Pilot-scale evaluation of partial denitrification/anammox on nitrogen removal from low COD/N real sewage based on a modified process

Regulation of microcurrent on carbon and nitrogen metabolism in denitrification under low carbon-to-nitrogen ratio: Optimizing carbon flux distribution

Synergy of autotrophic and heterotrophic denitrification can achieve low-carbon and high-efficient nitrogen removal. However, it remains unclear how microcurrent-driven hydrogen autotrophic denitrification regulates carbon flux distribution (nitrogen reduction, poly-β-hydroxyalkanoate (PHA) storage, and cell growth) in heterotrophic denitrification. This work compared biofilm reactor with biofilm electrode reactor under different carbon-to-nitrogen (C/N) ratios (10 - 3). At C/N ratio of 3, microcurrent accelerated nitrate reduction rate by 0.35 mg/(L·min) and reduced nitrite accumulation by 10.29 mg/L, thus decreasing nitrogen reduction proportion by 11.21%. Meanwhile, PHA storage and cell growth proportions increased by 0.03% and 11.18%, respectively. PHA was initially synthesized and subsequently utilized for nicotinamide adenine dinucleotide and energy production, while cell growth preferentially utilized limited carbon sources to maintain system stability. Increased abundance of hydrogen autotrophic denitrifiers, heterotrophic denitrifiers, and PHA storage bacteria confirmed optimization of microcurrent on carbon flux distribution. These findings advanced the understanding of microcurrent regulation on carbon flux.

Enhancing nitrogen removal from low-strength municipal wastewater by partial denitrification coupled with anammox in plug flow pure biofilm system

Biofilm has been widely utilized to retain anaerobic ammonium-oxidizing bacteria (AnAOB). However, most research on partial denitrification/anammox (PD/A) focuses on hybrid systems, with limited insights into systems dominated by complete biofilms. This study established pure biofilm system within plug-flow reactor to treat low strength municipal wastewater. Strategies such as two-point feeding significantly increased the activity of AnAOB within anoxic biofilm, reaching 2.0 mg N/g Mixed Liquor Suspended Solids/h. The ΔNO3--N/ΔNH4+-N indicated a more balanced relationship between denitrification and anammox. The reactor operated stably for 240 days, achieving a substantial nitrogen removal efficiency of 75.5 ± 3.6 % with influent carbon/nitrogen ratio of 3.2. Candidatus_Brocadia, whose abundance surged to 2.6 %, contributed 42.5 % to nitrogen removal, predominantly colonized the interior. Partial denitrification bacteria (Dechloromonas, Thauera) preferred the outer layer, exhibiting sustaining nitrite supply for AnAOB growth. This study highlights the potential of pure biofilm system for mainstream anammox application.

A chemically enhanced primary treatment and anammox-based process for sustainable municipal wastewater treatment: The advantage and application prospects

Low-carbon nitrogen removal, bioenergy production, and phosphorus recovery are key goals for sustainable municipal wastewater treatment. Traditional activated sludge processes face an energy demand conflict. Anaerobic ammonium oxidation (Anammox) offers a solution to this issue, with the A-B process providing a sustainable approach. Stable and cost-effective nitrite supply for mainstream anammox has gained attention, while the interactions between A-B stage processes are also crucial. This paper reviews the benefits and challenges of mainstream anammox, bioenergy, and phosphorus recovery. A combined process of chemically enhanced primary treatment, partial denitrification and anammox is identified as effective for sustainable treatment. Additionally, the stable nitrite supply from the sidestream partial nitrification provides a 54% nitrogen removal contribution to the mainstream anammox. Anaerobic digestion with sulfate reduction is proposed as an efficient method for simultaneous bioenergy and phosphorus recovery from iron-enhanced primary sludge. The recycling of iron and sulfate reduces excess sludge and cuts costs. A novel wastewater treatment scheme, supported by a mass balance analysis, is presented; the proposed process is capable of recovering >50% of the carbon and phosphorus, while reduced 40% dosing of Fe and S chemicals, reducing the cost of chemical dosing and treatment of the digestate while meeting the high-quality effluent. The paper also explores the potential for transitioning from conventional activated sludge processes and suggests directions for future research.

Achieving efficient autotrophic nitrogen removal in anaerobic membrane bioreactor plus membrane aerated biofilm reactor by regulating nutrient ratios

It is feasible to integrate an anaerobic membrane bioreactor with a membrane aerated biofilm reactor to efficiently implement the sulfate reduction, simultaneous nitrification and autotrophic denitrification process. However, the effect of parameters on nutrient removal and environmental impacts of the process are unclear. In this study, the reactor performance was mainly influenced by the chemical oxygen demand to sulfate (COD/S) ratio and the ammonium to sulfate (N/S) ratio in long-term operation. Significant models were developed to optimize the two factors using the response surface methodology. Under optimal conditions (COD/S ratio of 2.5 and N/S ratio of 0.3), the system could remove above 86 % COD, 99 % ammonium, and 92 % total inorganic nitrogen. Moreover, this process could reduce energy consumption by 30 % and global warming potential by 50 % compared with traditional anaerobic/oxic activated sludge process. These findings provide guidance for the application of this technology in sulfate-containing municipal sewage treatment.

Increasing biomass concentration facilitates simultaneous nitrogen removal and sludge reduction under low C/N conditions

To overcome the issues of limited carbon source and high sludge production in partial denitrification/anammox (PD/A) process, the effects of mixed liquor suspended solids (MLSS) and carbon/nitrogen ratio (C/N) on PD/A were investigated through parallel experiments. Nitrogen removal efficiencies decreased significantly when C/N was reduced (1.5 → 0.75). When MLSS was doubled, the nitrogen removal efficiencies in the two parallel reactors increased from 75.3 %, 72.9 % to 86.9 %, 89.7 %, respectively, and sludge yields decreased obviously. Combining with in-situ test, it was speculated when MLSS increased, fermentation was enhanced, providing substrate for partial denitrification. Thauera, involved in partial denitrification, decreased obviously with reduced C/N, but increased from 9.93 % to 38.16 % when MLSS doubled, which could promote the PD/A process. Terrimonas and Ignavibacterium (fermentative bacteria) increased from 1.26 %, 5.22 % to 6.62 %, 6.30 %, respectively. These results proved that increasing MLSS under low C/N ratios promoted fermentation in PD/A system, facilitating efficient nitrogen removal and sludge reduction.

Pilot-scale implementation of mainstream anammox for municipal wastewater treatment against cold temperature

Applying anammox to municipal wastewater treatment promises enormous energy and resource savings; however, seasonally cold conditions pose a considerable challenge, impeding its future applications towards non-tropical regions. In this study, we establish a pilot-scale wastewater treatment plant (50 m3/d) in northern China and implement the partial denitrification coupling anammox process on actual municipal wastewater. Despite seasonal cooling, the nitrogen removal efficiency remains high, ranging from 75.0 ± 4.6% at 27.8-20.0 °C to 70.4 ± 4.5% at 10-7.5 °C. This process exhibits remarkable low-temperature tolerance, achieving an in-situ anammox rate of 32.7 ± 4.7 g-N/(m3·d) at 10-7.5 °C and contributing up to 39.7 ± 6.7% to nitrogen removal. Further 15N stable isotope tracing and kinetic tests reveal that the partial denitrification is capable of supplying increasingly abundant NO2- to anammox with decreasing temperature, enabling robust mainstream anammox against seasonal cooling. From 27.8 °C to 7.5 °C, anammox bacteria not only survive but thrive under mainstream conditions, with absolute and relative abundances increasing by 429.1% and 343.5%, respectively. This pilot-scale study sheds fresh light on extending mainstream anammox towards non-tropical regions, taking a necessary step forward toward the sustainability goals of the wastewater treatment sector.

Molecular evidence of internal carbon-driven partial denitrification in a mainstream pilot A-B system coupled with side-stream EBPR treating municipal wastewater

Achieving mainstream short-cut nitrogen removal via nitrite has become a carbon and energy efficient way, but still remains challenging for low-strength municipal wastewaters. This study integrated sidestream enhanced biological phosphorus removal system in a pilot-scale adsorption/bio-oxidation (A-B) process (named A-B-S2EBPR system) and nitrite accumulation was successfully achieved for treating the municipal wastewater. Nitrite could accumulate to 5.5 ± 0.3 mg N/L in the intermittently aerated tanks of B-stage with the nitrite accumulation ratio (NAR) of 79.1 ± 6.5 %. The final effluent concentration and removal efficiency of total inorganic nitrogen (TIN) were 4.6 ± 1.8 mg N/L and 84.9 ± 5.6 %, respectively. In-situ process performance of nitrogen conversions, routine batch nitrification/denitrification activity tests and functional gene abundance of nitrifiers collectively suggested that the nitrite accumulation was mainly caused by partial denitrification rather than out-selection of nitrite oxidizing bacteria (NOB). Moreover, the single-cell Raman spectroscopy analysis first demonstrated that there was a specific microbial population that could utilize polyhydroxyalkanoates (PHA) as the potential internal carbon source during the partial denitrification process. The integration of S2EBPR brings unique features to the conventional A-B process, such as extended anaerobic retention time, lower oxidation-reduction potential (ORP), much higher and complex volatile fatty acids (VFAs) etc., which can largely reshape the microbial communities. The dominant genera were Acinetobacter and Comamonadaceae, which accounted for (17.8 ± 15.5)% and (6.7 ± 3.4)%, respectively, while the relative abundance of conventional nitrifiers was less than 0.2%. This study provides insights into phylogenetic and phenotypic shifts of microbial communities when incorporating S2EBPR into the sustainable A-B process to achieve mainstream short-cut nitrogen removal.

Deciphering the roles of nitrogen source in sharping synchronous metabolic pathways of linear alkylbenzene sulfonate and nitrogen in a membrane biofilm for treating greywater

Nitrogen (N) source is an important factor affecting biological wastewater treatment. Although the oxygen-based membrane biofilm showed excellent greywater treatment performance, how N source impacts the synchronous removal of organics and N is still unclear. In this work, how N species (urea, nitrate and ammonia) affect synchronous metabolic pathways of organics and N were evaluated during greywater treatment in the membrane biofilm. Urea and ammonia achieved efficient chemical oxygen demand (>97.5%) and linear alkylbenzene sulfonate (LAS, >98.5%) removal, but nitrate enabled the maximum total N removal (80.8 ± 2.6%). The nitrate-added system had poor LAS removal ratio and high residual LAS, promoting the accumulation of effluent protein-like organics and fulvic acid matter. N source significantly induced bacterial community succession, and the increasing of corresponded functional flora can promote the transformation and utilization of microbial-mediated N. The nitrate system was more conducive to the accumulation of denitrification related microorganisms and enzymes, enabling the efficient N removal. Combining with high amount of ammonia monooxygenase that contributing to LAS and N co-metabolism, LAS mineralization related microbes and functional enzymes were generously accumulated in the urea and ammonia systems, which achieved the high efficiency of organics and LAS removal.

A critical review of ozone-based electrochemical advanced oxidation processes for water treatment: Fundamentals, stability evaluation, and application

In recent years, electrochemical advanced oxidation processes (EAOPs) combined with ozonation have been widely utilized in water/wastewater treatment due to their excellent synergistic effect, high treatment efficiency, and low energy consumption. A comprehensive summary of these ozone-based EAOPs is still insufficient, though some reviews have covered these topics but either focused on a specific integrated process or provided synopses of EAOPs or ozone-based AOPs. This review presents an overview of the fundamentals of several ozone-based EAOPs, focusing on process optimization, electrode selection, and typical reactor designs. Additionally, the service life of electrodes and improvement strategies for the stability of ozone-based EAOPs that are ignored by previous reviews are discussed. Furthermore, four main application fields are summarized, including disinfection, emerging contaminants treatment, industrial wastewater treatment, and resource recovery. Finally, the summary and perspective on ozone-based EAOPs are proposed. This review provides an overall summary that would help to gain insight into the ozone-based EAOPs to improve their environmental applications.

Exploring the zero-valent iron (ZVI) mediated transformation of dissolved organic nitrogen (DON) in anammox system using FT-ICR MS and fluorescence spectroscopy

With fluorescence spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), this study evaluated the performance of zero-valent iron (ZVI) enhanced anammox process for treating fulvic acids (FA)-containing wastewater and investigated the fate of dissolved organic nitrogen (DON) at the molecular level. The results showed that ZVI increased the total organic carbon (TOC) removal rate of the anammox system by approximately 10% and reduced the organic matter fluorescence intensity by 21%. Specifically, microbial humic-like C1 and terrestrial humic-like C4 components were preferentially degraded by ZVI among the fluorescence components (C1-C4). Moreover, ZVI significantly altered the transformation pathways of DON molecules in anammox. The number of precursors molecular formulas increased from 1617 to 2002, attributed to the elimination of high molecular weight (MW) (>500 Da) molecules. Specifically, DON molecules with high O/C were degraded under the effect of ZVI, while some high MW were resisted. The original products generated from anammox consortia metabolism and FA degradation underwent secondary reactions with ZVI, thereby decreased the O/C of products molecular formulas. Furthermore, anammox process was subjected to the most carboxylic acid reaction, including decarboxylation and reduction of carboxylic acids. For CHON molecules, a significant number of reactions involving the reduction of nitro groups contributed to the reduction of wastewater toxicity, which improved the overall performance.

A temperature-resilient anammox process for efficient treatment of rare earth element tailings wastewater via synergistic nitrite supply of partial nitritation and partial denitrification

Rare earth elements result in substantial tailings wastewater with high ammonium and nitrate during extraction. In this study, a temperature-resilient Anammox process was employed for efficient treatment of rare earth element tailings wastewater through implementing synergistic nitrite supply by partial nitritation (PN) and partial denitrification (PD). Enhancing temperature resilience of Anammox process relies on dynamic management of DO and COD inputs to shift the dominant nitrite supplier from PN to PD, stable PD (NAR ≥ 90 %) can boost nitrogen removal by Anammox to 97.8 %. The nitrogen removal rate and nitrogen removal efficiency at 10.6 °C could maintain at 0.12 kgN/m3·d-1 and 92.5 %, respectively. Microbial analysis reveals that Nitrosomonas, Thauera, and Candidatus_Kuenenia are the predominant genera responsible for nitrite supply and nitrogen removal, localized within the gas channels of granules, flocs, and micro-granules, respectively. Keeping the influent C/NO3--N ratio below 1.7 is ideal to prevent overgrowth of Thauera and maintain system stability.

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

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

Treatment of low-C/N nitrate wastewater using a partial denitrification-anammox granule system: Granule reconstruction, stability, and microbial structure analyses

Industrial wastewater discharged into sewer systems is often characterized by high nitrate contents and low C/N ratios, resulting in high treatment costs when using conventional activated sludge methods. This study introduces a partial denitrification-anammox (PD/A) granular process to address this challenge. The PD/A granular process achieved an effluent TN level of 3.7 mg/L at a low C/N ratio of 2.3. Analysis of a typical cycle showed that the partial denitrification peaked within 15 min and achieved a nitrate-to-nitrite transformation ratio of 86.9%. Anammox, which was activated from 15 to 120 min, contributed 86.2% of the TN removal. The system exhibited rapid recovery from post-organic shock, which was attributed to significant increases in protein content within TB-EPS. Microbial dispersion and reassembly were observed after coexistence of the granules, with Thauera (39.12%) and Candidatus Brocadia (1.25%) identified as key functional microorganisms. This study underscores the efficacy of PD/A granular sludge technology for treating low-C/N nitrate wastewater.

S0-driven partial denitrification coupled with anammox (S0PDA) enables highly efficient autotrophic nitrogen removal from wastewater

This study proposed a novel strategy that integrates S0 particles (diameter: 2-3 mm) and granular sludge to establish S0-driven partial denitrification coupled with anammox (S0PDA) process for autotrophic nitrogen removal from NH4+- and NO3--containing wastewaters. This process was evaluated using an up-flow anoxic sludge bed bioreactor, operating continuously for 240 days. The influent concentrations of NH4+ and NO3- were 29.9 ± 2.7 and 50.2 ± 2.7 mg-N/L, respectively. Throughout the operation, the hydraulic retention time was shortened from 4.0 h to 2.0 h, while the effluent concentrations of NH4+ and NO3- were maintained at a desirable level of 1.45-1.51 mg-N/L and 4.46-6.52 mg-N/L, respectively. Despite an autotrophic process, the nitrogen removal efficiency and rate reached up to 88.5 ± 2.0 % and 1.75 ± 0.07 kg-N/(m3·d), respectively, indicating the remarkable robustness of the S0PDA process. Autotrophic anammox and sulfur-oxidizing bacteria (Candidatus Brocadia and Thiobacillus) were the predominant bacterial genera involved in the S0PDA process. Candidatus Brocadia was primarily enriched in the granular sludge, with a relative abundance of 6.70 %. Thiobacillus occupied a unique niche on the S0 particles, with a relative abundance as high as 57.6 %, of which Thiobacillus thioparus with partial denitrification function (reducing NO3- to NO2- without further reduction to N2) accounted for 78.0 %. These findings challenge the stereotype of low efficiency in autotrophic nitrogen removal from wastewater, shedding fresh light on the applications of autotrophic processes.

Rapid establishment of municipal sewage partial denitrification-anammox for nitrogen removal through inoculation with side-stream anammox biofilm without domestication

Mainstream partial denitrification anammox was achieved through inoculation of side-stream mature partial nitritation anammox biofilm without domestication. The contribution of anammox to nitrogen removal was 29.4 %. Moreover, prolonging anoxic hydraulic retention time and introducing side-stream nitrite under different carbon/nitrogen ratios enriched anammox bacteria. The abundance of anammox bacteria increased by ∼ 10 times ((2.19 ± 0.17) × 1012 copies gene / g dry sludge) with a total relative abundance of 18.51 %. During 258 days of operation, the contribution of anammox to nitrogen removal gradually increased to 68.8 %. The total nitrogen in the effluent decreased to 8.84 mg/L with a total nitrogen removal efficiency of 76.4 % under a carbon/nitrogen ratio of 3. This paper proposes a novel way to rapidly achieve mainstream partial denitrification anammox via inoculation with side-stream mature partial nitritation anammox biofilm. This method achieves advanced nitrogen removal from municipal wastewater, even under low carbon/nitrogen ratios.

Partial denitrifying phosphorus removal coupling with anammox (PDPRA) enables synergistic removal of C, N, and P nutrients from municipal wastewater: A year-round pilot-scale evaluation

Applying anaerobic ammonium oxidation (anammox) in municipal wastewater treatment plants (MWWTPs) can unlock significant energy and resource savings. However, its practical implementation encounters significant challenges, particularly due to its limited compatibility with carbon and phosphorus removal processes. This study established a pilot-scale plant featuring a modified anaerobic-anoxic-oxic (A2O) process and operated continuously for 385 days, treating municipal wastewater of 50 m3/d. For the first time, we propose a novel concept of partial denitrifying phosphorus removal coupling with anammox (PDPRA), leveraging denitrifying phosphorus-accumulating organisms (DPAOs) as NO2- suppliers for anammox. 15N stable isotope tracing revealed that the PDPRA enabled an anammox reaction rate of 6.14 ± 0.18 μmol-N/(L·h), contributing 57.4 % to total inorganic nitrogen (TIN) removal. Metagenomic sequencing and 16S rRNA amplicon sequencing unveiled the co-existence and co-prosperity of anammox bacteria and DPAOs, with Candidatus Brocadia being highly enriched in the anoxic biofilms at a relative abundance of 2.46 ± 0.52 %. Finally, the PDPRA facilitated the synergistic conversion and removal of carbon, nitrogen, and phosphorus nutrients, achieving remarkable removal efficiencies of chemical oxygen demand (COD, 83.5 ± 5.3 %), NH4+ (99.8 ± 0.7 %), TIN (77.1 ± 3.6 %), and PO43- (99.3 ± 1.6 %), even under challenging operational conditions such as low temperature of 11.7 °C. The PDPRA offers a promising solution for reconciling the mainstream anammox and the carbon and phosphorus removal, shedding fresh light on the paradigm shift of MWWTPs in the near future.

Research progress of anaerobic ammonium oxidation (Anammox) process based on integrated fixed-film activated sludge (IFAS)

The integrated fixed-film activated sludge (IFAS) process is considered one of the cutting-edge solutions to the traditional wastewater treatment challenges, allowing suspended sludge and attached biofilm to grow in the same system. In addition, the coupling of IFAS with anaerobic ammonium oxidation (Anammox) can further improve the efficiency of biological denitrification. This paper summarises the research progress of IFAS coupled with the anammox process, including partial nitrification anammox, simultaneous partial nitrification anammox and denitrification, and partial denitrification anammox technologies, and describes the factors that limit the development of related processes. The effects of dissolved oxygen, influent carbon source, sludge retention time, temperature, microbial community, and nitrite-oxidising bacteria inhibition methods on the anammox of IFAS are presented. At the same time, this paper gives an outlook on future research focus and engineering practice direction of the process.

Formation of autotrophic nitrogen removal granular sludge driven by the dual-partition airlift internal circulation: Insights from performance assessment, community succession, and metabolic mechanism

Granular sludge has been recognized as an effective method for the application and industrialization of the anammox-based process due to its good biomass retention capacity and environmental tolerance. In this study, a one-stage autotrophic nitrogen removal (ANR) dual-partition system with airlift internal circulation was implemented for 320 days. A high nitrogen removal efficiency of 84.6% was obtained, while the nitrogen removal rate reached 1.28 g-N/L/d. ANR granular sludge dominated by Nitrosomonas and Candidatus Brocadia was successfully cultivated. Results showed that activity and abundance of functional flora first increased with granulation process, but eventually declined slightly when particle size exceeded the optimal range. Total anammox activity was observed to be significantly correlated with protein content (R2 = 0.9623) and nitrogen removal performance (R2 = 0.8796). Correlation network revealed that AnAOB had complex interactions with other bacteria, both synergy for nitrogen removal and competition for substrate. Changes in abundances of genes encoding the Carbohydrate Metabolism, Energy Metabolism, and Membrane Transport suggested energy production and material transfer were possibly blocked with further sludge granulation. Formation of ANR granular sludge promoted the interactions and metabolism of functional microorganisms, and the complex nitrogen metabolic pathways improved the performance stability. These results validated the feasibility of granule formation in the airlift dual-partition system and revealed the response of the ANR system to sludge granulation.

Rapid start-up strategy and microbial population evolution of anaerobic ammonia oxidation biofilm process for low-strength wastewater treatment

The implementation of the anaerobic ammonium oxidation (anammox) process in treating low-strength wastewater is limited by the difficulty in enriching anammox bacteria (AnAOB). Here, the first enrichment of AnAOB at a high nitrogen (N) loading rate (NLR) as a strategy was proposed to achieve the rapid start-up of the anammox biofilm process treating low-strength wastewater. The long-term stability of the anammox biofilm process after start-up operating at a low NLR of 0.2-0.4 kg N/(m3⋅d) was evaluated. Results showed that the N removal efficiency was up to 75 % under a low NLR of 0.2 kg N/(m3⋅d) condition. Low-strength organic matter promoted the metabolic coupling between partial denitrifying bacteria (PDB) and AnAOB. The genus Candidatus Brocadia as AnAOB (18 %-27 %) can coexist with Limnobacter (PDB, 9 %-12 %) for efficient N removal. This study offers a rapid start-up strategy of anammox biofilm process in treating low-strength wastewater.

Optimized start-up strategies for elemental sulfur packing bioreactor achieving effective autotrophic denitrification

The start-up efficiency of the elemental sulfur packing bioreactor (S0PB) is constrained by the slow growth kinetics of autotrophic microorganisms, which is essentially optimized. This study aims to optimize start-up procedures and offer scientific guidance for the practical applications of S0PB. Through comparing the start-up efficiencies under various conditions related to inoculation, backwashing, and EBCT, it was found that these conditions did not significantly influence start-up time, but they did impact denitrification performance in detail. Using activated sludge as the inoculum was not recommended as the 2.5 ± 0.2 mg-N/L higher nitrite accumulation and 26.0 ± 5.1 % lower TN removal rate, compared to self-enrichment. Starting with a long-to-short EBCT (1 → 0.33 h) achieved higher nitrate removal of 11.5 ± 0.6 mg-N/L and eliminated nitrite accumulation compared to constantly short EBCT (0.33 h) conditions. Daily and postponed backwashing were suggested for long-to-short EBCT and constantly short EBCT start-up, respectively. Enrichment of Sulfurimonas was beneficial for the effective nitrite reduction process.

How to Provide Nitrite Robustly for Anaerobic Ammonium Oxidation in Mainstream Nitrogen Removal

Innovation in decarbonizing wastewater treatment is urgent in response to global climate change. The practical implementation of anaerobic ammonium oxidation (anammox) treating domestic wastewater is the key to reconciling carbon-neutral management of wastewater treatment with sustainable development. Nitrite availability is the prerequisite of the anammox reaction, but how to achieve robust nitrite supply and accumulation for mainstream systems remains elusive. This work presents a state-of-the-art review on the recent advances in nitrite supply for mainstream anammox, paying special attention to available pathways (forward-going (from ammonium to nitrite) and backward-going (from nitrate to nitrite)), key controlling strategies, and physiological and ecological characteristics of functional microorganisms involved in nitrite supply. First, we comprehensively assessed the mainstream nitrite-oxidizing bacteria control methods, outlining that these technologies are transitioning to technologies possessing multiple selective pressures (such as intermittent aeration and membrane-aerated biological reactor), integrating side stream treatment (such as free ammonia/free nitrous acid suppression in recirculated sludge treatment), and maintaining high activity of ammonia-oxidizing bacteria and anammox bacteria for competing oxygen and nitrite with nitrite-oxidizing bacteria. We then highlight emerging strategies of nitrite supply, including the nitrite production driven by novel ammonia-oxidizing microbes (ammonia-oxidizing archaea and complete ammonia oxidation bacteria) and nitrate reduction pathways (partial denitrification and nitrate-dependent anaerobic methane oxidation). The resources requirement of different mainstream nitrite supply pathways is analyzed, and a hybrid nitrite supply pathway by combining partial nitrification and nitrate reduction is encouraged. Moreover, data-driven modeling of a mainstream nitrite supply process as well as proactive microbiome management is proposed in the hope of achieving mainstream nitrite supply in practical application. Finally, the existing challenges and further perspectives are highlighted, i.e., investigation of nitrite-supplying bacteria, the scaling-up of hybrid nitrite supply technologies from laboratory to practical implementation under real conditions, and the data-driven management for the stable performance of mainstream nitrite supply. The fundamental insights in this review aim to inspire and advance our understanding about how to provide nitrite robustly for mainstream anammox and shed light on important obstacles warranting further settlement.

Carbon-Restricted Anoxic Zone as an Overlooked Anammox Hotspot in Municipal Wastewater Treatment Plants

The anoxic zone serves as the core functional unit in municipal wastewater treatment plants (MWWTPs). Unfortunately, in most cases, the downstream range of the anoxic zone is severely lacking in available organic carbon and thus contributes little to the removal of nutrients. This undesirable range is termed the "carbon-restricted anoxic zone", representing an insurmountable drawback for traditional MWWTPs. This study uncovers a previously overlooked role for the carbon-restricted anoxic zone: a hotspot for anaerobic ammonium oxidation (anammox). In a continuous-flow pilot-scale plant treating municipal wastewater (55 m3/d), virgin biocarriers were introduced into the carbon-restricted anoxic zone (downstream 25% of the anoxic zone with BOD5 of 5.9 ± 2.3 mg/L). During the 517-day monitoring, anammox bacteria highly self-enriched within the biofilms, with absolute and relative abundance reaching up to (9.4 ± 0.1) × 109 copies/g-VSS and 6.17% (Candidatus Brocadia), respectively. 15N isotopic tracing confirmed that anammox overwhelmingly dominated nitrogen metabolism, responsible for 92.5% of nitrogen removal. Following this upgrade, the contribution ratio of the carbon-restricted anoxic zone to total nitrogen removal increased from 9.2 ± 4.1% to 19.2 ± 4.2% (P < 0.001), while its N2O emission flux decreased by 84.5% (P < 0.001). These findings challenge stereotypes about the carbon-restricted anoxic zone and highlight the multiple environmental implications of this newfound anammox hotspot.

Effect mechanism of micron-scale zero-valent iron enhanced pyrite-driven denitrification biofilter for nitrogen and phosphorus removal

This study aims to explore the effect mechanism of micron-scale zero-valent iron (mZVI) to improve nitrogen and phosphorus removal in a pyrite (FeS2)-driven denitrification biofilter (DNBF) for the secondary effluent treatment. Two similar DNBFs (DNBF-A with FeS2 as fillers and DNBF-B with the mixture mZVI and FeS2 as carrier) were developed. The results showed that NO3--N, total nitrogen (TN) and PO43--P removal efficiencies were up to 91.64%, 67.44% and 80.26% in DNBF-B, which were obviously higher than those of DNBF-A (with NO3--N, TN and PO43--P removal efficiencies of 38.39%, 44.89% and 53.02%, respectively). Kinetic analysis of both PO43--P and NO3--N showed an increase in the rate constant (K) for DNBF-B compared to DNBF-A. The addition of mZVI not only improved the electron transport system activity (ETSA), but also achieved system Fe(II)/Fe(III) redox cycle in DNBF-B. In addition, the high-throughput sequencing analysis indicated that the addition of mZVI could obviously stimulate the enrichment of functional bacteria, such as Thiobacillus (11.99%), Mesotoga (7.50%), JGI-0000079D21 (6.37%), norank_f__Bacteroidetes_vadinHA17 (6.19%), Aquimonas (5.93%) and Arenimonas (3.97%). These genus played the important role in nitrogen and phosphorus removal in DNBF-B. Addition mZVI in the FeS2-driven denitrification biofilter is highly promising for TN and TP removal during secondary effluent treatment.

Study on H2SO4-modified corn straw biochar as substrate material of constructed wetland

The high value resource utilization of corn straw is a long-term problem at present and in the future. Biochar preparation is an important utilization way of corn straw. The research on city tail water treated by constructed wetland (CW) with biochar was carried out to further increase the wastewater treatment capacity of the CW. Surface characterization, structural characteristics, and adsorption of straw biochar modified by different acids were measured. The study found that the ability of H2SO4 to remove ash from biochar was stronger than other acids and H2SO4-biochar was easy to be cleaned without H2SO4 residue. The performance of biochar modified by H2SO4 was obviously better than other acids, and the biochar adsorption was enhanced. The modification of biochar substrate modified by H2SO4 in CW reduced the change of electrical conductivity (EC) and promoted denitrification. H2SO4-modified biochar promoted the absorption of N and P by Iris pseudacorus L. The compound modification effect of straw biochar was obvious. The results revealed the acid modification characteristics of straw biochar, which were beneficial for increasing the wastewater treatment rate by CW. This study will promote the sustainable development of CW.

Pilot-scale demonstration of self-enrichment of anammox bacteria in a two-stage nitrification-denitrification suspended sludge system treating municipal wastewater under extremely low nitrogen loading rate

In suspended sludge system, efficient enrichment and retention of anammox bacteria are crucial obstacles in mainstream wastewater treatment by anammox process. In this study, anammox bacteria was self-enriched in a pilot-scale suspended sludge system of two-stage nitrification-denitrification process serving municipal wastewater treatment. With the low ammonia (NH4+-N) of 9.3 mg/L, nitrate (NO3--N) of 15.6 mg/L and COD/NO3--N of 2.2 under extremely low nitrogen loading rate of 0.012 kg N/m3/d, anammox activity bloomed after its abundance increasing from 5.9 × 107 to 4.6 × 109 copies/g dry sludge. Significant NH4+-N removal was occurred and maintained stably in the denitrification reactor with anammox bacteria accounting for 1.13%, even under temperature decreasing to 20.0℃. The adequately anoxic environment, efficient retention with the static settlement, and NO2- production via NO3- reduction provided favorable environment for anammox bacteria. This study demonstrated the feasibility and great potential in mainstream anammox application without seeding specific sludge.

Multi-omics analysis revealed the selective enrichment of partial denitrifying bacteria for the stable coupling of partial-denitrification and anammox process under the influence of low strength magnetic field

The microbial consortium involving anaerobic ammonium oxidation (anammox) and partial denitrification (PD), known as PD-anammox, is an emerging energy-efficient and lower carbon nitrogen removal process from wastewater. However, maintaining a stable PD process by locking nitrate reduction until nitrite was challenging. This study established the first stable connection of anammox with constant nitrite generation by PD bacteria under a low-strength (1.3 mT) magnetic field (MF). When the nitrogen loading rate was 1.81 kg-N/m3/d, the nitrogen removal efficiency of the control reactor (R1) was 75%, lower than that of the experimental reactor (R2), which was 85%. The expression of Thauera and Zoogloea, potential PD bacteria was substantially lower in R1 (5.75% and 1.21%, respectively) than in R2 (10.25 and 6.61%, respectively), according to a meta-transcriptomic analysis. At the same time, the mRNA expression of anammox genera Candidatus Brocadia and Candidatus Kuenenia was 33.53% and 3.83% in R1 and 22.86% and 1.87% in R2. Moreover, carbon and nitrogen metabolism pathways were more abundant under the influence of low-strength MF. The selective enrichment of PD bacteria can be attributed to the increased expression of carbon metabolic pathways like the citrate cycle, glycolysis/gluconeogenesis, and pyruvate metabolism. Interestingly, the control reactor was dominated by a hydroxylamine-dependent anammox process while a low-strength MF-enhanced nitric-oxide-dependent anammox process. For successful anammox-centered nitrogen removal from wastewater, this study demonstrated that low-strength MF is a convenient and applicable technique to lock the nitrate reduction until nitrite.

New insights into enhanced biodegradation of 4-bromphenol in a nitrate-reducing system: Process performance and mechanism

Due to the recalcitrant nature of halogenated phenol, conventional anaerobic bioprocess is often limited by low removal efficiency and poor process stability. At the presence of electron acceptors such as nitrate, 4-bromophenol (4-BP) removal efficiency is significantly higher than that in the anaerobic control system, but the mechanism involved is still unclear. Therefore, an up-flow nitrate-reducing bioreactor (NRBR) was designed and consecutively performed for 215 days to explore the synergistic mechanism for BPs biodegradation and nitrate reduction. Complete 4-BP biodegradation could be obtained in NRBR at HRT and 4-BP loading rate of 24 h and 0.29 mol m - 3d - 1, while the TOC removal and nitrate reduction efficiencies were as high as 91.33±2.11% and 98.31±1.33%, respectively. Population evolution analyses revealed that the microorganisms involved in 4-BP debromination and biodegradation (Candidatus Peregrinibacteria, Denitratisoma, Anaerolineaceae and Ignavibacterium) as well as nitrate reduction (Denitratisoma, Anaerolineaceae, Limnobacter and Ignavibacterium) were significantly enriched in NRBR. Major intermediates during 4-BP biodegradation, including 4-bromocatechol, 4‑bromo-6-oxo-hexanoic acid and succinic acid were identified, while a distinct 4-BP biodegradation pathway via hydration, aromatic-ring cleavage, hydrolysis debromination and oxidation was expounded. Metagenomic analysis indicated that oxidation (had, pht4, boh, butA), hydrolysis debromination ((S)-2-haloacid dehalogenase) and bio-mineralization (gabD, sdhA) of 4-BP were largely enhanced in NRBR. Moreover, carbon, nitrogen, energy and amino acid metabolisms were significantly facilitated with the injection of nitrate in order to provide energy and electron, thus enhanced microbial activities and enzymatic reactions in NRBR. The proposed mechanism provides new insights into our mechanistic understanding of halogenated phenol biodegradation and the development of sustainable bioremediation strategies.

Optimizing nitrogenous organic wastewater treatment through integration of organic capture, anaerobic digestion, and anammox technologies: sustainability and challenges

With China's recent commitment to reducing carbon emissions and achieving carbon neutrality, anaerobic digestion and anaerobic ammonium oxidation (anammox) have emerged as promising technologies for treating nitrogenous organic wastewater. Anaerobic digestion can convert organic matter into volatile fatty acids (VFAs), methane, and other chemicals, while anammox can efficiently remove nitrogen with minimal energy consumption. This study evaluates the principles and characteristics of enhanced chemical flocculation and bioflocculation, as well as membrane separation, for capturing organic matter. Additionally, the paper evaluates the production of acids and methane from anaerobic digestion, exploring the influence of various factors and the need for control strategies. The features, challenges, and concerns of partial nitrification-anammox (PN/A) and partial denitrification-anammox (PD/A) are also outlined. Finally, an integrated system that combined organic capture, anaerobic digestion, and anammox is proposed as a sustainable and effective solution for treating nitrogenous organic wastewater and recovering energy and resources.

From hybrid process to pure biofilm anammox process: Suspended sludge biomass management contributing to high-level anammox enrichment in biofilms

The application of mainstream anammox is highly desirable for municipal wastewater treatment. However, enrichment of anammox bacteria (AnAOB) is challenging, particularly given the vicious competition from denitrifying bacteria (DB). Here, suspended sludge biomass management, a novel operational strategy for hybrid process (suspended sludge/biofilm), was investigated for 570 days based on a modified anaerobic-anoxic-oxic system treating municipal wastewater. By successively decreasing the suspended sludge concentration, the traditional hybrid process was successfully upgraded to a pure biofilm anammox process. During this process, both the nitrogen removal efficiency (NRE) and rate (NRR) were significantly improved (P < 0.001), from 62.1 ± 4.5% to 79.2 ± 3.9% and from 48.7 ± 9.7 to 62.3 ± 9.0 g N/(m³·d), respectively. Mainstream anammox was improved in the following: Candidatus Brocadia was enriched from 0.70% to 5.99% in anoxic biofilms [from (9.94 ± 0.99) × 10⁸ to (1.16 ± 0.01) × 10¹⁰ copies/g VSS, P < 0.001]; the in situ anammox reaction rate increased from 8.8 ± 1.9 to 45.5 ± 3.2 g N/(m³·d) (P < 0.001); the anammox contribution to nitrogen removal rose from 9.2 ± 2.8% to 67.1 ± 8.3% (P < 0.001). Core bacterial microbiome analysis, functional gene quantification, and a series of ex situ batch experiments demonstrated that the stepwise decreases in suspended sludge concentration effectively mitigated the vicious competition of DB against AnAOB, enabling high-level AnAOB enrichment. This study presents a straightforward and effective strategy for enriching AnAOB in municipal wastewater, shedding fresh light on the application and upgradation of mainstream anammox.

Starting-up performances and microbial community shifts in the coupling process (SAPD-A) with sulfide autotrophic partial denitrification (SAPD) and anammox treating nitrate and ammonium contained wastewater

A novel coupling process (SAPD-A) with sulfide autotrophic partial denitrification (SAPD) (NO₃^--N→NO₂^--N) and anaerobic ammonium oxidation (Anammox) was developed using anaerobic sequencing batch reactor (ASBR) in this work. The integrated process comprised two stages. Firstly, the starting-up of SAPD process succeeded by gradually increasing the influent nitrate and sulfide in 95 days. The average nitrate removal efficiency (NRE) and NO₂^--N accumulation rates were 71.24% ± 0.21% and 46.44% ± 0.53% at SAPD process (days 75-95). Then, successful coupling process (SAPD-A) was implemented in two stages (stage I and stage II of SAPD-A). In stage I, it is feasible to promote the successful construction of SAPD-A process by elevating influent ammonium only based on SAPD system, making the NRE increased from 44.45% ± 0.46% (day 95) to 64.62% ± 0.12% at the end of stage I in SAPD-A system (day 126). Meanwhile, the ammonium nitrogen removal efficiency (ARE) and total nitrogen removal efficiency (TN-RE) also rose up to 42.46% ± 2.02% and 63.28% ± 0.54% respectively. Furthermore, the average ARE, NRE and TN-RE during the stage II in the bioreactor could reach 65.17% ± 1.45%, 74.50% ± 0.81% and 77.81% ± 0.37% by loading some biofilters (with of approximate 10% of the volume of the bioreactor) attached anaerobic ammonium oxidation bacteria (AnAOB). High-throughput sequencing results showed that the dominant genera concerning nitrogen removal were norank_f_norank_o_Fimbriimonadates (with the abundance of 2.88-8.54%), norank_ o_ norank _ c_ OM190 (2.48-4.41%), norank_f_norank_o_norank_c_WWE3 (11.01-17.69%), subgroup_10 (1.97-3.81%), Limnobacter（2.17-3.49%）, norank_f_n orank_ o_norank_ c_OLB14 (2.03-5.23%), norank-f-PHOS-HE36 (2.18-5.5%), Ellin6067 (1.34-2.24%) and Candidatus_ Brocadia (1.95-2.42%) during the whole starting-up period of coupling SAPD-A process. Batch experiments revealed that the sulfide was fully oxidized within 2 h, with the maximum reaction rate of 38.30 ± 1.53 mg (L h)^-1 in the first 1 h. Simultaneously, the concentration of nitrate sharply decreased from 53.08 ± 0.23 mg L^-1 to 24.16 ± 0.42 mg L^-1 with the reaction rate of 66.41 ± 2.12 mg (L h)^-1 in 0.5 h. Also, the ammonium concentration significantly declined from 47.88 ± 0.34 mg L^-1 to 10.98 ± 0.39 mg L^-1 in 8 h. Anammox process was responsible for the dominant nitrogen removal in the coupling SAPD-A system.

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

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

Reduction of metronidazole in municipal wastewater and protection of activated sludge system using a novel immobilized Aspergillus tabacinus LZ-M

Metronidazole (MNZ) accumulation inhibits municipal wastewater treatment bio-systems, and an effective solution to augment anaerobic activated sludge (AAS) is required. This research discovered that Aspergillus tabacinus LZ-M could degrade 77.39% of MNZ at 5 mg/L. MNZ was metabolized into urea, and the enzymes involved in its degradation were aminotransferase, methyltransferase, monooxygenase, and CN cleavage hydrolase. The strain was immobilized in polyurethane foam and used in AAS for the treatment of MNZ-containing municipal wastewater. The results showed that, using immobilized LZ-M, MNZ was completely removed, and the degradation efficiency of wastewater's chemical oxygen demand (COD) was increased from 11.7% to 83.31%. The extracellular polymer and ROS levels indicated that MNZ's toxicity on AAS was reduced. Furthermore, bioaugmentation stabilized its microbial community, and decreased MNZ resistance genes. These observations confirm that the immobilized fungi are effective in protecting AAS against antibiotic contamination in the treatment process of municipal wastewater.

Recovery trajectories of the bacterial community at distances in the receiving river under wastewater treatment plant discharge

Microbes in rivers are an important part of the biogeochemical cycle in aquatic ecosystems, and understanding the major factors that influence the composition of microbial communities has an important role in assessing and improving ecosystem functioning. A high-throughput 16 S rRNA gene sequencing technique was employed to sequence bacterial communities in 21 sediment samples and 21 water samples from an urban river WWTP (wastewater treatment plant) discharge. A systematic study of changes in bacterial community composition in downstream river sediment and water was conducted. The study found that compared with the bacterial diversity in the natural upstream area of the wastewater outfall, the bacterial diversity in the sediment lower reaches decreased significantly, while the bacterial abundance and diversity in the water increased significantly. The Mantel test and redundancy analysis showed that the downstream distance and physicochemical properties were significantly related to the succession of bacterial communities in the sediment downstream of the WWTP discharge. Among them, TOC (total organic carbon) was the most important factor affecting the change in the bacterial community in the downstream sediment. The physicochemical properties were significantly correlated with the succession of bacterial communities in the water downstream of the WWTP discharge. Among them, TN (total nitrogen), PO₄^3--P (phosphorus phosphate) and TP (total phosphorus) were the main factors that affected the change in the bacterial community in the downstream water. Key taxa in the co-occurrence network at different distances downstream reflected the depth of the effect of the WWTP effluent on the bacterial community. The bacterial community in the lower reaches of the river sediment showed a strong recovery ability under the influence of pollutants, while the bacterial community in the lower reaches of the river water was difficult to recover under the influence of pollutants. In general, pollutants contained in effluent are the key to changing the composition of bacterial communities in the lower reaches of the river, but exogenous bacteria in effluent are not. This study provides a basis for further improving the effluent discharge standards of WWTPs in the future.

Stable enhanced nitrogen removal from low COD/N municipal wastewater via partial nitrification-anammox in the traditional continuous anoxic/oxic process through bio-augmentation of partial nitrification sludge under decreasing temperatures

The application of partial nitrification-anammox (PNA) in continuous flow processes for treating low COD/N (C/N) sewage remains a critical challenge. Here, a traditional continuous anoxic/oxic (A/O) process was operated to investigate nitrogen removal from municipal wastewater by the bio-augmentation of partial nitrification sludge combined with the inoculation of biocarriers under decreasing temperatures. Stable enhanced nitrogen removal via PNA was achieved. The average total inorganic nitrogen in influent and effluent was 44.3 and 7.1 mg N/L under a low C/N ratio (3.4) and a short hydraulic retention time (8.2 h). The bio-augmentation of partial nitrification sludge enhanced the PNA process under low temperatures (16.9 ± 0.6 °C). The nitrogen removal efficiency remained stable at 83.3 ± 5.7 % as the temperature decreased from 29.1 to 16.3 °C, and the relative abundance of Ca. Brocadia in carrier biofilms increased from 2.22 % to 4.31 % and 3.27 % in two aerobic chambers after 70 days of operation.

Excellent anammox performance driven by stable partial denitrification when encountering seasonal decreasing temperature

Effluent quality deterioration caused by seasonal temperature reductions in wastewater treatment systems using partial anammox technology is a challenge that cannot be ignored. Here, relationships of denitrification and anammox under decreasing temperature were investigated in an anoxic moving bed biofilm reactor (MBBR). Compared with stable partial-denitrification (NO3- → NO2-), the NO2- reduction to N2 was considerably inhibited when the temperature decreased, conversely helping to the competition of NO2- for anammox. Namely, this transformation provided sufficient substrates for anammox bacteria. Although the TIN removal decreased slightly, anammox contribution was robustly maintained at 91.3 ± 6.6 %, even increased. High-throughput sequencing results revealed that anammox bacteria were enriched (0.56 % to 1.22 %). Moreover, qPCR results showed that increased ratio of hzsB/(nirK + nirS) further supported anammox gained an enhancement. This study demonstrated partial-denitrification/anammox process using anoxic MBBR could maintain stable autotrophic nitrogen removal contribution when encountering temperature decrease, providing a new perspective on the application of mainstream anammox.

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

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

Denitrification performance, bioelectricity generation and microbial response in microbial fuel cell - constructed wetland treating carbon constraint wastewater

For the deep reduction of nitrogen, the microbial fuel cell-constructed wetland (MFC-CW) was conducted for treating carbon constraint wastewater. Results indicated that nitrogen removal decreased from 94.96% to 24.96% with influent COD/TN (chemical oxygen demand/total nitrogen) from 4 to 0. MFC-CW was seriously affected by low organic wastewater. Wetland plants contributed to denitrification, with TN removal increasing from 46.13% to 64.87%. The bioenergy output showed a linear relationship with influent COD, and the maximum power density of 1.17 mW/m² was obtained. Correlation analysis indicated that functional genera of Paenibacillus, Trichococcus, norank_KD4-96, norank_OLB14 played a crucial role in nitrogen removal. Influent COD and wetland plants affected carbon and nitrogen metabolisms, and key genes related to denitrification were more abundant in the cathode. Findings illustrated the nitrogen metabolism in MFC-CW with carbon constraint wastewater and will extend the application of MFC-CW in secondary effluent treatment from wastewater treatment plants.

Enhancing nitrogen removal through directly integrating anammox into mainstream wastewater treatment: Advantageous, issues and future study

Anaerobic ammonium oxidation (anammox) has great potential to be applied to the process of nitrogen removal from mainstream wastewater. However, directly applying complete anammox to the mainstream is typically hindered by low temperatures, a low ammonia concentration, and high organic matter concentrations. Directly integrating anammox into mainstream treatment by enhancing the in-situ enrichment of anammox bacteria in wastewater treatment plants (WWTPs) could effectively improve the nitrogen removal efficiency and reduce the treatment cost. A certain anammox bacteria abundance in full-scale WWTPs provides the feasibility of directly integrating anammox into mainstream treatment and realizing partial mainstream anammox. The technical development status of partial anammox and the mechanisms of achieving partial mainstream anammox by aeration and organic control are summarized. This review provides an enhanced understanding of this novel technical route of partial mainstream anammox treatment for improving the quality, performance, and prospects for this technology to be used in upgrading WWTPs.

Nitrogen removal characteristics of biofilms in each area of a full-scale AAO oxidation ditch process

Plastic carriers were installed in different areas of a full-scale anaerobic/anoxic/aerobic (AAO) oxidation ditch process, and the dynamics of nitrogen removal, biofilm morphologies, and microorganism species were investigated. The results showed that the biofilm at the front of the aerobic area (dissolved oxygen [DO] = 0.93 mg L^-1) provided the best denitrification, with specific nitrate and nitrite reduction rates of 10.16 and 3.78 mg·(g·h)^-1, respectively. The biofilm in the middle of the aerobic area (DO = 1.27 mg L^-1) exhibited the best nitrification performance, with a maximum specific ammonia oxidation rate of 3.21 mg·(g·h)^-1. The biofilm at the end of the aerobic area (DO = 0.01 mg L^-1) exhibited the highest anammox potential with a maximum specific anammox rate of 0.67 mg·(g·h)^-1. No correlation was observed between the specific nitrogen removal rates and abundance of nitrogen-removing microorganisms at the genus level. Biofilm denitrification during the process was primarily completed by heterotrophic denitrifying bacteria (Thauera, Acinetobacter, Hyphomicrobium, and Thermomonas). Aerobic denitrifying bacteria (0.19% Thauera and 0.34% Hyphomicrobium) were identified as the main denitrifying bacteria in the middle of the aerobic area. The dominant nitrifying microorganisms in the middle of the aerobic area were Nitrosomonas (0.50%) and Nitrospira (1.04%). A biofilm in the end of the aerobic area exhibited specific anammox potential, which may have been related to the dominance of 0.024% Candidatus Brocadia. Kinetic analysis revealed that adding plastic carriers to the front and middle of the aerobic area contributed to stable nitrogen removal efficiency.

A review of ammonia removal using a biofilm-based reactor and its challenges

Extensive growth of industries leads to uncontrolled ammonia releases to environment. This can result in significant degradation of the aquatic ecology as well as significant health concerns for humans. Knowing the mechanism of ammonia elimination is the simplest approach to comprehending it. Ammonia has been commonly converted to less hazardous substances either in the form of nitrate or nitrogen gas. Ammonia has been converted into nitrite by ammonia-oxidizing bacteria and further reduced to nitrate by nitrite-oxidizing bacteria in aerobic conditions. Denitrification takes place in an anoxic phase and nitrate is converted into nitrogen gas. It is challenging to remove ammonia by employing technologies that do not incur particularly high costs. Thus, this review paper is focused on biofilm reactors that utilize the nitrification process. Many research publications and patents on biofilm wastewater treatment have been published. However, only a tiny percentage of these projects are for full-scale applications, and the majority of the work was completed within the last few decades. The physicochemical approaches such as ammonia adsorption, coagulation-flocculation, and membrane separation, as well as conventional biological treatments including activated sludge, microalgae, and bacteria biofilm, are briefly addressed in this review paper. The effectiveness of biofilm reactors in removing ammonia was compared, and the microbes that effectively remove ammonia were thoroughly discussed. Overall, biofilm reactors can remove up to 99.7% ammonia from streams with a concentration in range of 16-900 mg/L. As many challenges were identified for ammonia removal using biofilm at a commercial scale, this study offers future perspectives on how to address the most pressing biofilm issues. This review may also improve our understanding of biofilm technologies for the removal of ammonia as well as polishing unit in wastewater treatment plants for the water reuse and recycling, supporting the circular economy concept.

Advanced nitrogen removal from real municipal wastewater by multiple coupling nitritation, denitritation and endogenous denitritation with anammox in a single suspended sludge bioreactor

Achieving advanced nitrogen removal based on anammox for treating mainstream municipal wastewater in a single suspended sludge bioreactor is a challenging research topic. In this study, multiple coupling nitritation, denitritation and endogenous denitritation with anammox (PNA-(E)PDA) was simultaneously achieved in a 10 L step-feed bioreactor, which enhanced stable nitrogen removal. After 223 days of operation, the total nitrogen concentrations of the influent and effluent were 70.7 ± 6.1 and 4.3 ± 1.8 mg/L, respectively, when treating municipal wastewater even at C/N ratio of 2.24 with only 5 h of aerobic time (DO: 0.5-0.8 mg/L). After the evolution of nitritation/anammox to PNA-(E)PDA, the contribution of anammox to nitrogen removal increased to 78.6% and the anammox activity increased from 4.3 ± 0.2 to 15.2 ± 0.7 mg NH₄⁺-N/gVSS/d. qPCR results showed that the abundance of anammox bacteria increased from 4.1 × 10⁹ to 4.5 × 10¹⁰ copies/ (g VSS). High-throughput sequencing further revealed that the relative abundance of Candidatus Brocadia, the dominant anammox genus, increased from 0.09 to 0.46%. Based on the strong competitiveness of anammox on nitrite, this novel PNA-(E)PDA process provides a potential strategy for enriching anammox bacteria in municipal wastewater treatment plants.

Metagenomics uncovers the effect of nitrogen-doped graphene on anammox consortia and microbial function

The effect of 50 mg/L nitrogen-doped graphene (N-G) on anammox microbial guild was studied by metagenomics in this paper. The continuous experiment results showed the average NRE improved by 17.57% with N-G addition. The metagenomic analysis revealed N-G significantly increased the relative abundance of dominant AnAOB (Candidatus Kuenenia) from 18.10% to 28.30%. And the FISH assay further manifested N-G promoted the growth of AnAOB biomass. Meanwhile, metagenomics indicated that N-G enriched the abundance of genes (Hzs, Hdh, NosZ, NorB, NirK, NirS and NrfA) involved in nitrogen metabolism to varying degrees. Furthermore, N-G not only improved the microbial functionality in terms of "Metabolism", but markedly upregulated the abundance of c-di-GMP synthesized genes and genes related to quinolone signal molecule, which contributed to more EPS content and better sludge settleability. In brief, this study provided a novel perspective for anammox biomass enrichment, which may be valuable for practical engineering applications of anammox.

Sustainable upgrading of biological municipal wastewater treatment based on anammox: From microbial understanding to engineering application

Anaerobic ammonium oxidation (anammox) has drawn increasing attention as a promising option to energy-neutral wastewater treatment. While anammox process still faces challenges in the low-strength and organics-contained municipal wastewater due to its susceptibility and the technical gaps in substrate supply. Effective strategies for extensive implementation of anammox in municipal wastewater treatment plants (WWTPs) remain poorly summarized. In view of the significance and necessity of introducing anammox into mainstream treatment, the growing understanding not only at level of microbial interactions but also on view of upgrading municipal WWTPs with anammox-based processes need to be considered urgently. In this review, the critical view and comprehensive analysis were offered from the perspective of microbial interactions within partial nitrification- and partial denitrification-based anammox processes. To minimize the microbial competition and enhance the cooperation among anammox bacteria and other functional bacteria, targeted control strategies were systematically evaluated. Based on the comprehensive overview of recent advances, the combination of flexible regulation of input organic carbon with anaerobic/oxic/anoxic process and the integration of sludge fermentation with anoxic biofilms in anaerobic/anoxic/oxic process were proposed as promising solutions to upgrade municipal WWTPs with anammox technology. Furthermore, a new perspective of coupling anammox with denitrifying dephosphatation was proposed as a promising method for in-depth nutrients removal from carbon-limit municipal wastewater in this study. This review provides the critical and comprehensive viewpoints on anammox engineering in municipal wastewater and paves the way for the anammox-based upgrading of municipal WWTPs.

Efficient nitrogen removal from mature landfill leachate in a step feed continuous plug-flow system based on one-stage anammox process

A continuous plug-flow multistage anoxic/oxic (A/O) system based on one-stage partial nitrification coupled anammox (PNA) process with integrated fixed-film activated sludge (IFAS) was established and operated over 400 days. A step feed strategy effectively controlled free ammonia concentration and alleviated impacts on ammonia oxidizing bacteria (AOB) and anammox bacteria (AnAOB). During day 301-405, 98.1% of total inorganic nitrogen was removed from mature landfill leachate, whereas chemical oxygen demand (COD) removal efficiency was 52.9%. With the enrichment of AnAOB in oxic biofilm, nitrogen removal via the anammox pathway reached 94.3%-95.0%. During system operation, the dominant anammox genus shifted from Candidatus_Brocadia to Candidatus_Kuenenia. Fluorescent in situ hybridization (FISH) indicated AnAOB encapsulated by AOB colonies were mainly distributed inside of the biofilm, which promoted nitrite utilization by the anammox process. This innovative system and the results are of great value to practical applications.

Enhancing the treatment performance of partial denitrification/Anammox process at high nitrogen load: Effects of immobilized strain HFQ8C/Non the sludge characteristics

A novel strategy based on quorum sense (QS) was proposed to improve the treatment performance of the partial denitrification/Anammox (PD/A) process at high loads by adding immobilized Pseudomonas sp. HFQ8_C/N, which could release high concentrations of N-butyryl-DL-homoserine lactone (C4-HSL), N-octanoyl-DL- homoserine lactone (C8-HSL) and N-decanoyl-DL-homoserine lactone (C10-HSL). The results showed that adding immobilized HFQ8_C/N improved the sludge activity and settleability, contributing to higher nitrogen removal efficiency at the high nitrogen loading rate (NLR). It was proved that C4-HSL promoted the abundances of Thauera and Candidatus Kuenenia at NLR 1.68-2.52 kg N/(m³·d), while C10-HSL promoted the abundance of Candidatus Brocadia. Besides, C8-HSL and C10-HSL played different regulation roles in the production of protein (PN) in tightly bound extracellular polymeric substances (TB-EPS) at different loads, improving the sludge settleability. This study provided a new way to improve the treatment performance of high-load PD/A processes.