Pilot-scale evaluation of anammox-based mainstream nitrogen removal from municipal wastewater

Learning from microorganisms: using new insights in microbial physiology for sustainable nitrogen management

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To reduce nitrate to N₂ distinct nitrogen-oxide-reducing microorganisms function together.

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Detecting nirS, nirK or narG genes cannot be directly linked to NO and N₂O emission.

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Nitrogen-oxide-reducing specialists can be exploited to reduce NO and N₂O emission from wwtp.

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Aerobic methanotrophs and methane stripping must be considered for the application of N-DAMO.

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Ammonium recovery could be a more sustainable alternative to nitrogen removal.

Diverse nitrogen-transforming microorganisms with a wide variety of physiological properties are employed for biological nitrogen removal from wastewater. There are many technologies that achieve the required nitrogen discharge standards; however, greenhouse gas emissions and energy consumption constitute the bulk of the environmental footprint of wastewater treatment plants. In this review, we highlight current and proposed approaches aiming to achieve more energy-efficient and environment-friendly biological nitrogen removal, discuss whether new discoveries in microbial physiology of nitrogen-transforming microorganisms could be used to reduce greenhouse gas emissions, and summarize recent advances in ammonium recovery from wastewater.

Critical Analysis of Biomass Retention Strategies in Mainstream and Sidestream ANAMMOX-Mediated Nitrogen Removal Systems

ANAMMOX (anaerobic ammonium oxidation) represents an energy-efficient process for biological nitrogen removal, particularly from wastewater streams with low chemical oxygen demand (COD) to nitrogen (C/N) ratios. Its widespread application, however, is still hampered by a lack of access to biomass-enriched with ANAMMOX bacteria (AMX), slow growth rates of AMX, and their sensitivity to inhibition. Although the coupling of ANAMMOX processes with partial nitrification is already widespread, especially for sidestream treatment, maintaining a functional population density of AMX remains a challenge in these systems. Therefore, strategies that maximize retention of AMX-rich biomass are essential to promote process stability. This paper reviews existing methods of biomass retention in ANAMMOX-mediated systems, focusing on (i) granulation; (ii) biofilm formation on carrier materials; (iii) gel entrapment; and (iv) membrane technology in mainstream and sidestream systems. In addition, the microbial ecology of different ANAMMOX-mediated systems is reviewed.

Deammonification process in municipal wastewater treatment: Challenges and perspectives

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

Membrane aerated biofilm reactors for mainstream partial nitritation/anammox: Experiences using real municipal wastewater

This study investigated the potential of Membrane-Aerated Biofilm Reactors (MABRs) for mainstream nitrogen removal via partial nitration/anaerobic ammonium oxidation (anammox). Four laboratory-scale MABRs were operated with real municipal wastewater characterized by low concentrations of nitrogen (varying between 31 and 120 mg-NH4-N L-1) and the presence of biodegradable organic carbon (soluble COD (sCOD) between 7 and 230 mg-O2 L-1). Two reactors were operated with different aeration strategies (intermittent vs. continuous), the other two with differences in biomass retention (recirculation or removal of detached biomass). Keeping a constant HRT caused instabilities due to difficulties with setting the optimal oxygen flux for the respective surface loadings (1.6-6 g-NH4-N m-2 d-1). Operating the MABRs with a constant surface loading (2 g-NH4-N m-2 d-1) resulted in higher and more stable total nitrogen (TN) removal independent of the aeration strategy. The intermittently aerated MABR improved from an average TN removal of 23%-69%, the continuously aerated MABR from 20% to 50% TN removal. Independent of the feeding strategy, the continuously aerated reactor removed slightly more ammonium (80-95%) compared to the intermittently aerated reactor (74-93%). Limiting the oxygen supply by intermittent aeration proofed successful to favor partial nitritation and anammox. Continuous aeration did not achieve stable suppression of nitrite oxidizing bacteria (NOB). Of the removed ammonium, approx. 26% were left in the effluent as nitrate (only 10% with intermittent aeration). Recirculation of the detached biomass resulted in reattachment onto the biofilm or membrane surface. This recirculation led to significantly higher biomass retention times and thus to better performance. Removing detached biofilm from the reactor caused a slightly lower TN removal of 33% compared to 45% with reattachment, while average ammonium removal was 58% compared to 63%, respectively. Scouring events had a significant impact on the overall operation, resulting in short term losses of TN removal capacities of 50-100%. The microbial community composition was different depending on the aeration strategy and biomass retention. The continuously aerated reactor contained significantly more AOB than the intermittently aerated MABR. The reactor with biomass retention contained less ammonium oxidizing bacteria (AOB), compared to the reactor with low biomass retention. In all MABRs, anammox bacteria established in the biofilm after an initial drop in abundance.

The role of the external mass transfer resistance in nitrite oxidizing bacteria repression in biofilm-based partial nitritation/anammox reactors

A model-based study was developed to analyse the behaviour of Moving Bed Biofilm Reactor (MBBR) and Integrated Fixed-Film Activated Sludge (IFAS) reactor configurations for the removal of nitrogen in the main water line of municipal wastewater treatment plants via partial nitritation/anammox (PN/AMX). The basic principles and underlying mechanisms linking operating conditions to process performance were investigated, with particular focus on nitrite oxidizing bacteria (NOB) repression and resulting volumetric conversion rates. The external mass transfer resistance is a major factor differentiating granular sludge PN/AMX processes from MBBR or IFAS systems. The external mass transfer resistance was found to promote the metabolic coupling between anammox (AMX) and ammonia oxidizing bacteria (AOB), crucial for NOB repression in the biofilm. Operation at low bulk DO prevents NOB proliferation in the flocs of IFAS systems as AMX activity limits nitrite availability (the so-called AMX nitrite sink). Importantly, the effectiveness of the AMX nitrite sink strongly depends on the AMX sensitivity to oxygen. Also, over a broad range of operational conditions, the seeding of AOB from the biofilm played a crucial role in maintaining their activity in the flocs. From a practical perspective, while low DO promotes NOB repression, lower nitrogen loads have to be applied to maintain the same effluent quality. Thus, a trade-off between NOB repression and volumetric conversion capacity needs to be defined. To this end, IFAS allow for higher volumetric rates, but the window of operating conditions with effective NOB repression is smaller than that for MBBR. Ultimately, this study identified the principles controlling NOB in MBBR and IFAS systems and the key differences with granular reactors, allowing for the interpretation of (seemingly contradictory) published experimental results.

Review of characteristics of anammox bacteria and strategies for anammox start-up for sustainable wastewater resource management

Wastewater management has experienced different stages, including pollutant removal, resource recovery, and water nexus. Within these stages, anaerobic ammonia oxidation-based biotechnology can be incorporated for nitrogen removal, which can help achieve sustainable wastewater management, such as reclamation and ecologization of wastewater. Here, the physiology, metabolism, reaction kinetics and microbial interactions of anammox bacteria are discussed, and strategies to start-up the anammox system are presented. Anammox bacteria are slow growers with a high doubling time and a low reaction rate. Although most anammox bacteria grow autotrophically, some types can grow mixotrophically. The reaction stoichiometric coefficients can be affected by loading rates and other biological reactions. Microbial interactions also contribute to enhanced biological nitrogen removal and promote activities of anammox bacteria. The start-up of the anammox process is the key aspect for its practical application, which can be realized through seed selection, system stimulation, and biomass concentration enhancement.

WITHDRAWN: Membrane Aerated Biofilm Reactors for mainstream partial nitritation / anammox: Experiences using real municipal wastewater

The Publisher regrets that this article is an accidental duplication of a published article,  http://dx.doi.org/10.1016/j.wroa.2020.100066. The duplicate article has therefore been withdrawn. The full Elsevier Policy on Article Withdrawal can be found at https://www.elsevier.com/about/our-business/policies/article-withdrawal.

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

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

Application of the Anammox Process for Treatment of Liquid Phase Digestate

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

Mainstream partial nitritation/anammox with integrated fixed-film activated sludge: Combined aeration and floc retention time control strategies limit nitrate production

Implementation of mainstream partial nitritation/anammox (PN/A) can lead to more sustainable and cost-effective sewage treatment. For mainstream PN/A reactor, an integrated fixed-film activated sludge (IFAS) was operated (26 °C). The effects of floccular aerobic sludge retention time (AerSRT_floc), a novel aeration strategy, and N-loading rate were tested to optimize the operational strategy. The best performance was observed with a low, but sufficient AerSRT_floc (~7d) and continuous aeration with two alternating dissolved oxygen setpoints: 10 min at 0.07-0.13 mg O₂ L^-1 and 5 min at 0.27-0.43 mg O₂ L^-1. Nitrogen removal rates were 122 ± 23 mg N L^-1 d^-1, and removal efficiencies 73 ± 13%. These conditions enabled flocs to act as nitrite sources while the carriers were nitrite sinks, with low abundance of nitrite oxidizing bacteria. The operational strategies in the source-sink framework can serve as a guideline for successful operation of mainstream PN/A reactors.

A novel universal primer pair for prokaryotes with improved performances for anammox containing communities

Abundance profiling via 16S rRNA targeted next generation sequencing (NGS) is a common procedure to characterize mixtures of prokaryotic populations inhabiting an environment. Depending on the variable region/s addressed, different maps can be obtained due to their different information content. In this work, we focussed on wastewater microbial communities and we compared several recently developed universal primers that addressed regions V1-V3, V3-V4 and V4. They all proved to have good performance over a wide range of microbial phyla, but the phylum Planctomycetes was not optimally covered, especially for members of the Brocadiales family. Such bacteria are at the basis of the novel nitrogen removal strategy based on anammox process. To overcome this limitation we performed an extensive bioinformatic analysis that allowed the design of a primer (Pro341FB) that shows increased sensitivity for this specific phylum with respect to the previously proposed Pro341F primer. Upon validation using a 16S NGS survey on microbial communities from different wastewater treatment plant (activated sludge systems, anaerobic digesters, aerobic and anaerobic granules) we demonstrated that Pro341FB is able to reveal up to 5 times more members of the Candidatus Brocadiales family (plus many other previously under-covered prokaryotes) than Pro341F, without affecting its excellent previous coverage.

Effective nitrogen removal from ammonium-depleted wastewater by partial nitritation and anammox immobilized in granular and thin layer gel carriers

This study investigates the effect of physicochemical conditions on the partial nitritation and anammox treatment by immobilized ammonia oxidizers under ammonium-deplete conditions. The impact of oxygen and temperature was studied by measuring the activity of immobilized aerobic and anaerobic ammonia-oxidizing organisms (Ammonia-oxidizing bacteria (AOB) and archaea (AOA), and Anammox bacteria) embedded in polyvinyl alcohol - sodium alginate (PVA-SA) beads and in thin layer poly-ethylene glycol hydrogels. Beads and flat hydrogels were incubated in a fluidized bed reactor (FBR) and in two flow cells, respectively. Both systems were fed with synthetic wastewater (15 mg N-NH₄⁺/L) at different temperatures (20 °C and/or 30 °C) and different dissolved oxygen (DO) concentrations (0.1, 0.3, 0.5 and/or 1 mg/L) over 152 and 207 days, respectively. The FBR system had a maximum removal rate of 1.7 g-N/m³/d at 0.1 mg O₂/L, corresponding to 80% removal efficiency, while a high aerobic ammonia-oxidizing activity but a partial oxygen inhibition of Anammox bacteria were observed at higher DO concentrations. In both flow cells, nitrogen removal efficiency was highest (80%) at 30 °C and 1 mg O₂/L while removal was less favorable at lower DO and lower temperature. Our results indicate a potential use of hydrogel beads for an energy efficient technology with reduced aeration demand for treating low ammonia wastewater, while layered hydrogels are a possible first step for biological treatments of wastewater using tangential flow. In addition, we provide blueprint drawings of the flow cells, which may be used to 3D-print the apparatus for other applications.

Performance of the anammox process treating low-strength municipal wastewater under low temperatures: Effect of undulating seasonal temperature variation

In the anammox process treating low-strength municipal wastewater, the effect of common seasonal temperature variation (15.1 °C-22.2 °C) on performance was studied. In autumn and winter, the nitrogen removal rate (NRR) decrement of 0.038kgN/(m³·d) (17.9 °C → 15.1 °C) was nearly threefold higher than 0.014kgN/(m³·d) (22.2 °C → 17.9 °C), which showed that lower temperature laid more negative impact on nitrogen removal. ¹⁵N isotope tracing tests confirmed that the contribution of denitrification to nitrogen removal was far less than anammox, and anammox contributed more at 15.1 °C (91.7%) than 21.9 °C (78.9%). Anammox bacteria could adapt to lower temperature after short-term acclimatization, especially the dominant genus Ca. Brocadia increased from 1.8% to 2.5% and its abundance was significantly correlated with nitrogen consumption (p < 0.05). Above findings suggest that the adaptability of Ca. Brocadia could provide the possibility to maintain nitrogen removal performance at lower temperature. In spring, the improved maximum anammox activity from 2.85 to 3.23mgNH₄⁺-N/(gVSS·h) indicated the recovered removal capacity.

Nutrient Limitation Causes Differential Expression of Transport- and Metabolism Genes in the Compartmentalized Anammox Bacterium Kuenenia stuttgartiensis

Anaerobic ammonium-oxidizing (anammox) bacteria, members of the "Candidatus Brocadiaceae" family, play an important role in the nitrogen cycle and are estimated to be responsible for about half of the oceanic nitrogen loss to the atmosphere. Anammox bacteria combine ammonium with nitrite and produce dinitrogen gas via the intermediates nitric oxide and hydrazine (anammox reaction) while nitrate is formed as a by-product. These reactions take place in a specialized, membrane-enclosed compartment called the anammoxosome. Therefore, the substrates ammonium, nitrite and product nitrate have to cross the outer-, cytoplasmic-, and anammoxosome membranes to enter or exit the anammoxosome. The genomes of all anammox species harbor multiple copies of ammonium-, nitrite-, and nitrate transporter genes. Here we investigated how the distinct genes for ammonium-, nitrite-, and nitrate- transport were expressed during substrate limitation in membrane bioreactors. Transcriptome analysis of Kuenenia stuttgartiensis planktonic cells showed that four of the seven ammonium transporter homologs and two of the nine nitrite transporter homologs were significantly upregulated during ammonium-limited growth, while another ammonium transporter- and four nitrite transporter homologs were upregulated in nitrite limited growth conditions. The two nitrate transporters were expressed to similar levels in both conditions. In addition, genes encoding enzymes involved in the anammox reaction were differentially expressed, with those using nitrite as a substrate being upregulated under nitrite limited growth and those using ammonium as a substrate being upregulated during ammonium limitation. Taken together, these results give a first insight in the potential role of the multiple nutrient transporters in regulating transport of substrates and products in and out of the compartmentalized anammox cell.

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

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

Simultaneous partial nitritation and denitritation coupled with polished anammox for advanced nitrogen removal from low C/N domestic wastewater at low dissolved oxygen conditions

Simultaneous partial nitritation and denitritation (SPND) coupled with anammox was established in this study to treat domestic wastewater. Two lab-scale bioreactors, namely SPND-SBR and ANA-UASB, were used in the two-stage system. In SPND-SBR, stable nitrogen removal efficiency of 51.1% was achieved with a high ammonia oxidation rate of 0.117 kg N/(m3·d). Besides, successful out-selection of nitrite-oxidizing bacteria (NOB) under low-DO of 0.1 mg/L during the steady period, resulting in an average effluent NO2--N/NH4+-N ratio of 1.04. In ANA-UASB, the abundance of Candidatus Brocadia and Candidatus Kuenenia increased from 8.21% and 4.01% to 21.33% and 6.41% with low influent substrate contents of only 38 mg N/L. The effluent total inorganic nitrogen (TIN) was only 8.4 ± 1.1 mg N/L and the nitrogen removal efficiency reached 88.24%. Overall, the study demonstrated that the novel low-DO two-stage process for nitrogen removal is a promising technique for wastewater of low C/N ratio.

Assessment of the fate of organic micropollutants in novel wastewater treatment plant configurations through an empirical mechanistic model

Novel wastewater treatment plants (WWTPs) are expected to be less energetically demanding than conventional ones. However, scarce information is available about the fate of organic micropollutants (OMPs) in these novel configurations. Therefore, the objective of this work is to assess the fate of OMPs in three novel WWTP configurations by using a plant-wide simulation that integrates multiple units. The difference among the three configurations is the organic carbon preconcentration technology: chemically enhanced primary treatment (CEPT), high-rate activated sludge (HRAS) combined or not with a rotating belt filter (RBF); followed by a partial-nitritation (PN-AMX) unit. The simulation results show that the three selected novel configurations lead mainly to comparable OMPs removal efficiencies from wastewater, which were similar or lower, depending on the OMP, than those obtained in conventional WWTPs. However, the presence of hydrophobic OMPs in the digested sludge noticeably differs among the three configurations. Whereas the configuration based on sole HRAS to recover organic carbon leads to a lower presence of OMPs in digested sludge than the conventional WWTP, in the other two novel configurations this presence is noticeable higher. In conclusion, novel WWTP configurations do not improve the OMPs elimination from wastewater achieved in conventional ones, but the HRAS-based WWTP configuration leads to the lowest presence in digested sludge so it becomes the most efficient alternative.

Long-term stability of partial nitritation-anammox for treatment of municipal wastewater in a moving bed biofilm reactor pilot system

Nitrogen removal from the mainstream of municipal wastewater with partial nitritation-anammox (PNA) would be highly beneficial with regard to the uses of energy and organic carbon. However, the challenges of process instability, low nitrogen removal rates (NRR) and unwanted aerobic nitrite oxidation need to be solved to reach large-scale implementation. Here, we have operated pilot-scale moving bed biofilm reactors (MBBRs) for mainstream treatment, together with sidestream treatment of sludge liquor from anaerobic digestors, for over 900 days to investigate process stability, reactor performance and microbial community structure at realistic conditions. The MBBR biofilm contained stable and high relative abundances of anammox bacteria (10-32%) consisting of two major Brocadia sp. populations, and several populations of aerobic ammonia-oxidising bacteria (AOB) within Nitrosomonas sp. (0.2-3.1%), as assessed by 16S rDNA amplicon sequencing. In addition, nitrite-oxidising bacteria (NOB) consisting of Nitrospira sp. (0.4-0.8%) and Nitrotoga sp. (up to 0.4%) were present. Nitrogen was removed at a peak rate of 0.66 g N m^-2 d^-1 (0.13 kg N m^-3 d^-1) with a nitrate production over ammonium consumption of 15% by the NOB, at operation with continuous aeration at 15 °C. However, during most periods with continuous aeration, the NRR was lower (≈ 0.45 g N m^-2 d^-1), with larger relative nitrate production (≈40%), presumably due to problems to maintain stable residual ammonium concentrations during wet-weather mainstream flows. Changing reactor operation to intermittent aeration decreased the NRR but did not help in suppressing the NOB. The study shows that with MBBRs, stable mainstream PNA can be attained at realistic NRR, but with need for post-treatment of nitrate, since effective NOB suppression was hard to achieve.

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.

Improving wastewater management using free nitrous acid (FNA)

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

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.

Application of an enrichment culture of the marine anammox bacterium "Ca. Scalindua sp. AMX11" for nitrogen removal under moderate salinity and in the presence of organic carbon

Seawater can be directly used for toilet flushing in coastal areas to reduce our dependence on desalination and freshwater resources. The presence of high-salt content in the generated wastewater from seawater toilet flushing could limit the performance of conventional biological nitrogen removal processes. Anaerobic ammonium oxidation (anammox) process is regarded as one of the most energy-efficient process for nitrogen removal from N-rich waste streams. In this study, we demonstrated the application of a novel marine anammox bacterium (Candidatus Scalindua sp. AMX11) in a membrane bioreactor (MBR) to treat moderate-saline (∼1.2% salinity) and N-rich organic (2 mM acetate) solution, prepared using real seawater. The MBR showed stable performance with nitrogen removal rate of 0.3 kg-N m^-3 d^-1 at >90% N-removal efficiency. Furthermore, results of ¹⁵N stable isotope experiments revealed that anammox bacteria was mainly responsible for respiratory ammonification through NO₃^- reduction to NH₄⁺ via NO₂^-, and the by-products of respiratory ammonification were used as substrates by anammox bacteria. The dominant role of anammox bacteria in nitrogen removal under saline and organic conditions was further confirmed by genome-centric combined metagenomics and meta-transcriptomic approach. Taken together, these results highlight the potential application of marine anammox bacteria for treating saline wastewater generated from seawater toilet flushing practices.

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

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

A review on mainstream deammonification of municipal wastewater: Novel dual step process

The conventional biological nitrogen removal process is receiving increasing pressure partially due to its energy-negative operation. To address this challenge, various mainstream deammonification processes have been explored for energy-neutral municipal wastewater treatment, whereas these processes appear challenging to be sustainably and stably achieved in conventional process configurations. Therefore, this review aimed to provide a comprehensive analysis of the state-of-the-art of mainstream deammonification, while highlighting the major technical challenges. It appeared that recently developed novel dual step process, i.e. A-B processes, could provide a feasible engineering option for mainstream deammonification, where A-stage is designed for COD capture with the aim to enhance energy recovery, and B-stage is tailored for nutrient removal/recovery. This indeed may lead to a promising integrated mainstream deammonification process towards energy-efficient and environmentally sustainable nitrogen removal. Meanwhile, this review also offered an opinion on future municipal wastewater treatment, aiming for concurrent water reclamation and energy recovery.

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

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

Application of the Anammox in China-A Review

Anaerobic ammonia oxidation (anammox) has been one of the most innovative discoveries for the treatment of wastewater with high ammonia nitrogen concentrations. The process has significant advantages for energy saving and sludge reduction, also capital costs and greenhouse gases emissions are reduced. Recently, the use of anammox has rapidly become mainstream in China. This study reviews the engineering applications of the anammox process in China, including various anammox-based technologies, selection of anammox reactors and attempts to apply them to different wastewater treatment plants. This review discusses the control and implementation of stable reactor operation and analyzes challenges facing mainstream anammox applications. Finally, a unique and novel perspective on the development and application of anammox in China is presented.

Metagenomics reveals microbial community differences lead to differential nitrate production in anammox reactors with differing nitrogen loading rates

Nitrate production during anammox can decrease total nitrogen removal efficiency, which will negatively impact its usefulness for the removal of nitrogen from waste streams. However, neither the performance characteristics nor physiological shifts associated with nitrate accumulation in anammox reactors under different nitrogen loading rates (NLRs) is well understood. Consequently, these parameters were studied in a lower NLR anammox reactor, termed R1, producing higher than expected levels of nitrate and compared with a higher NLR reactor, termed R2, showing no excess nitrate production. While both reactors showed high NH₄⁺-N removal efficiencies (>90%), the total nitrogen removal efficiency (<60%) was much lower in R1 due to higher nitrate production. Metagenomic analysis found that the number of reads derived from anammox bacteria were significantly higher in R2. Another notable trend in reads occurrence was the relatively higher levels of reads from genes predicted to be nitrite oxidoreductases (nxr) in R1. Binning yielded 27 high quality draft genomes from the two reactors. Analysis of bin occurrence found that R1 showing both a decrease in anammox bacteria and an unexpected increase in nxr. In-situ assays confirmed that R1 had higher rates of nitrite oxidation to nitrate and suggested that it was not solely due to obligate NOB, but other nxr-containing bacteria are important contributors as well. Our results demonstrate that nitrate accumulation can be a serious operational concern for the application of anammox technology to low-strength wastewater treatment and provide insight into the community changes leading to this outcome.

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.

Mechanism of stable sewage nitrogen removal in a partial nitrification-anammox biofilm system at low temperatures: Microbial community and EPS analysis

Stable sewage nitrogen removal with nitrogen removal efficiency of 87.5 ± 2.2% was achieved in a partial nitrification-anammox (PNA) biofilm system at low temperatures (12.8-16.3 °C). High-throughput sequencing analysis indicated that the microbial community structure in the sequencing batch biofilm reactor (SBBR) remained reasonably stable. Candidatus Brocadia was the only detected anammox genus and remained stable at 0.3-0.5%. Some psychrotolerant microorganisms that could secrete cryoprotective extracellular polymeric substances (EPS), including Flavobacterium and Thermomonas, were enriched at low temperatures. This could be conducive to the stable operation of the PNA-SBBR. Moreover, according to the EPS composition and characteristics analysis, the secretion of tightly-bound EPS that bound to the cell surface containing plentiful protein was stimulated at low temperatures, further improving the system stability. Overall, the reasonably stable microbial community structure, enrichment of psychrotolerant microorganisms, and increased secretion of EPS could play important roles for stable sewage nitrogen removal at low temperatures.

Achieving Stable Partial Nitritation in an Acidic Nitrifying Bioreactor

Partial nitritation providing a suitable effluent for subsequent anammox is a critical step in a two-stage autotrophic nitrogen removal system. This study demonstrates an innovative approach for attaining partial nitritation in an acidic bioreactor operating at a slightly low pH (i.e., 5-6). This approach is based on our hypothesis in this study that acid-tolerant ammonia-oxidizing bacteria (AOB) can produce nitrite and protons to self-sustain free nitrous acid (FNA, NO₂^- + H⁺ ↔ HNO₂) at a parts per million level, as an inhibitor of nitrite-oxidizing bacteria (NOB). With influent nitrogen of about 200 mg/L and operating conditions of high dissolved oxygen, long sludge retention time, and moderate temperature, a lab-scale acidic bioreactor with FNA up to 2 mg of HNO₂-N/L successfully established stable nitrite accumulation in the effluent for 200 days, with an average ratio [NO₂^-/(NO₂^- + NO₃^-)] exceeding 95%. A 16S rRNA amplicon sequencing analysis showed that Nitrosospira was the dominant AOB in the biomass of the bioreactor, while Nitrosomonas and Nitrospira, two typical nitrifying genera in neutral wastewater treatment, both disappeared after the startup of partial nitritation. Kinetic characterization revealed that Nitrosospira had a substrate affinity of 11.4-16.5 mg of total ammonia (NH₄⁺ + NH₃)/L. It also revealed that less than 3.5 mg of HNO₂-N/L FNA did not inhibit AOB activity significantly. Acidic operation is economically attractive because it can be achieved via acidophilic ammonia oxidation without adding chemical acid. However, hazardous gas, nitric oxide (NO), should be removed from gas produced by acidic nitrifying bioreactors.

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.

Metagenomics and metatranscriptomics analyses reveal oxygen detoxification and mixotrophic potentials of an enriched anammox culture in a continuous stirred-tank reactor

The metabolisms of anaerobic ammonium oxidation (anammox) bacteria related to ammonia oxidation with nitrite reduction and autotrophic carbon fixation have been extensively observed. However, little is known about the specific metabolic pathways associated with oxygen detoxification and organic carbon utilization. To this end, we obtained high abundance of anammox species (∼50%) in a lab-scale continuous stirred-tank reactor (CSTR) at room temperature without strict anaerobic condition. The draft genome of the dominant anammox bacteria affiliated to Ca. Brocadia sp. was recovered. Its metabolic pathways and genes expression were reconstructed and examined through metagenomic and metatranscriptomic analyses. Interestingly, the results suggested that this anammox lineage likely performs oxygen detoxification with genes encoding superoxide dismutase (SOD) and cytochrome c peroxidase (Ccp). Moreover, the Ccp-activated hydrogen peroxide (intermediate of oxygen detoxification) reduction might be energetically beneficial for the observed acetate conversion related to cell synthesis of Ca. Brocadia sp. This study offers a comprehensive understanding on the diverse metabolic activities in anammox species affiliated to Ca. Brocadia sp., and expanded the applicability of anammox process.

Membrane Aerated Biofilm Reactors - How longitudinal gradients influence nitrogen removal - A conceptual study

Membrane-aerated biofilm reactors are becoming more important for nitrogen removal in the wastewater sector. One-dimensional (1D) models are widely used to study the performance of such systems; however, 1D models are not able to simulate the longitudinal gradients that exist in the reactor. Although there is experimental evidence that points to the existence of longitudinal gradients simple modeling approaches that consider these gradients are not yet developed. This study proposes a novel multi-compartment model that simulates the longitudinal substrate and oxygen gradients. It assesses the effects of temperature, biofilm thickness, number of compartments, and flow configuration (liquid and gas phase) on the modeling results. Additionally, it compares the capabilities of a traditional 1D model with those of the novel multi-compartment model. Our results show that a classical 1D model predicts a lower total dissolved nitrogen concentration (TDN) in the effluent in contrast to the predictions of the multi-compartment model. In the worst-case scenario, the TDN predicted by the traditional 1D model was three times lower than the prediction of the multi-compartment model. The results delivered by the models differ also in the axial gradients. The traditional 1D model, for example, predicted an oxygen concentration at the membrane surface of 0.4 mg-O₂/l while the multi-compartment model predicted a concentration of 2.9 mg-O₂/l. Finally, the results of this study show that the longitudinal oxygen gradient has an important effect on both, biomass distribution and effluent TDN, whereas the longitudinal substrate exclusively affected the effluent TDN.

Towards mainstream deammonification of municipal wastewater: Partial nitrification-anammox versus partial denitrification-anammox

The mainstream deammonification has been believed as a viable technology for the energy-neutral municipal wastewater treatment, which can be realized through two approaches known as partial nitrification-anammox (PN/AMX) and partial denitrification-anammox (PDN/AMX). However, large-scale applications of these deammonification processes for municipal wastewater treatment have been rarely reported thus far. Given such a situation, this review examined the mainstream PN/AMX and PDN/AMX processes with the focus on their engineering feasibility, economic viability and potential challenges. It was revealed that soluble COD and stable nitrite production were the main challenges for mainstream deammonification. Pre-capture of COD was essential for mitigating the competition between denitrifiers and anammox bacteria on nitrite, while NOB suppression and partial denitrification control to nitrite stage were critical issues for stable nitrite production in PN and PDN processes respectively. Compared to nitrification-denitrification, the unit oxygen demand for nitrogen removal in PN/AMX and PDN/AMX could be reduced by 57.3% and 47.7%, while the sludge production could also be cut off by 83.7% and 66.3% in PN/AMX and PDN/AMX respectively. These clearly showed the greater economic viability and environmental sustainability of PN/AMX against PDN/AMX. Consequently, more effort is needed to improve the engineering feasibility of large-scale mainstream deammonification for municipal wastewater treatment.

Performance of partial nitritation-anammox processes at mainstream conditions in an IFAS system

The partial nitritation-anammox processes implementation in the main line of wastewater treatment plants would lead them closer to the energy autarky. With this purpose, an integrated fixed film activated sludge (IFAS) reactor was operated at pilot scale. Efficient nitrogen removal (72 ± 11%) was achieved for anaerobically pre-treated municipal wastewater at low temperature (21 - 15 °C), with a nitrogen removal rate of 37 ± 3 g N/(m³·d) at 15 °C. The ammonium oxidizing bacteria were more abundant in the activated sludge, while anammox bacteria were primarily located in biofilm attached onto the carriers surface. Nitrite oxidizing bacteria (NOB) activity was similar between both fractions and its specific activity decreased more than that of other populations when the operating temperature was reduced. Furthermore, the IFAS operational strategy (aerobic/anoxic periods) allowed an efficient NOB activity suppression inside the reactor, which accounted only for the 10 - 20% of the maximum potential activity.

Reducing Cost and Environmental Impact of Wastewater Treatment with Denitrifying Methanotrophs, Anammox, and Mainstream Anaerobic Treatment

In water resource recovery facilities, sidestream biological nitrogen removal via anaerobic ammonium oxidation (anammox) is more energy and cost efficient than conventional nitrification-denitrification. However, under mainstream conditions, nitrite oxidizing bacteria (NOB) out-select anammox bacteria for nitrite produced by ammonium oxidizing bacteria (AOB). Therefore, nitrite production is the bottleneck in mainstream anammox nitrogen removal. Nitrate-dependent denitrifying anaerobic methane oxidizing archaea (n-damo) oxidize methane and reduce nitrate to nitrite. The nitrite supply challenge in mainstream anammox implementation could be solved with a microbial community of AOB, NOB, n-damo, and anammox with methane from anaerobic sludge digestion or a mainstream anaerobic membrane bioreactor (AnMBR). The cost and environmental impact of traditional nitrification/dentrification relative to AOB/anammox and AOB/anammox/n-damo systems, with and without an AnMBR, were compared with a stoichiometric model. AnMBR implementation reduced costs and emission rates at moderate to high nutrient loading by lowering aeration and sludge handling demands while increasing methane available for cogeneration. AnMBR/AOB/anammox systems reduced cost and GHG emission by up to $0.303/d/m³ and 1.72 kg equiv. CO₂/d/m³, respectively, while AnMBR/AOB/anammox/n-damo systems saw a similar reduction of at least $0.300/d/m³ and 1.65 kg equiv. CO₂/d/m³ in addition to alleviating the necessity to stop nitrification at nitrate, allowing easier aeration control.

Effective nitrogen removal in a two-stage partial nitritation-anammox reactor treating municipal wastewater - Piloting PN-MBBR/AMX-IFAS configuration

A novel partial nitritation-anammox (PNA) reactor configuration was piloted for 250 days. Primary effluent from full-scale municipal wastewater treatment plant was treated in a two-stage biofilm system incorporating innovative process control for cold partial nitritation. Partial nitritation was combined with carbon removal in a moving bed biofilm reactor (MBBR) to achieve high-rate treatment and nitritation was obtained with dissolved oxygen to total ammonium nitrogen (DO/TAN) ratio control and free ammonia (FA) for inhibition of nitratation. Effluent from MBBR was directed to an integrated fixed-film activated sludge (IFAS) reactor where nitrogen was removed via anammox. MBBR achieved partial nitritation at 2.0 ± 0.3 g-N m^-2 d^-1 and nitrogen removal in the IFAS reactor reached 0.45 ± 0.1 g-N m^-2 d^-1 (55 g-N m^-3 d^-1). The process performed well at 19 ± 3 °C with an average effluent total inorganic nitrogen (TIN) concentration of 11 ± 4 mg L^-1.

Nitrite oxidizing bacteria (NOB) contained in influent deteriorate mainstream NOB suppression by sidestream inactivation

Sidestream sludge treatment approaches have been developed in recent years to achieve mainstream nitrite shunt or partial nitritation, where NOB are selectively inactivated by biocidal factors such as free nitrous acid (FNA) or free ammonium (FA) in a sidestream reactor. The existence of NOB in raw wastewater has been increasingly realized and could pose critical challenge to stable NOB suppressions in those systems. This study, for the first time, evaluated the impact of influent NOB on the NOB suppressions in a mainstream nitrite shunt system achieved through sidestream sludge treatment. An over 500-day sequential batch reactor operation with six experimental phases rigorously demonstrated the negative effects of influent NOB on mainstream NOB control. Continuously seeding of NOB contained in influent stimulated NOB community shifts, leading to different extents of ineffective NOB suppression. The role of primary wastewater treatment in NOB removal from raw wastewater was also investigated. Results suggest primary settling and High Rate Activated Sludge system could remove a large part of NOB contained in raw wastewater. Primary treatment for raw wastewater is necessary for ensuring stable mainstream NOB suppressions.

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.

Controlling nitritation in a continuous split-feed/aeration biofilm nitrifying bioreactor

This study explored the stability of partial ammonium oxidation at low feed concentration (50 g N/m³), suitable for anammox process, in continuous fixed bed up-flow biofilm reactors with external recirculation-aeration. The reactors, filled with crushed basalt, were fed with synthetic medium at 20-25 °C at constant flow-rate with limiting dissolved oxygen concentration controlled by the recirculation ratio (R). Successful nitritation was achieved at R ≅ 4-6 with approx. 50% of NH₄⁺ oxidized to NO₂^- with <5% NO₃^-accumulation. q-PCR analysis along the reactor showed ammonia oxidizing bacteria being the prevalent nitrifiers over the three-fourths of the bed in the flow direction, negligible denitrifiers and absent ammonium oxidizing archaea. A numerical model for predicting the concentration of the nitrogen species and DO was formulated. The model successfully predicted the experimental results and displayed good sensitivity to intrinsic oxygen uptake parameters. The proposed numerical model can serve both as an operational and design tool.

Quantify the contribution of anammox for enhanced nitrogen removal through metagenomic analysis and mass balance in an anoxic moving bed biofilm reactor

In this study, enhanced nitrogen removal through in situ enrichment of anammox bacteria was successfully obtained in a full-scale municipal wastewater treatment plant (WWTP). The WWTP was an anaerobic-anoxic-oxic (AAO) process and upgraded by adding moving carriers into the anoxic zone. Enhanced nitrogen removal was obtained during almost two years of operation. The significant nitrogen removal might be associated with the in situ enrichment of anammox bacteria on the adding carriers, as revealed by the comprehensive results of molecular analysis and ¹⁵N-stable isotope tracing tests. Quantitative PCR results indicated that anammox bacteria in the anoxic-carrier biofilms presented a higher abundance than flocculent sludge (16S rRNA: P < 0.005; HzsB: P < 0.042). The 16S rRNA amplicon sequencing showed significant differences in the phylum Planctomycetes (P < 0.001) between anoxic-carrier biofilms and flocculent sludge. And metagenomic sequencing analysis further revealed the anammox relative abundance in the anoxic-carrier biofilms was significantly higher than the reported level in the flocculent sludge of conventional WWTPs. In addition, ¹⁵N-stable isotope tracing tests showed that anammox could be combined with nitrate reduction by the anoxic-carrier biofilms. Thus, enriched anammox bacteria might contribute to nitrogen loss and lead to improvements in the nitrogen removal, which was also supported by the mass balance analysis of organic carbon, nitrogen, and phosphorus of the WWTP. Overall, this study suggests that anoxic-carrier biofilms might be a candidate to enhance nitrogen removal through partial anammox in municipal WWTPs.

Integrated upflow anaerobic fixed-bed and single-stage step-feed process for mainstream deammonification: A step further towards sustainable municipal wastewater reclamation

The high energy consumption and excessive waste activated sludge (WAS) production have become the major concerns on the municipal wastewater treatment with conventional biological processes. To tackle these emerging issues, this study demonstrated the feasibility of a novel process integrating an upflow anaerobic fixed-bed reactor (UAFBR) followed by a continuous step-feed reactor for mainstream deammonification towards improved energy efficiency, minimized sludge production and cost-effective ammonium removal. The results showed that 48.8% of the influent chemical oxygen demand (COD) was directly converted to methane gas in UAFBR with minimized sludge production, while 80% of total nitrogen (TN) was removed in the step-feed reactor. Mass balance on the step-feed reactor revealed that the oxic chambers contributed 51.6% of the removed ammonium oxidation to mainly nitrite, while the produced nitrite was immediately removed via anammox with the ammonium supplied by the step-feed in the following anoxic chambers where about 87.1% TN removal occurred. Moreover, it was found that sustainable repression of nitrite oxidizing bacteria (NOB) was achieved without compromising the activity of ammonia oxidizing bacteria (AOB). The anammox bacteria were effectively retained in the anoxic chambers and showed a high specific anammox activity of 0.42 g N/(g VSS·day). These suggest that the step-feed configuration can offer a feasible engineering option towards single-stage mainstream deammonification. It appears that the integrated process developed in this study sheds light on the possible way towards sustainable, energy self-sufficient municipal wastewater reclamation.

Stability of aerobic granular sludge in a pilot scale sequencing batch reactor enhanced by granular particle size control

Aerobic granulation was successfully achieved in a pilot scale sequencing batch reactor within 40 days. Then, stability of different particle size granules was explored according to their activity and resistance to ultrasonic crushing. Different particle size granules (0.3-0.6 mm, 0.6-1 mm, 1-1.43 mm, 1.43-2 mm, 2-3 mm and 3-4 mm) were exposed under different ultrasonic power separately. It was found that the granules with 2-3 mm always had the maximum granulation rates after ultrasonic crushing. Meanwhile, activity data showed that the 2-3 mm granules had the lowest specific oxygen utilization rates, which indicated that they were easier to maintain stability as the increase of their particle sizes was the slowest. So, 500 mL mixed liquid of the reactor were taken out and sieved to obtain the 2-3 mm granules, which were subsequently returned to the reactor to increase their proportion. Through the manual regulation, the proportion of 2-3 mm granules kept increasing which gradually became dominant in the reactor. Under the strategy of 86 days of operation, the aerobic granules were regular and compact, which had good removal effects of the real wastewater. The results indicated that the stability of the system could be greatly enhanced by the method, which provided a new strategy to maintain the granular stability.

Energetic and economic assessment of sludge thermal hydrolysis in novel wastewater treatment plant configurations

Novel wastewater treatment plants (WWTPs) are aimed to be more energetically efficient than conventional ones. Their first step is a chemical oxygen demand (COD) preconcentration stage with different alternatives, such as rotating belt filters (RBF), chemically enhanced primary treatment (CEPT), high-rate activated sludge (HRAS), or combinations thereof, in which energy requirements are substantially reduced. The COD recovered as sludge allows a noticeable increase of biogas production in anaerobic digestion (AD). In conventional WWTPs, sludge anaerobic biodegradability can be significantly enhanced by applying sludge pretreatment methods, such as thermal hydrolysis (TH), before AD. However, considering that novel-sludges are more anaerobically biodegradable than conventional ones, the impact of TH on their methane production is expected to result significantly lower. In this study, an energetic and economic assessment of applying TH in novel WWTPs was performed. We found that TH is only justified to reduce operational costs as long as sludge TS concentration in the feeding to the TH unit is higher than 1-2%. The HRAS-based WWTP is the scenario that leads to the lowest treatment costs (below 1c €/ m³ wastewater if sludge is thickened over 10% of TS). However, the WWTP based on CEPT for COD preconcentration leads to the lowest electricity consumption (below 0.01 kWh/m³ of wastewater), but even in the most favourable conditions the energy autarky was not achievable. Results show that the main impact of TH is mainly due to sludge disposal savings (270,000-430,000 €/year for a 500,000 inhabitants WWTP) rather than the increase of energy production (achieves maximum savings of 35,000-60,000 €/year). Payback time is very dependent on the WWTP size, ranging from 15 to 30 years for a 100,000 inhabitants WWTP and from 2 to 4 years for a 1,000,000 inhabitants WWTP.

Biomass segregation between biofilm and flocs improves the control of nitrite-oxidizing bacteria in mainstream partial nitritation and anammox processes

The control of nitrite-oxidizing bacteria (NOB) challenges the implementation of partial nitritation and anammox (PN/A) processes under mainstream conditions. The aim of the present study was to understand how operating conditions impact microbial competition and the control of NOB in hybrid PN/A systems, where biofilm and flocs coexist. A hybrid PN/A moving-bed biofilm reactor (MBBR; also referred to as integrated fixed film activated sludge or IFAS) was operated at 15 °C on aerobically pre-treated municipal wastewater (23 mg_NH4-N L^-1). Ammonium-oxidizing bacteria (AOB) and NOB were enriched primarily in the flocs, and anammox bacteria (AMX) in the biofilm. After decreasing the dissolved oxygen concentration (DO) from 1.2 to 0.17 mg_O2 L^-1 - with all other operating conditions unchanged - washout of NOB from the flocs was observed. The activity of the minor NOB fraction remaining in the biofilm was suppressed at low DO. As a result, low effluent NO₃^- concentrations (0.5 mg_N L^-1) were consistently achieved at aerobic nitrogen removal rates (80 mg_N L^-1 d^-1) comparable to those of conventional treatment plants. A simple dynamic mathematical model, assuming perfect biomass segregation with AOB and NOB in the flocs and AMX in the biofilm, was able to qualitatively reproduce the selective washout of NOB from the flocs in response to the decrease in DO-setpoint. Similarly, numerical simulations indicated that flocs removal is an effective operational strategy to achieve the selective washout of NOB. The direct competition for NO₂^- between NOB and AMX - the latter retained in the biofilm and acting as a "NO₂-sink" - was identified by the model as key mechanism leading to a difference in the actual growth rates of AOB and NOB (i.e., μ_NOB < μ_AOB in flocs) and allowing for the selective NOB washout over a broad range of simulated sludge retention times (SRT = 6.8-24.5 d). Experimental results and model predictions demonstrate the increased operational flexibility, in terms of variables that can be easily controlled by operators, offered by hybrid systems as compared to solely biofilm systems for the control of NOB in mainstream PN/A applications.

Towards mainstream anammox: lessons learned from pilot-scale research at WWTP Dokhaven

The aim of this research was to study the biological feasibility of the Partial Nitritation/Anammox (PN/A) technology to remove nitrogen from municipal mainstream wastewaters. During stable process operations at summer temperatures (23.2 ± 1.3°C), the total nitrogen removal rate was 0.223 ± 0.029 kg N (m³d)^-1 while at winter temperatures (13.4 ± 1.1°C) the total nitrogen removal rate was 0.097 ± 0.016 kg N (m³ d)^-1. Nitrite-oxidizing bacteria (NOB) suppression was successfully achieved at the complete temperature range of municipal mainstream wastewater. Despite the presence of NOB as observed in activity tests, their activity could be successfully suppressed due to a relative low dissolved oxygen concentration. An overcapacity of ammonia-oxidizing bacteria and anammox activity was always present. Long-term stability is a focus point for future research, especially in relation to the stability of the biological oxygen demand removing step, preceding the PN/A reactor.

Enhancing mainstream nitrogen removal by employing nitrate/nitrite-dependent anaerobic methane oxidation processes

Due to serious eutrophication in water bodies, nitrogen removal has become a critical stage for wastewater treatment plants (WWTPs) over past decades. Conventional biological nitrogen removal processes are based on nitrification and denitrification (N/DN), and are suffering from several major drawbacks, including substantial aeration consumption, high fugitive greenhouse gas emissions, a requirement for external carbon sources, excessive sludge production and low energy recovery efficiency, and thus unable to satisfy the escalating public needs. Recently, the discovery of anaerobic ammonium oxidation (anammox) bacteria has promoted an update of conventional N/DN-based processes to autotrophic nitrogen removal. However, the application of anammox to treat domestic wastewater has been hindered mainly by unsatisfactory effluent quality with nitrogen removal efficiency below 80%. The discovery of nitrate/nitrite-dependent anaerobic methane oxidation (n-DAMO) during the last decade has provided new opportunities to remove this barrier and to achieve a robust system with high-level nitrogen removal from municipal wastewater, by utilizing methane as an alternative carbon source. In the present review, opportunities and challenges for nitrate/nitrite-dependent anaerobic methane oxidation are discussed. Particularly, the prospective technologies driven by the cooperation of anammox and n-DAMO microorganisms are put forward based on previous experimental and modeling studies. Finally, a novel WWTP system acting as an energy exporter is delineated.

Integration of Green Energy and Advanced Energy-Efficient Technologies for Municipal Wastewater Treatment Plants

Wastewater treatment can consume a large amount of energy to meet discharge standards. However, wastewater also contains resources which could be recovered for secondary uses under proper treatment. Hence, the goal of this paper is to review the available green energy and biomass energy that can be utilized in wastewater treatment plants. Comprehensive elucidation of energy-efficient technologies for wastewater treatment plants are revealed. For these energy-efficient technologies, this review provides an introduction and current application status of these technologies as well as key performance indicators for the integration of green energy and energy-efficient technologies. There are several assessment perspectives summarized in the evaluation of the integration of green energy and energy-efficient technologies in wastewater treatment plants. To overcome the challenges in wastewater treatment plants, the Internet of Things (IoT) and green chemistry technologies for the water and energy nexus are proposed. The findings of this review are highly beneficial for the development of green energy and energy-efficient wastewater treatment plants. Future research should investigate the integration of green infrastructure and ecologically advanced treatment technologies to explore the potential benefits and advantages.