biodegradable COD (bCOD) rather than total COD (TCOD) should be used for calculating the COD:TN ratio during the study of the effects of nitrogen removal from real wastewaters using the anammox process. A not-competitive model could fit the anammox inhibition with caffeine concentrations at 50–500 mg/L with maximum specific anammox activity (SAAmax) of 0.594 mg-N/mg-volatile suspended solids (VSS)/d and inhibitory constant (Ki) of 480.97 mg/L.

Feeding low-level benzethonium chloride can promote the start-up, fast recovery and long-term stable maintenance of partial nitrification for low-ammonium wastewater

The establishment of stable partial nitrification (PN) is beneficial to promote the application of anaerobic ammonium oxidation, especially for low-ammonium wastewater. This study demonstrated an innovative approach for achieving PN through feeding low-level benzethonium chloride (BZC). PN was started and maintained for 125 days after the sequential feeding of 0.2 and 1 mg/L BZC for 50 days. The damaged PN recovered rapidly within eight days by feeding 2 mg/L BZC, and it thrived for more than 172 days, indicating that nitrite-oxidizing bacteria did not adapt to BZC. The removal of BZC changed from adsorption to degradation gradually. Increased extracellular polymeric substances secretion and altered protein secondary structures explained sludge granulation during BZC feeding, which may be closely related to long-term stable maintenance of PN. PICRUSt2 revealed the underlying microbial mechanisms in depth. Overall, this research proposed a novel scheme to achieve robust PN treating low-ammonium wastewater through feeding low-level BZC.

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

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

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

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

Emerging onsite electron donors for advanced nitrogen removal from anammox effluent of leachate treatment: A review and future applications

Partial nitrification-anammox process is promising in leachate treatment, but the 11% residue nitrate limits the total nitrogen removal efficiency. Denitrification or partial denitrification and anammox are both practical polishing processes of anammox effluent, requiring extra electron donors. Fortunately, there are organic matter, sulfide and methane in leachate or produced by leachate treatment, which can serve as onsite electron donors. In this review, the mechanisms and processes using these three kinds of electron donors for residue nitrate reduction in anammox effluent of leachate are systematically summarized and discussed. It can be concluded that, biodegradable organic matter is an effective electron donor, sulfide is a promising electron donor, methane is a potential electron donor. Two possible applications in future based on anammox treatment of fresh and mature leachate using sulfide and methane as onsite electron donors are proposed. Through sulfide reutilization, energy-saving with about 14% of aeration reduction can be achieved.

The evolving trends of landfill leachate treatment research over the past 45 years

Landfilling is one of the most prevalent waste management strategies on a global scale. However, one major drawback of landfills is the production of highly contaminated leachate which can harm ecosystems and pollute water resources which municipalities depend on. Various treatment options have been explored for treating leachate throughout the years. This paper aims to investigate landfill leachate treatment studies through bibliometric and visual analysis of articles collected from the Scopus database from 1975 to 2019. The results show that more than one thousand papers have been published in 159 journals. Nearly 3000 authors have published on the topic, most commonly from China, the USA, Malaysia, and Brazil. The keywords co-occurrence network led to the formation of 5 topical clusters, after which they were closely examined and titled "oxidation processes," "nitrogen removal processes," "constructed wetlands/coagulation," "adsorption," and "membrane-based processes" based on their contents. Finally, the overlay network was used to discern the hottest research trends. The results showed that advanced coagulation-based processes (such as peroxicoagulation) and studies focusing on improving membrane-based processes through reduction of membrane fouling are focal points which will be trending into the future.

Improving stability of mainstream Anammox in an innovative two-stage process for advanced nitrogen removal from mature landfill leachate

This study presents an innovative mainstream Anammox based on multiple NO₂^--N supplement pathways to treat actual mature landfill leachate over 180 days. Desirable effluent quality of 11.8 mg/L total nitrogen (TN) and nitrogen removal efficiency of 98.8% were achieved despite fluctuation conditions of 1.5-fold influent substrates and 8.0-fold dissolved oxygen overload. Nitrogen mass balance confirmed Anammox was the dominant nitrogen removal pathway, contributing up to 87.9%. Functional genes of ammonia monooxygenase (amoA), hydrazine synthase (hzsB), and ratio of nitrate/nitrite reductase were highly detected. Anammox genera, Candidatus_Kuenenia (4.1%) and Candidatus_Brocadia (5.3%) were dominant in two functional systems, respectively, due to the different affinity of nitrite, oxygen, and organic carbon. As an economical and sustainable technology, the innovative process enabled a 95.1% decrease in organic carbon demand, a 61.5% reduction in aeration energy consumption, and 77.6% lower biomass production compared with traditional nitrification-denitrification process.

New insights into co-treatment of mature landfill leachate with municipal sewage via integrated partial nitrification, Anammox and denitratation

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

Simultaneous nitrogen and phosphorus removal from simulated digested piggery wastewater in a single-stage biofilm process coupling anammox and intracellular carbon metabolism

A Single-stage biofilm process coupling Anammox and Intracellular Carbon metabolism (SAIC) was constructed for treating simulated digested piggery wastewater with low carbon/nitrogen ratio (C/N) in this study. TN removal in SAIC system increased by more than 12.77% compared to the reference, and the maximum total phosphorus (TP) removal efficiency reached to 83.70% (C/N = 1.5). Denitrification driven by intracellular carbon, mainly poly-β-hydroxybutyrate (PHB, 78.57%), contributed 32.60% of TN elimination at most, and at least 67.40% should be attributed to anammox. Phosphorus was thought to be mainly removed through biological route, while chemical precipitation also explained around 10% of removed TP. Furthermore, commensalism of glycogen accumulating organisms (GAOs), phosphate accumulating organisms (PAOs), nitrifiers and anammox bacteria was revealed by combining 16S rRNA amplicon sequencing and metagenomics. As a result, multiple metabolic pathways including anammox, (partial) nitrification, endogenous (partial) denitrification and biological P-removal played synergistic effect in SAIC system.

A survey on experiences in leachate treatment: Common practices, differences worldwide and future perspectives

The necessity for landfill leachate treatment is a requisite to reduce the environmental impact related to municipal solid waste landfills and different aspects must be considered while deciding for an appropriate treatment process. For example, it was demonstrated that the landfill leachate stabilization in tropical regions is achieved right after its first year of operation, requiring technologies capable of treating leachates of a higher recalcitrant character if compared to those leachates from temperate regions and same landfill age. In view of its complexity and variability, stand-alone processes (either biological or physicochemical) are often ineffective in attaining the threshold values for its discharge in receiving bodies. Due to that fact, full-scale facilities have adopted integrated routes, harvesting the benefits of both biological and physicochemical processes. The implementation of membrane bioreactors followed by polishing membrane separation process (nanofiltration and reverse osmosis) seems to be a trend in leachate treatment by full-scale treatment plants. This technology is widely employed in China, European countries, and tropical countries as Brazil, generally with a treatment cost lower than the costs related to its disposal in domestic effluent collection systems. From the technologies already employed by full-scale facilities, four integrated routes were proposed for a sensitive analysis considering the treatment of a landfill leachate of different physicochemical characteristics. From all routes, those employing the membrane separation process as a polishing step had a better efficacy in attaining the threshold values for leachate disposal, being that an interesting alternative for leachate polishing by full-scale facilities.

An Analysis of Operation Conditions and Microbial Characteristics in Swine Wastewater Treatment Plants with Spontaneously Enriched Anammox Bacteria

The spontaneous enrichment of anammox bacteria has been reported in swine wastewater treatment facilities. However, their causative conditions and microbial characteristics, which this study aims to explain, are poorly understood. We discovered eight treatment facilities where the collected red biofilms exhibited high anammox activity levels at 57–843 µmol-N2/g-ignition loss (IL)/h and anammox DNA concentrations of 4.3 × 108–1.6 × 1012 copies/g-IL. The facilities used various wastewater treatment methods—six of them employed a multi-stage continuous reactor, whereas aeration tanks were continuously aerated at another combination of six facilities. Levels of dissolved oxygen (DO) in these tanks were fairly low at ≤1 mg/L. Pyrosequencing of the biofilms indicated the presence of 3–62.5% Planctomycetes, and the dominant anammox in each biofilm comprised three operational taxonomic units (OTUs) similar to Candidatus Jettenia asiatica, Ca. Brocadia fulgida, and Ca. B. caroliniensis. This suggested that some particular species of anammox bacteria naturally thrive when operating a swine wastewater treatment facility at low DO levels. The frequent enrichment of anammox biofilms at the sampled sites indicated that these treatment facilities were good seed sources of anammox; therefore, anammox treatment would be a viable method for the removal of nitrogen from swine wastewater.

Spontaneous mainstream anammox in a full-scale wastewater treatment plant with hybrid sludge retention time in a temperate zone of China

Spontaneous anammox bacteria enrichment at mainstream conditions was reported in a full-scale Wastewater Treatment Plant (WWTP) in a temperate zone of China. The mainstream anammox was observed after WWTP process retrofit, which constructed a hybrid sludge retention time (SRT) system by providing moving carriers in the anaerobic/anoxic tank and was initially designed to enhance the denitrification process in a conventional anaerobic/anoxic/oxic process. The hybrid SRT system achieved 86.0 ± 4.6% total nitrogen (TN) removal via combined mainstream anammox and conventional denitrification. Autotrophic denitrification via mainstream anammox was confirmed by various shreds of evidence including high-throughput sequencing, specific anammox activity test, and ¹⁵ N isotopic tracing. Long-term anammox bacteria existence in the biofilm of the carrier in anoxic zones was detected in a much higher relative abundance compared with other spots. The contribution of anammox activity to TN removal was estimated at around 20%-30%. The reasons leading to spontaneous anammox enrichment were mainly attributed to the carriers for slow-growing bacteria growth and dissolved oxygen gradient in the anoxic tank (caused by intermittent aeration) for nitrite production. The insights of this full-scale case study provide important perspectives for future mainstream anammox application, and also the design of an energy-neutral WWTP process. PRACTITIONER POINTS: Spontaneous mainstream anammox in a full-scale WWTP after its retrofit in a temperate zone of China was reported. Anammox bacteria enrichment and long-term stability on moving carriers at mainstream conditions was achieved by modified hybrid SRT system. The hybrid SRT system achieve stable nitrogen removal even in cold winter and high BOD/N situation by combining mainstream anammox with conventional denitrification. Long term full-scale operation demonstrated excellent nitrogen removal with about 20%-30% contribution of mainstream anammox. This full-scale case study provided perspectives for future optimizing mainstream anammox application, and also energy-neutral WWTP process design.

Scaling-Up and Long-Term Operation of a Full-Scale Two-Stage Partial Nitritation-Anammox System Treating Landfill Leachate

(1) Background: Biological treatment of leachate in landfill sites using anaerobic ammonium oxidation (anammox) is challenging because of the intrinsic characteristics of this complex wastewater. In this work, the scale-up and subsequent full-scale implementation of the PANAMMOX® technology (LEQUIA Research Group, Girona, Catalonia, Spain) are presented as a case study to achieve long-term nitrogen (N) removal from mature leachate mostly through a completely autotrophic pathway. (2) Methods: The treatment system consists of two sequencing batch reactors (SBRs) running in series to individually operate partial nitritation (PN) and anammox (A). Following biological treatment, physicochemical oxidation (i.e., Fenton-based process) was used to remove the remaining non-biodegradable organic matter. A cost analysis comparative was conducted in relation to the former technology used on-site for treating the leachate. (3) Results: The scale-up of the process from pilot- to full-scale was successfully achieved, finally reaching an average removal of 7.4 kg N/d. The composition of the leachate changed over time, but especially once the landfill site stopped receiving solid waste (this fact involved a marked increase in the strength of the leachate). The adjustment of the alkalinity-to-ammonium ratio before feeding PN-SBR helped to improve the N-removal efficiency. Values of conductivity above 25 mS/cm in A-SBR could negatively affect the performance of the anammox process, making it necessary to consider a dilution strategy according to the on-line monitoring of this parameter. The analysis of the operational costs showed that by implementing the PANAMMOX® technology (LEQUIA Research Group, Girona, Catalonia, Spain) in the landfill site, savings up to 32% were achievable. (4) Conclusions: Treatment of mature landfill leachate in such a two-stage PN-A system was demonstrated as feasible and economically appealing despite the complexity of this industrial wastewater. Accurate expert supervision of the process was a key factor to reaching good performances.

Robust Nitritation of Anaerobic Digester Centrate Using Dual Stressors and Timed Alkali Additions

Nitrogen is commonly removed from wastewater by nitrification to nitrate followed by nitrate reduction to N₂. Shortcut N removal saves energy by limiting ammonia oxidation to nitrite, but nitrite accumulation can be unstable. We hypothesized that repeated short-term exposures of ammonia-oxidizing communities to free ammonia (FA) and free nitrous acid (FNA) would stabilize nitritation by selecting against nitrite-oxidizing bacteria (NOB). Accordingly, we evaluated ammonium oxidation of anaerobic digester centrate in two bench-scale sequencing batch reactors (SBRs), seeded with the same inoculum and operated identically but with differing pH-control strategies. A single stressor SBR (SS/SBR) using pH set-point control produced HNO₃, while a dual stressor SBR (DS/SBR) using timed alkalinity addition (TAA) produced HNO₂ (ammonium removal efficiency of 97 ± 2%; nitrite accumulation ratio of 98 ± 1%). The TAA protocol was developed during an adaptation period with continuous pH monitoring. After adaptation, automated TAA enabled stable nitritation without set-point control. In the SS/SBR, repeatedly exposing the community to FA (8-10 h/exposure, one exposure/cycle) selected for FA-tolerant ammonia-oxidizing bacteria (Nitrosomonas sp. NM107) and NOB (Nitrobacter sp.). In the DS/SBR, repeatedly exposing the community to FA (2-4 h/exposure, three exposures/cycle) and FNA (4-6 h/exposure, two exposures/cycle) selected for FA- and FNA-resistant AOB (Nitrosomonas IWT514) and against NOB, stabilizing nitritation.

Probing the dynamics of three freshwater Anammox genera at different salinity levels in a partial nitritation and Anammox sequencing batch reactor treating landfill leachate

Partial nitritation/Anammox was applied to treat NaCl-amended landfill leachate. The reactor established robust nitrogen removal of 85.7 ± 2.4% with incremental salinity from 0.61% to 3.10% and achieved 0.91-1.05 kg N/m³/d at salinity of 2.96%-3.10%. Microbial community analysis revealed Nitrosomonas, Nitrospira, and denitrifiers occupied 4.1%, <0.2% and 10.9%, respectively. Salinity variations impelled the dynamics of Anammox bacteria. Jettenia shifted to Brocadia and Kuenenia at salinity of 0.61%-0.81%. Kuenenia outcompeted Brocadia and occupied 51.5% and 50.9% at salinity of 1.48%-1.54% and 2.96%-3.10%, respectively. High nitrite affinity and fast growth rate were proposed as key factors fostering Brocadia overgrew Jettenia. Functionalities of sodium-motive-force facilitated energy generation and intracellular osmotic pressure equilibrium regulation crucially determined Kuenenia's dominance at elevated salinity. Co-occurrence network further manifested beneficial symbiotic relationships boosted Kuenenia's preponderance. Knowledge gleaned deepen understanding on survival niches of freshwater Anammox genera at saline environments and lead to immediate benefits to its applications treating relevant wastewaters.

Optimization of the intermittent aeration to improve the stability and flexibility of a mainstream hybrid partial nitrification-anammox system

Intermittent aeration is favorable for maintaining a long-term sewage partial nitrification-anammox (PN/A) process but the underlying mechanism is not yet fully understood. In this study, mainstream PN/A was established in an integrated fixed film activated sludge (IFAS) PN/A reactor and nitrite oxidization bacteria (NOB) activity was continuously suppressed. The suppression of NOB was significantly affected by the dissolved oxygen (DO) concentration during the aeration period as well as the duration of anoxic period. NOB was more suppressed in the hybrid system under a low DO level (0.5 mg/L) than under a high DO level (1.5-1.8 mg/L). Meanwhile, shortening the anoxic time from 40 to 20 min and keeping low DO during the intermittent aeration cycle could still suppress NOB activity, increasing the nitrogen removal rate by 40%. Biomass segregation was also enhanced by low DO, which favors the NOB inhibition in IFAS PN/A system. Overall, under an optimized intermittent aeration, a stable and high nitrogen removal efficiency (80-89%) with a nitrogen removal rate of 0.101 kg-N/(m³·d). This study is useful to supports the application of PN/A in sewage treatment.

Achieving stable mainstream deammonification process by a novel combinatorial control strategy

In this study, a novel combinatorial control strategy was developed to guarantee a stable mainstream deammonification process, with three critical steps including (a) upflow airwater washing, (b) short-term increased nitrogen loading rate (NLR), and (c) low oxygen supply. Results showed that two upflow double-blanket filter (UDBF) reactors effectively performed the mainstream deammonification process with the nitrogen removal efficiency (NRE) 84.5 ± 2.2% and 84.6 ± 1.6%, respectively and nitrogen removal rate (NRR) 123.8 ± 2.9 and 125.5 ± 6.2 g N·(m³·d)^-1, respectively. Statistically, temperature and C/N were considered as two vital factors affecting the nitrogen removal pathways, which co-explained 80.9% and 78.4% of the maximum possible contribution of heterotrophic denitrification in both reactors. The deammonification process accounted for more than 59.8% of TN removal in R2 and 54.8% in R1, which cooperated well with heterotrophic denitrification for efficient performance in treating municipal sewage.

Rapid start-up of partial nitrification process using benzethonium chloride-a novel nitrite oxidation inhibitor

Benzethonium chloride (BZC) is an antibacterial compound with extensive applications in various anti-infective products. However, the feasibility of attaining partial nitrification of municipal wastewater using BZC has not been reported. In this work, BZC was used for the first time to attain partial nitrification. Batch experiments indicated nitrite oxidizing bacteria (NOB) was more vulnerable to BZC than ammonia oxidizing bacteria (AOB). When activated sludge was treated only once with 0.023 g BZC·(g MLSS)^-1 for 18 h, partial nitrification was attained at the 29th cycle with NAR of 97.46% and sustained 91 cycles in stability tests. Complimentary DNA sequencing analysis revealed the suppression of Nitrospira was the reason for partial nitrification. Oligotyping analysis indicated AOB could likely resist to BZC by both the species shifts and development of tolerance, while most NOB species could not adapt to BZC. This study revealed the feasibility of BZC as a novel NOB inhibitor.

Unclassified Anammox bacterium responds to robust nitrogen removal in a sequencing batch reactor fed with landfill leachate

Treatment of landfill leachate was conducted in a lab-scale sequencing batch reactor (SBR). The SBR was started through inoculating activated sludge with controlling dissolved oxygen of 0.5-1.0 mg/L. Anammox reaction took place within around three months. The SBR established robust nitrogen removal with incremental NLRs of 0.25-2.17 kg N/m³/d. At the final phase, it achieved elevated nitrogen removals of 1.68-1.91 kg N/m³/d. 16S rRNA gene amplicon sequencing analysis revealed Nitrosomonas, unclassified Anammox bacterium, and diverse denitrifying populations coexisted and accounted for 4.02%, 20.05% and 34.69%, respectively. Phylogenic analysis and average nucleotide identity comparison jointly suggested the unclassified Anammox bacterium potentially pertained to a novel Anammox lineage. The functional profiles' prediction suggested sulfate reduction, arsenate reduction and eliminations of antibiotics and drugs likely occurred in the SBR. The finding from this study suggests contribution of unclassified Anammox bacteria in influencing nitrogen budget in natural and engineering systems is currently being underestimated.

In situ startup of a full-scale combined partial nitritation and anammox process treating swine digestate by regulation of nitrite and dissolved oxygen

A challenge during the startup of the combined partial nitritation and anammox process is how to balance dissolved oxygen control and nitrite accumulation for converting partial nitritation into anammox, maintaining stable partial nitritation and promoting growth of anammox bacteria. An innovative regulation strategy of nitrite dosing and dissolved oxygen control in this study was developed to achieve the rapid startup of a full-scale combined partial nitritation and anammox reactor within 77 days and the total nitrogen removal rate of reactor was 0.097 kg N/kgMLSS·d^-1, and the activity and gene copy concentration of anammox bacteria reached 0.307 kg N/kgMLVSS·d^-1 and 7.79 × 10⁹ copies/gMLVSS, respectively. Microbial community analysis revealed that Candidatus_Kuenenia and Nitrosomonas were the dominant nitrogen transformation bacteria with an abundance of 2.49% and 14.86%, respectively. This study offers a new method for rapid startup and spreading application of the full-scale anammox process.

Nanoarchitectured structure and population dynamics of anaerobic ammonium oxidizing (anammox) bacteria in a wastewater treatment plant

Studies on microbial community and population dynamics are essential for the successful development, monitoring and operation of biological wastewater treatment systems. Especially for novel or sustainable systems such as the anaerobic ammonium oxidizing (anammox) process that are not yet well explored. Here we collected granular microbial sludge samples and investigated a community of anammox bacteria over a period of four years, divided into eight stages in a full scale simultaneous partial nitrification, anammox and denitrification (SNAD) process for treating landfill leachate. Specific qPCR primers were designed to target and quantify the two most abundant anammox species, Candidatus Kuenenia stuttgartiensis (KS) and Candidatus Brocadia anammoxidans (BA). The two species were monitored and could explain the dynamic shift of the anammox community corresponding to the operating conditions. Using the newly designed KS-specific primer (KSqF3/KSqR3) and BA-specific primer (BAqF/BAqR), we estimated the amounts of KS and BA to be in the range of 6.2 × 10⁶ to 5.9 × 10⁸ and 1.1 × 10⁵ to 4.1 × 10⁷ copies μg^-1 DNA, respectively. KS was found to be the dominant species in all anammox granules studied and played an important role in the formation of granules. The KS/BA ratio was positively correlated to the size of granules in the reactor and ammonia nitrogen removal efficiency of the treatment plant.

Nitrogen management in landfill leachate using single-stage anammox process-illustrating key nitrogen pathways under an ecogenomics framework

Simultaneous functional gene expressions using mRNA, rate measurements, and biochemical analysis proved the consistent contribution of ammonia oxidizers, heterotrophic denitrifiers, and anammox bacteria in a single-stage attached growth partial nitritation/anammox system for nitrogen management in landfill leachate. Average removal efficiencies of ammonia-nitrogen, total inorganic nitrogen, and COD were 94%, 88%, and 26%, respectively, in the reactor. Off-gas N₂O fluxes increased at relatively higher dissolved oxygen. Batch activity tests revealed the occurrence of significant anammox activity even in the presence of high concentrations of organic carbon in the influent. mRNA based functional expressions of nitrite reductase (nirK and nirS) and hydrazine synthase (hzsA) suggested simultaneous active heterotrophic denitrification and anammox, respectively. 16S rRNA amplicon sequencing revealed Proteobacteria (36-56%), Planctomycetes (10-31%), and Bacteroidetes (6-39%) as dominant phyla in the reactor. Candidatus brocadia was observed as the most abundant genus representing anammox community.

Efficient transition from partial nitritation to partial nitritation/Anammox in a membrane bioreactor with activated sludge as the sole seed source

A lab-scale membrane bioreactor (MBR) was employed to carry out the partial nitritation/Anammox (PN/A) process from conventional activated sludge. Seed sludge was cultivated under microaerobic conditions for 10 days before seeding into the MBR. The bacterial community was analyzed on the basis of cloning and sequencing of 16S rRNA gene. Relative slow ammonia oxidation rates (3.2-13.0 mgN/L/d) were established in the microaerobic cultivation period. In the continuous MBR operation, the nitritation was achieved in the first 16 days and the reactor produced a balanced ratio between ammonia and nitrite which favored the proliferation of Anammox bacteria. Efficient transition from PN to PN/A was achieved in two months which was supported by appearance of reddish spots on the reactor inner wall and the concurrent consumption of ammonium and nitrite. The PN/A performed a robust and high-rate nitrogen removal capability and achieved a peak nitrogen removal of 1.81 kg N/m³/d. 16S rRNA gene-based analysis indicated that "Nitrosomonas sp." and "Candidatus Jettenia sp." accounted for ammonia oxidation and nitrogen depletion, respectively. Denitratisoma facilitated denitrification in the reactor. The present study suggested that a pre-cultivation of seed sludge under microaerobic conditions assists fast realization of PN and further convoyed efficient transition from PN to PN/A. Knowledge gleaned from this study is of significance to initiation, operation, and control of MBR-PN/As.

Energy-Efficient Single-Stage Nitrite Shunt Denitrification with Saline Sewage through Concise Dissolved Oxygen (DO) Supply: Process Performance and Microbial Communities

Single-stage nitrite shunt denitrification (through nitrite rather than nitrate) with low dissolved oxygen (DO) supply is a better alternative in terms of energy-efficiency, short-footprint, and low C/N-ratio requirement. This study investigates the optimal DO level with temperature effect, with saline sewage at the fixed hydraulic and solids retention times of 8 h and 8 d, respectively. Moreover, 16S rRNA gene sequencing analysis corresponding with total nitrogen (TN) and chemical oxygen demand (COD) removals in each operating condition were performed. Results showed that DO of 0.3 mg/L at 20 °C achieved over 60.7% and over 97.9% of TN and COD removal, respectively, suggesting that such condition achieved effective nitrite-oxidizing bacteria inhibition and efficient denitrification. An unexpected finding was that sulfur-reducing Haematobacter and nitrogen-fixing Geofilum and Shinella were highly abundant with the copredominance of ammonia-oxidizing Comamonas and Nitrosomonas, nitrite-oxidizing Limnohabitans, and denitrifying Simplicispira, Castellaniella, and Nitratireductor. Further, canonical correspondence analysis (CCA) with respect to the operating conditions associated with phenotype prediction via R-based tool Tax4Fun was performed for a preliminary diagnosis of microbial functionality. The effects of DO, temperature, nitrite, and nitrate in various extents toward each predominant microbe were discussed. Collectively, DO is likely pivotal in single-stage nitrite shunt denitrification, as well as microbial communities, for energy-efficient saline sewage treatment.

Biodegradable organic matter-containing ammonium wastewater treatment through simultaneous partial nitritation, anammox, denitrification and COD oxidization process

For both nitrogen and COD removal from biodegradable organic matter (BOM)-containing ammonium wastewater, the simultaneous partial nitritation, anammox, denitrification and COD oxidization (SNADCO) process is a promising solution. In this study, with the stable influent ammonium concentration of 250.0 mg/L (nitrogen loading rate of 0.5 kg/m³/d) and the variation of influent COD/NH₄⁺-N (C/N) ratio from 0.0 to 1.6, the performance of the SNADCO process in a one-stage carrier-packing airlift reactor with continuous mode was investigated for the first time. The results showed that until the C/N ratio of 0.8, both the well nitrogen and COD removal targets could be reached. Mass balance calculations indicated that the average nitrogen removal efficiency (NRE) of 80.9% achieved at the C/N ratio of 0.8 were due to both the anammox and denitrification pathways. Correspondingly, the achieved average COD removal efficiency of 94.6% was attributed to both the denitrification and COD oxidization pathways. Based on the specific sludge activity tests and Fluorescence in Situ Hybridization observation, anammox and denitrification bacteria were mainly distributed in the biofilm sludge, while ammonium oxidizing bacteria and ordinary heterotrophic organisms were mainly in the suspended sludge. At the C/N ratio of 1.6, the washout of suspended sludge became serious while the biofilm sludge was well retained, resulting in inefficient nitritation and a subsequent decrease in NRE. The microbial interaction analysis provided a clear explanation of the performance change of the SNADCO process under different C/N ratios. This research enriches the knowledge of the SNADCO process in BOM-containing ammonium wastewater treatment.

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

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

Insight into the impacts of organics on anammox and their potential linking to system performance of sewage partial nitrification-anammox (PN/A): A critical review

Partial nitrification-anammox (PN/A) is an energy-efficient process for nitrogen removal from sewage. The influent organics of sewage is usually pre-removed, reducing the risk for enriching anammox bacteria (AnAOB). However, recent studies demonstrate that optimum influent organics could improve nitrogen removal and operational stability of PN/A. Thus, the impact of organics on PN/A-based process should not be overlooked. In this review, the complicated impacts of organics-containing influent on anammox and their linking to apply PN/A are discussed. Firstly, the effect of organics on AnAOB metabolism and the competition relationship between AnAOB and heterotrophic bacteria are summarized. Secondly, the combined effects of influent organics and operational strategies on PN/A-based process were reviewed. Thirdly, how to control influent organics in the real application of PN/A were discussed. Lastly, recent development of the PN/A-based process combined with denitrification were reviewed. Overall, influent organics could be an essential factor for successful application of sewage PN/A.

The short- and long-term effects of formic acid on rapid nitritation start-up

The feasibility of achieving stable nitritation inoculating with activated sludge by adding formic acid was studied in this work. Short-term batch effects of formic acid on nitrification showed that the nitrite accumulation ratio (NAR) significantly increased from 0.3% to 83.7% with an increase of formic acid concentration from 0 to 50 mM at an initial ammonia concentration of 75 mg·L^-1, which was demonstrated to be due to the inhibition of nxrB transcription in nitrite oxidizing bacteria (NOB). The long-term effects of formic acid at 30 mM were constantly monitored in an aerobic sequencing batch reactor. During 27 days of operation, the NAR was rapidly raised and maintained approximately 90%. What's more, in the following 52 days without addition of formic acid, the NAR was kept above 91.3%. The sustained suppression of NOB genus Nitrospira coupling nxrB inhibition was the main reason to maintain stable nitritation. These results supported the feasibility of formic acid as an efficient nitritation regulator, thus providing a new approach for the development of the BNR process via nitrite pathway.

The effect of different denitrification and partial nitrification-Anammox coupling forms on nitrogen removal from mature landfill leachate at the pilot-scale

The effects on nitrogen removal from landfill leachate were compared between the denitrification (DN) direct coupling in Partial nitrification (PN)-Anammox (DN+(PN-Anammox)) and pre-DN followed by PN-Anammox (DN-PN-Anammox). Both processes can achieve coupling and high nitrogen removal. However, the DN+(PN-Anammox) process was not conducive to the treatment of high-COD wastewater. The total nitrogen removal rate (TNRR) and total nitrogen removal efficiency (TNRE) were stable at 0.31 kg/(m³·d) and 76.3%. When UASB was added to denitrification and transform the process into DN-PN-Anammox, the influent NH₄⁺-N and COD concentrations were increased to 2230 and 2612 mg/L, TNRR and TNRE reached 0.45 kg/(m³·d) and 96.7%, respectively. The DN-PN-Anammox process not only was able to make full use of degradable COD in wastewater to realize the NO₃^--N removal, but also benefited the growth of autotrophic bacteria. The DN-PN-Anammox reduced the oxygen supply and was more conducive to the treatment of highly-concentrated mature landfill leachate.

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.

A continuous-flow combined process based on partial nitrification-Anammox and partial denitrification-Anammox (PN/A + PD/A) for enhanced nitrogen removal from mature landfill leachate

A novel continuous-flow combined process of partial nitrification, Anammox (PN/A) and partial denitrification-Anammox (PD/A) was established to achieve enhanced nitrogen removal from landfill leachate. The NH₄⁺-N transformation rate and NO₂^--N accumulation rate in the PN reactor reached 93.4% and 91.5%, respectively. The nitrite generated from the PN reactor was combined with influent (38%) and fed into the Anammox reactor. The nitrate produced in the Anammox reactor was then discharged to PD/A reactor, where nitrate was transformed to nitrite and removed via Anammox. Under a COD/NO₃^--N ratio of 4.0, the NO₃^--N-to-NO₂^--N transformation ratio (NTR) and Anammox contribution rate reached 60.4% and 57.1% in PD/A reactor. The final effluent TN concentration was 15.7 mg/L, and the efficiency of TN removal could reach 98.8%. By combining PN/A with PD/A, enhanced nitrogen removal from landfill leachate was achieved successfully with an external carbon source addition (COD/NH₄⁺-N) of 0.28.

Treatment of anaerobically digested effluent from kitchen waste using combined processes of anaerobic digestion-complete nitritation-ANAMMOX based on reflux dilution

In this study, an anaerobically digested effluent from kitchen waste with high concentrations of chemical oxygen demand (COD) and ammonia nitrogen was treated using combined processes of anaerobic digestion (AD), complete nitritation (CN), and anaerobic ammonium oxidation (ANAMMOX). The COD and nitrogen removal efficiency of each treatment unit were investigated. The feasibility of using the final treatment effluent to dilute the original wastewater was also discussed. Findings showed that as a pretreatment step, AD resulted in the decline in biodegradability and increase in NH 4 + - N concentration. CN was successfully and stably achieved for 106 days with an average nitritation rate of 95% by maintaining the dissolved oxygen at 2-3 mg/L and hydraulic retention time of 24 hr under 30 ± 1°C. High NH 4 + - N and NO 2 - - N . removal efficiencies of over 88% and 96% were attained in the following ANAMMOX reactor. The reflux of ANAMMOX-treated effluent for the dilution of raw wastewater or an influent of CN and ANAMMOX ensured the stable operation of the combined system. PRACTITIONER POINTS: Anaerobic digestion effluent of kitchen waste had low COD/ NH 4 + - N ratio and poor biodegradability. Stable and efficient nitritation was realized by controlling DO, HRT and TEMP. High NH 4 + - N and NO 2 - -N removal efficiency were obtained by ANAMMOX process. Average nitrogen removal rate of 0.94 kg N/m³ /day were obtained by ANAMMOX. Reflux dilution with the effluent guaranteed the system's successful operation.

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.

Anammox-based processes: How far have we come and what work remains? A review by bibliometric analysis

Nitrogen contamination remains a severe environmental problem and a major threat to sustainable development worldwide. A systematic analysis of the literature indicates that the partial nitritation-anammox (PN/AMX) process is still actively studied as a viable option for energy-efficient and feasible technology for the sustainable treatment of N- rich wastewaters, since its initial discovery in 1990. Notably, the mainstream PN/AMX process application remains the most challenging bottleneck in AMX technology and fascinates the world's attention in AMX studies. This paper discusses the recent trends and developments of PN/AMX research and analyzes the results of recent years of research on the PN/AMX from lab-to full-scale applications. The findings would deeply improve our understanding of the major challenges under mainstream conditions and next-stage research on the PN/AMX process. A great deal of efforts has been made in the process engineering, PN/AMX bacteria populations, predictive modeling, and the full-scale implementations during the past 22 years. A series of new and excellent experimental findings at lab, pilot and full-scale levels including good nitrogen removal performance even under low temperature (15-10 °C) around the world were achieved. To date, pilot- and full-scale PN/AMX have been successfully used to treat different types of industrial sewage, including black wastewater, sludge digester liquids, landfill leachate, monosodium glutamate wastewater, etc. Supplementing the qualitative analysis, this review also provides a quantitative bibliometrics study and evaluates global perspectives on PN/AMX research published during the past 22 years. Finally, general trends in the development of PN/AMX research are summarized with the aim of conveying potential future trajectories. The current review offers a valuable orientation and global overview for scientists, engineers, readers and decision makers presently focusing on PN/AMX processes.

Hydroxylamine addition and real-time aeration control in sewage nitritation system for reduced start-up period and improved process stability

Sewage nitritation is a promising process for nitrogen removal, but its practical application is limited by long start-up period and unstable operation. In this study, hydroxylamine (NH₂OH) addition and real-time aeration control strategies were adopted for the promotion of sewage nitritation in a sequencing batch reactor. Initially, 4.5 mg/L NH₂OH was added into reactor every 24 h to establish nitritation, increasing the nitrite accumulation ratio (NAR) from 1.7% to 93% in 19 d. In the following period, NH₂OH addition was stopped and nitritation remained stable over 55 d, with NAR of 97% by real-time aeration control. The aeration duration was determined by characteristic points on pH curve. The main genera of nitrite oxidizing bacteria, Nitrobacter and Nitrospira, were both eliminated from the system, supporting the long-term stability of nitritation. Overall, NH₂OH addition and real-time aeration control is an excellent strategy for establishing and maintaining effective sewage nitritation.

Mass balance and bacterial characteristics in an in-situ full-scale swine wastewater treatment system occurring anammox process

A spontaneous development of full-scale anaerobic ammonium oxidation (anammox) process was seldom reported, and its operational parameters could supply references in actual applications. This engineered case indicated that anammox process was suitable for treating relatively high-strength ammonium and organics wastewater due to niche differentiation of biofilm. Results of isotope labelling showed that anammox contributed approximately 40% to N-loss in aerobic unit, but this value increased to 78.3% in anoxic tank. Mass balance showed that N-removal via anammox and denitrification pathways were 38.1 and 23.9 g m^-3 d^-1, and anammox rate was 1.6 times higher than denitrifiaction. The wild-type anammox granules had a high purity, with anammox accounting for 92.2%. Candidatus Brocadia was the predominant species. Mixing sludge had a higher oxygen tolerance compared with granules, although the latter had a higher anammox activity under anaerobic conditions. Moreover, physicochemical precipitation on the surface of granules may be related to granulation mechanism.

Improving Wastewater Nitrogen Removal and Reducing Effluent NOx - -N by an Oxygen-Limited Process Consisting of a Sequencing Batch Reactor and a Sequencing Batch Biofilm Reactor

A series of reactors including a sequencing batch reactor (SBR) and a sequencing batch biofilm reactor (SBBR) were used for nitrogen removal. The aim of this study was simultaneous removal of NH4+-N and NOx–-N from synthetic wastewater. In the novel proposed method, the effluent from SBR was sequentially introduced into SBBR, which contained 0.030 m3 biofilm carriers, so the system operated under a paired sequence of aerobic-anoxic conditions. The effects of different carbon sources and aeration conditions were investigated. A low dissolved oxygen (DO) level in the biofilm depth of the fixed-bed process (SBBR) simulated the anoxic phase conditions. Accordingly, a portion of NH4+-N that was not converted to NO3–-N by the SBR process was converted to NO3–-N in the outer layer of the biofilm in the SBBR process. Further, simultaneous nitrification and denitrification (SND) was achieved in the SBBR where NO2–-N was converted to N2 directly, before NO3–-N conversion (partial nitrification). The level of mixed liquid suspended solids (MLSS) was 2740 mg/l at the start of the experiments. The required carbon source (C: N ratio of 4) was provided by adding an internal carbon source (through step feeding) or ethanol. Firstly, as part of the system (SBR and SBBR), SBR operated at a DO level of 1 mg/l while SBBR operated at a DO concentration of 0.3 mg/l during Run-1. During Run-2, the system operated at the low DO concentration of 0.3 mg/l. When the source of carbon was ethanol, the nitrogen removal rate (RN) was higher than the operation with an internal carbon source. When the reactors were operated at the same DO concentration of 0.3 mg/l, 99.1 % of the ammonium was removed. The NO3–-N produced during the aerobic SBR operation of the novel method was removed in SBBR reactor by 8.3 %. The concentrations of NO3--N and NO2–-N in the SBBR effluent were reduced to 2.5 and 5.5 mg/l, respectively. Also, the total nitrogen (TN) removal efficiency was 97.5 % by adding ethanol at the DO level of 0.3 mg/l.

When C:N adjustment was carried out SND efficiency at C:N ratio of 6.5 reached to 99 %. The increasing nitrogen loading rate (NLR) to 0.554 kg N/m3 d decreased SND efficiency to 80.7 %.

Effects of short- and long-term exposures of humic acid on the Anammox activity and microbial community

Humic acid has a controversial effect on the biological treatment processes. Here, we have investigated humic acid effects on the Anammox activity by studying the nitrogen removal efficiencies in batch and continuous conditions and analyzing the microbial community using Fluorescence in situ hybridization (FISH) technique. The results showed that the Anammox activity was affected by the presence of humic acid at a concentration higher than 70 mg/L. In fact, in the presence of humic acid concentration of 200 mg/L, the Anammox activity decreased to 57% in batch and under continuous condition, the ammonium removal efficiencies of the reactor decreased from 78 to 41%. This reduction of Anammox activity after humic acid addition was highlighted by FISH analysis which revealed a considerable reduction of the abundance of Anammox bacteria and the bacteria living in symbiosis with them. Furthermore, a total inhibition of Candidatus Brocadia fulgida was observed. However, humic acid has promoted heterotrophic denitrifying bacteria which became dominant in the reactor. In fact, the evolution of the organic matter in the reactor showed that the added humic acid was used as carbon source by heterotrophic bacteria which explained the shift of metabolism to the favor of heterotrophic denitrifying bacteria. Accordingly, humic acid should be controlled in the influent to avoid Anammox activity inhibition.

Strategy of rapid start-up and the mechanism of de-nitrogen in landfill bioreactor

Nitrogen removal from landfill leachate via anaerobic ammonium oxidation (Anammox) process has been considered as an innovative and sustainable approach to the traditional nitrification and denitrification process. However, the various technologies for rapid start-up of Anammox are still being explored. In this study, two strategies (inoculating anaerobic sludge and without inoculation) were applied to treat landfill leachate based on biological nitrogen removal processes. The start-up and mechanism of de-nitrogen process in landfill bioreactor was explored using ¹⁵N stable isotopic tracing, quantitative polymerase chain reaction (qPCR) and high-throughput sequencing methods. Results showed that inoculating anaerobic sludge was beneficial to enhance the nitrogen removal at the initial stage (from day 10 to day 25), but no significant increase was found during days 25-55 (p > 0.05). ¹⁵N stable isotopic tracing demonstrated that the inoculation of sludge accelerated by denitrification other than Anammox. Inoculation of sludge was conducive to increase of ammonia-oxidizing bacteria (AOB)- amoA and niK genes. Thauera was the dominant genus for nitrogen removal due to inoculation of sludge in landfill bioreactor, whereas the abundance of Candidatus Kuenenia did not increase by inoculating the sludge. Moreover, seeding anaerobic sludge could not provide Anammox's ecological niches. The results will provide a scientific basis for the selection of suitable operational condition for the rapid start-up in the landfill bioreactor.

Metagenomics Response of Anaerobic Ammonium Oxidation (anammox) Bacteria to Bio-Refractory Humic Substances in Wastewater

Anammox-based processes have been widely applied for the treatment of wastewater (e.g., wastewater irrigation systems and constructed wetland) which consists of bio-refractory humic substances. Nonetheless, the impacts of bio-refractory humic substances on anammox consortia are rarely reported. In the present study, three identical lab-scale anammox reactors (i.e., HS0, HS1 and HS10), two of which were dosed with humic substances at 1 and 10 mg·L−1, respectively, were operated for nearly one year. The long-term operation of the reactors showed that the presence of humic substances in influent had no significant influence on nitrogen removal rates. Despite this, comparative metagenomics showed changes in anammox microbiota structure during the exposure to humic substance; e.g., the relative abundance of Candidatus Kuenenia was lower in HS10 (18.5%) than that in HS0 (22.8%) and HS1 (21.7%). More specifically, a lower level of humic substances (1 mg·L−1) in influent led to an increase of genes responsible for signal transduction, likely due to the role of humic substances as electron shuttles. In contrast, a high level of humic substances (10 mg·L−1) resulted in a slight decrease of functional genes associated with anammox metabolism. This may partially be due to the biodegradation of the humic substances. In addition, the lower dosage of humic substances (1 mg·L−1) also stimulated the abundance of hzs and hdh, which encode two important enzymes in anammox reaction. Overall, this study indicated that the anammox system could work stably over a long period under humic substances, and that the process was feasible for leachate treatment.