Mainstream anammox in a novel A-2B process for energy-efficient municipal wastewater treatment with minimized sludge production

Effective recovery of the nitritation process through hydrogen peroxide

This study successfully achieved stable nitritation by adding hydrogen peroxide (H2O2) to the nitrification sludge whose nitritation stability had been destroyed. The batch experiment demonstrated that, the activity of ammonia-oxidizing bacteria (AOB) was restored more rapidly than that of nitrite oxidizing bacteria (NOB) after the addition of H2O2, thereby selectively promoting AOB enrichment and NOB washout. When the H2O2 concentration was 6.25 mg/L, the NOB activity was significantly reduced and the nitrite accumulation rate (NAR) was more than 95% after 18 cycles of nitrifying sludge restoration. As a result, H2O2 treatment enabled a nitrifying reactor to recover stable nitritation performance via H2O2 treatment, with the NAR and ammonia removal efficiency (ARE) both exceeding 90%. High-throughput sequencing analysis revealed that H2O2 treatment was successful in restoring nitritation, as the relative abundance of Nitrosomonas in the nitrifying reactor increased from 6.43% to 41.97%, and that of Nitrolancea decreased from 17.34% to 2.37%. Recovering nitritation by H2O2 inhibition is a low operational cost, high efficiency, and non-secondary pollution nitritation performance stabilization method. By leveraging the varying inhibition degrees of H2O2 on AOB and NOB, stable nitrification can be efficiently restored at a low cost and without causing secondary pollution.

Emerging biotechnological applications of anaerobic ammonium oxidation

Anaerobic ammonium oxidation (anammox) is an energy-efficient method for nitrogen removal that opens the possibility for energy-neutral wastewater treatment. Research on anammox over the past decade has primarily focused on its implementation in domestic wastewater treatment. However, emerging studies are now expanding its use to novel biotechnological applications and wastewater treatment processes. This review highlights recent advances in the anammox field that aim to overcome conventional bottlenecks, and explores novel and niche-specific applications of the anammox process. Despite the promising results and potential of these advances, challenges persist for their real-world implementation. This underscores the need for a transition from laboratory achievements to practical, scalable solutions for wastewater treatment which mark the next crucial phase in the evolution of anammox research.

Toxic effect of copper ions on anammox in IFFAS process filled with ZVI-10 modified carriers

The inhibitory effects of heavy metals on anammox bacteria (AnAOB) have attracted attention worldwide. However, most are conducted in activated sludge rather than biofilm systems. The toxic effect and resistance response of anammox biofilm are not predictable from those of free-living AnAOB. Zero valent iron (ZVI) has been demonstrated to enhance anammox performance, but whether ZVI can promote AnAOB resistance to heavy metal stress remains unclear. Herein, the toxic effect of copper ions (Cu(II)) on anammox in integrated floating-film activated sludge (IFFAS) process filled with 10 wt% ZVI modified carriers (R1) was investigated. Results indicated half inhibiting concentration (IC50) of Cu(II) in R1 was 9.13 mg/L, which was much higher than that in R0 filled with conventional carriers made of high density polyethylene (HDPE) (3.94 mg/L). Long-term effect of Cu(II) demonstrated that Cu(II) concentrations less than 1.0 mg/L could not inhibit anammox biofilm significantly, whereas R1 performed better anammox process than R0 under the stress of 0.1-1.0 mg/L Cu(II). The ZVI modified carriers induced more extracellular polymeric substances (EPS) to trap Cu(II) to attenuate the toxicity to AnAOB. Besides, the activities of functional enzymes related to anammox (NIR and HDH), as well as heme-c contents, were always higher in R1 than R0 regardless of the Cu(II) dosage. Candidatus Kuenenia was identified as the predominant AnAOB, which had stronger resistance to Cu(II) stress compared to other genera in the IFFAS process. Metal resistance genes (MRGs) analysis identified AnAOB induced multi-responses to resist Cu(II) stress, such as the up-regulation of copC, cutA, cutC, cutF, cueR and cueO, to synthesize more proteins with functions of copper exocytosis, conjugation and oxidation.

Larger hydroxyapatite aggregation from Ca2+ adhesion in ANAMMOX granular sludge caused by high dissolved oxygen

Anaerobic ammonia oxidation (ANAMMOX), a sustainable biological process, is promising to remove NH4+-N from municipal sewage. In this study, results showed that the anammox granular sludge morphology changes with the alternation of dissolved oxygen (DO), mainly attributing to the adhesion of calcium ions (Ca2+) to the surface of sludge particles. Diverse characterization methods revealed that gray adhesions in the form of hydroxyapatite covered the original holes on the anammox granular sludge surface, including scanning Electron Microscopy (SEM), digital camera images, Energy Dispersive Spectrometer (EDS), and X-ray diffraction (XRD). Ex-situ degradation of NH4+-N and NO2--N yielded diverse outcomes. The protein to polysaccharide ratio (PN/PS) in the total extracellular polymeric substances (EPS) across 4 size groups demonstrated a decrease under O2 exposure. Microbial community analysis indicated norank_f_A4b and Nitrolancea being the most abundant genus under O2 exposure at day 1 and day 100, respectively. These findings offer an effective strategy to prevent size-larger granular sludge from deteriorating through changing DO and Ca2+ in municipal wastewater in ANAMMOX.

A comprehensive literature mining and analysis of nitrous oxide emissions from different innovative mainstream anammox-based biological nitrogen removal processes

The biological nitrogen removal (BNR) process in wastewater treatment plants generates a substantial volume of nitrous oxide (N2O), which possesses a potent greenhouse gas effect. A limited number of studies have systematically investigated the N2O emissions of anammox-based systems with different BNR processes under mainstream conditions. Based on extensive big data statistical analysis, it had been revealed that simultaneous nitritation, anammox and denitrification (SNAD), partial nitritation anammox (PNA) and partial denitrification anammox (PDA), exhibit significantly lower N2O emission factors when compared to traditional BNR processes. The median values for N2O emission factors were determined to be 1.01 %, 1.15 % and 1.43 % for SNAD, PNA and PDA, respectively. Based on nitrogen removal data and N2O emission factors, the N2O emissions from PNA, SNAD and PDA processes were calculated to be 0.016 g·d-1, 0.037 g·d-1 and 0.008 g·d-1, respectively. Furthermore, the machine learning models (SVM and ANN) exhibited excellent predictive performance for N2O emissions in the BNR processes. However, after removing environmental factors, the R2 value of the SVM model sharply decreased. The SHAP feature analysis demonstrated the significant impact of environmental factors on the accuracy of predictive performance in machine learning models. Spearman correlation analysis was employed to investigate the relationship between N2O emissions and operational factors as well as microbial communities. The results demonstrated a negative correlation between HRT, temperature and C/N with N2O emissions. Moreover, strong associations were observed between Nitrosomonas, Nitrospira, Denitratisoma, Thauera species and N2O emissions. The contribution of N2O production via AOB pathways played a key role that was quantitatively calculated to be 93 %, 80 % and 48 % in the PNA, SNAD and PDA processes, respectively. These findings highlight the potential of these innovative BNR processes in mitigating N2O emissions.

A critical review of improving mainstream anammox systems: Based on macroscopic process regulation and microscopic enhancement mechanisms

Full-scale anaerobic ammonium oxidation (anammox) engineering applications are vastly limited by the sensitivity of anammox bacteria to the complex mainstream ambience factors. Therefore, it is of great necessity to comprehensively summarize and overcome performance-related challenges in mainstream anammox process at the macro/micro level, including the macroscopic process variable regulation and microscopic biological metabolic enhancement. This article systematically reviewed the recent important advances in the enrichment and retention of anammox bacteria and main factors affecting metabolic regulation under mainstream conditions, and proposed key strategies for the related performance optimization. The characteristics and behavior mechanism of anammox consortia in response to mainstream environment were then discussed in details, and we revealed that the synergistic nitrogen metabolism of multi-functional bacterial genera based on anammox microbiome was conducive to mainstream anammox nitrogen removal processes. Finally, the critical outcomes of anammox extracellular electron transfer (EET) at the micro level were well presented, carbon-based conductive materials or exogenous electron shuttles can stimulate and mediate anammox EET in mainstream environments to optimize system performance from a micro perspective. Overall, this review advances the extensive implementation of mainstream anammox practice in future as well as shedding new light on the related EET and microbial mechanisms.

Anammox granule destruction and reconstruction in a partial nitrification/anammox system under hydroxylamine stress

Nitrite oxidizing bacteria (NOB) outcompeting anammox bacteria (AnAOB) poses a challenge to the practical implementation of the partial nitrification/anammox (PN/A) process for municipal wastewater. A granules-based PN/A bioreactor was operated for 260 d with hydroxylamine (NH2OH) added halfway through. qPCR results detected the different amounts of NOB among granules and flocs and the dynamic succession during operation. CLSM images revealed a unique layered structure of granules that NOB located inside led to the inhibition effect of NH2OH delayed. Besides, the physical and morphological characteristics revealed that anammox granules experienced destruction. AnAOB took the broken granules as an initial biofilm aggregate to reconstruct new granules. RT-qPCR and high throughput sequencing results suggested that functional gene expression and community structure were regulated for the AnAOB metabolism process. Correspondingly, the rapid proliferation (0.52 → 1.99%) of AnAOB was realized, and the nitrogen removal rate achieved a nearly quadruple improvement (0.21 → 0.83 kg-N/m3·d). This study revealed that anammox granules can self-reconstruct in the PN/A system when granules are disintegrated under NH2OH stress, broadening the feasibility of applying PN/A process.

Effect of free nitrous acid on extracellular polymeric substances production and membrane fouling in a nitritation membrane bioreactor

The membrane bioreactor (MBR) with nitritation based nitrogen removal processes has attracted growing interest in recent years, although membrane fouling in the nitritation MBR is a challenging issue. In this study, the inhibitory effect of free nitrous acid (FNA) on microbial extracellular polymeric substances (EPS) production and membrane fouling in a nitritation MBR was investigated. Results showed that EPS played a critical role in the biofouling process, and EPS production was affected by FNA concentration. As FNA concentration increased from 5.10 × 10-3 mg N/L to 1.34 × 10-2 mg N/L, protein (PN) and polysaccharide (PS) contents increased from 8.20 to 60.28 mg/g VSS and 4.74-30.46 mg/g VSS, respectively. However, when FNA concentration was 1.48 × 10-2 mg N/L, PN and PS reduced by 20.0% and 10.9%, respectively, indicating that the higher FNA concentration could reduce EPS production. The EPS reduction could be attributed to reduction in the loosely bound (LB) and tightly bound (TB) EPS but not the soluble microbial products (SMP). It was further revealed that higher FNA concentrations up to 1.48 × 10-2 mg N/L consequently mitigate trans-membrane pressure (TMP) rate in terms of dTMP/dt by 25.5% in the nitritation MBR. High throughput sequencing analysis revealed that the increase in FNA led to enrichment of Nitrosomonas but reduction in heterotrophic bacteria. This study showed that the appropriate FNA concentration affected EPS production and hence membrane fouling, leading to the possibility of membrane fouling mitigation by in-situ generated FNA in the nitritation MBR.

Towards low energy-carbon footprint: Current versus potential P recovery paths in domestic wastewater treatment plants

With the unprecedented exhaustion of natural phosphorus (P) resource and the high eutrophication potential of the associated-P discharge, P recovery from the domestic wastewater is a promising way and has been putting on agenda of wastewater industry. To address the concern of P resource recovery in an environmentally sustainable way is indispensable especially in the carbon neutrality-oriented wastewater treatment plants (WWTPs). Therefore, this review aims to offer a critical view and a holistic analysis of different P removal/recovery process in current WWTPs and more P reclaim options with the focus on the energy consumption and greenhouse gas (GHG) emission. Unlike P mostly flowing out in the planned/semi-planned P removal/recovery process in current WWTPs, P could be maximumly sequestered via the A-2B- centered process, direct reuse of P-bearing permeate from anaerobic membrane bioreactor, nano-adsorption combined with anaerobic membrane and electrochemical P recovery process. The A-2B- centered process, in which the anaerobic fixed bed reactor was designated for COD capture for energy efficiency while P was enriched and recovered with further P crystallization treating, exhibited the lowest specific energy consumption and GHG emission on the basis of P mass recovered. P resource management in WWTPs tends to incorporate issues related to environmental protection, energy efficiency, GHG emission and socio-economic benefits. This review offers a holistic view with regard to the paradigm shift from "simple P removal" to "P reuse/recovery" and offers in-depth insights into the possible directions towards the P-recovery in the "water-energy-resource-GHG nexus" plant.

Multi-parameter control-based operation strategy for mainstream deammonification in an integrated anaerobic biofilm reactor-step feed MBR

The mainstream deammonification of municipal wastewater has been recognized as one of the greatest challenges in wastewater engineering. The conventional activated sludge process has disadvantages of high energy input and sludge production. To tackle this situation, an innovative A-B process, where an anaerobic biofilm reactor (AnBR) was functioned as the A stage for energy recovery, and a step-feed membrane bioreactor (MBR) was functioned as the B stage for mainstream deammonification, was constructed for carbon-neutral wastewater treatment. For addressing the challenge associated with selective retention of ammonia-oxidizing bacteria (AOB) over nitrite oxidizing bacteria (NOB), a multi-parameter control-based operation strategy was developed with synergistic control of influent COD redistribution, dissolved oxygen (DO) concentration and sludge retention time (SRT) in the innovative AnBR - step-feed MBR system. Results showed that more than 85% of wastewater COD could be removed with the direct production of methane gas in the AnBR. A relatively stable partial nitritation, which is a prerequisite of anammox, was achieved with the successful suppression of NOB, leading to 98% of ammonium-N and 73% of total nitrogen removed. Anammox bacteria could well survive and enrich in the integrated system, and the contribution of anammox to the total nitrogen removal was more than 70% at optimal conditions. Reactions network involved in the nitrogen transformation in the integrated system was further constructed through the mass balance and microbial community structure analyses. Consequently, this study demonstrated a practically feasible process configuration with high operation and control flexibility towards stable mainstream deammonification of municipal wastewater.

Novel Use of a Ferric Salt to Enhance Mainstream Nitrogen Removal from Anaerobically Pretreated Wastewater

This study aims to demonstrate a new technology roadmap to support the ongoing paradigm shift in wastewater management from pollutant removal to resource recovery. This is achieved by developing a novel use of an iron salt (i.e., FeCl₃) in an integrated anaerobic wastewater treatment and mainstream anammox process. FeCl₃ was chosen to be dosed in a proposed sidestream unit rather than in a primary settler or a mainstream reactor. This causes acidification of returned activated sludge and enables stable suppression of nitrite-oxidizing bacterial activity and excess sludge reduction. A laboratory-scale system, which comprised an anaerobic baffled reactor, a continuous-flow anoxic-aerobic (A/O) reactor, and a secondary settler, was designed to treat real domestic wastewater, with the performance of the system comprehensively monitored under a steady-state condition. The experimental assessments showed that the system had good effluent quality, with total nitrogen and phosphorus concentrations of 12.6 ± 1.3 mg N/L and 0.34 ± 0.05 mg P/L, respectively. It efficiently retained phosphorus in excess sludge (0.18 ± 0.03 g P/g dry sludge), suggesting its potential for further recovery. About half of influent organic carbon was recovered in the form of bioenergy (i.e., methane). This together with low energy consumption revealed that the system could produce a net energy of about 0.11 kWh/m³-wastewater, assessed by an energy balance analysis.

In a quest for high-efficiency mainstream partial nitritation-anammox (PN/A) implementation: One-stage or two-stage?

Partial nitritation-anammox (PN/A) process is known as an energy-efficient technology for wastewater nitrogen removal, which possesses a great potential to bring wastewater treatment plants close to energy neutrality with reduced carbon footprint. To achieve this goal, various PN/A processes implemented in a single reactor configuration (one-stage system) or two separately dedicated reactors configurations (two-stage system) were explored over the past decades. Nevertheless, large-scale implementation of these PN/A processes for low-strength municipal wastewater treatment has a long way to go owing to the low efficiency and effectiveness in nitrogen removal. In this work, we provided a comprehensive analysis of one-stage and two-stage PN/A processes with a focus on evaluating their engineering application potential towards mainstream implementation. The difficulty for nitrite-oxidizing bacteria (NOB) out-selection was revealed as the critical operational challenge to achieve the desired effluent quality. Additionally, the operational strategies of low oxygen commonly adopted in one-stage systems for NOB suppression and facilitating anammox bacteria growth results in a low nitrogen removal rate (NRR). Introducing denitrification into anammox system was found to be necessary to improve the nitrogen removal efficiency (NRE) by reducing the produced nitrate with in-situ utilizing the organics from wastewater itself. However, this may lead to part of organics oxidized with additional oxygen consumed in one-stage system, further compromising the NRR. By applying a relatively high dissolved oxygen in PN reactor with residual ammonium control, and followed by a granules-based anammox reactor feeding with a small portion of raw municipal wastewater, it appeared that two-stage system could achieve a good effluent quality as well as a high NRR. In contrast to the widely studied one-stage system, this work provided a unique perspective that more effort should be devoted to developing a two-stage PN/A process to evaluate its application potential of high efficiency and economic benefits towards mainstream implementation.

Composite carrier enhanced bacterial adhesion and nitrogen removal in partial nitrification/anammox process

The rapid start-up and stable operation of one-stage (Partial nitrification/anammox) PN/A process for low-ammonium wastewater are difficult to be achieved, and many carriers are designed to solve this problem. Here, a composite carrier was developed, in which sepiolite and non-woven fabrics were assembled in polypropylene spherical shells. At the start-up phase, P_A reactor using the composite carriers reached a higher nitrogen removal rate of 134.50 ± 19.60 mg·N·L^-1d^-1, in contrast to that of 48.85 ± 19.64 mg·N·L^-1d^-1 in the P_B reactor without sepiolite carriers. When the final influent ammonium concentration of PN/A process is 100 mg/L, the total nitrogen removal efficiency can reach 72 ± 0.03 %. High biomass immobilization ability of composite carrier was evidenced by the greater adsorption trend between sludge and sepiolite than that between sludge and non-woven fabrics, where hydrophobic interaction and Van der Waals interaction played a major role. Extracellular protein (PN) content and the ratio of PN and extracellular polysaccharide of samples in P_A were significantly higher than those in P_B, verifying higher biofilm formation ability on the composite carrier. The composite carrier also increased the abundance of dominant bacteria in PN/A process, especially AOB, the relative abundance of which reached 46.11 %. And it increased the abundance of essential functional genes for nitrogen conversion as their perfect acid neutralizing effects. This study is of great significance in improving the start-up speed and stable operation of this process.

Design strategy and mechanism of nitrite oxidation suppression of elevated loading rate partial nitritation system

There is a current need for a low operational intensity, effective and small footprint system to achieve stable partial nitritation for subsequent anammox treatment at mainstream municipal wastewaters. This research identifies a unique design strategy using an elevated total ammonia nitrogen (TAN) surface area loading rate (SALR) of 5 g TAN/m2.d to achieve cost-effective, stable, and elevated rates of partial nitritation in a moving bed biofilm reactor (MBBR) system under mainstream conditions. The elevated loaded partial nitritation MBBR system achieves a TAN surface area removal rate (SARR) of 2.01 ± 0.07 g TAN/m2.d and NO2−-N: NH4+-N stoichiometric ratio of 1.15:1, which is appropriate for downstream anammox treatment. The elevated TAN SALR design strategy promotes nitrite-oxidizing bacteria (NOB) activity suppression rather than a reduction in NOB population as the reason for the suppression of nitrite oxidation in the mainstream elevated loaded partial nitritation MBBR system. NOB activity is limited at an elevated TAN SALR likely due to thick biofilm embedding the NOB population and competition for dissolved oxygen (DO) with ammonia-oxidizing bacteria for TAN oxidation to nitrite within the biofilm structure, which ultimately limits the uptake of DO by NOB in the system. Therefore, this design strategy offers a cost-effective and efficient alternative for mainstream partial nitritation MBBR systems at water resource recovery facilities.

Biofilm stratification and autotrophic-heterotrophic interactions in a structured bed reactor (SBRIA) for carbon and nitrogen removal

The structured-bed reactor with intermittent aeration (SBRIA) is a promising technology for simultaneous carbon and nitrogen removal from wastewater. An in depth understanding of the microbiological in the reactor is crucial for its optimization. In this research, biofilm samples from the aerobic and anoxic zones of an SBRIA were analyzed through 16S rRNA sequencing to evaluate the bacterial community shift with variations in the airflow and aeration time. The control of the airflow and aeration time were essential to guarantee reactor performances to nitrogen removal close to 80%, as it interfered in nitrifying and denitrifying communities. The aeration time of 1.75 h led to establishment of different nitrogen removal pathways by syntrophic relationships between nitrifier, denitrifier and anammox species. Additionally, the predominance of these different species in the internal and external parts of the biofilm varied according to the airflow.

Reactivated biofilm coupling n-DAMO with anammox achieved high-rate nitrogen removal in membrane aerated moving bed biofilm reactor

As a promising technology, the combination of nitrate/nitrite-dependent anaerobic methane oxidation (n-DAMO) with Anammox offers a solution to achieve effective and sustainable wastewater treatment. However, this sustainable process faces challenges to accumulate sufficient biomass for reaching practical nitrogen removal performance. This study developed an innovative membrane aerated moving bed biofilm reactor (MAMBBR), which supported sufficient methane supply and excellent biofilm attachment, for cultivating biofilms coupling n-DAMO with Anammox. Biofilms were developed rapidly on the polyurethane foam with the supply of ammonium and nitrate, achieving the bioreactor performance of 275 g N m^-3 d^-1 within 102 days. After the preservation at -20 °C for 8 months, the biofilm was successfully reactivated and achieved 315 g N m^-3 d^-1 after 188 days. After reactivation, MAMBBR was applied to treat synthetic sidestream wastewater. Up to 99.9% of total nitrogen was removed with the bioreactor performance of 4.0 kg N m^-3 d^-1. Microbial community analysis and mass balance calculation demonstrated that n-DAMO microorganisms and Anammox bacteria collectively contributed to nitrogen removal in MAMBBR. The MAMBBR developed in this study provides an ideal system of integrating n-DAMO with Anammox for sustainable wastewater treatment.

Water-energy-greenhouse gas nexus of a novel high-rate activated sludge-two-stage vertical up-flow constructed wetland system for low-carbon wastewater treatment

Municipal wastewater treatment which is associated with high energy consumption and excessive greenhouse gas (GHG) emissions, has been facing severe challenges toward carbon emissions. In this study, a high-rate activated sludge-two-stage vertical up-flow constructed wetland (HRAS-TVUCW) system was developed to reduce carbon emissions during municipal wastewater treatment. Through carbon management, optimized mass and energy flows were achieved, resulting in high treatment efficiency and low operational energy consumption. The carbon emission of the HRAS-TVUCW system (i.e., 0.21 kg carbon dioxide equivalent/m³ wastewater) was 4.1-folds lower than that of the conventional anaerobic/anoxic/aerobic (A²O) process. Meanwhile, the recovered energy from the HRAS-TVUCW system increased its contribution to carbon neutrality to 40.2%, 4.6-folds higher than that of the A²O process. Results of functional microbial community analysis at the genus level revealed that the controlled dissolved oxygen allocation led to distinctive microbial communities in each unit of HRAS-TVUCW system, which facilitated denitrification efficiency increase and carbon emissions reduction. Overall, the HRAS-TVUCW system could be considered as a cost-effective and sustainable low-carbon technology for municipal wastewater treatment.

A critical review on microbial ecology in the novel biological nitrogen removal process: Dynamic balance of complex functional microbes for nitrogen removal

The novel biological nitrogen removal process has been extensively studied for its high nitrogen removal efficiency, energy efficiency, and greenness. A successful novel biological nitrogen removal process has a stable microecological equilibrium and benign interactions between the various functional bacteria. However, changes in the external environment can easily disrupt the dynamic balance of the microecology and affect the activity of functional bacteria in the novel biological nitrogen removal process. Therefore, this review focuses on the microecology in existing the novel biological nitrogen removal process, including the growth characteristics of functional microorganisms and their interactions, together with the effects of different influencing factors on the evolution of microbial communities. This provides ideas for achieving a stable dynamic balance of the microecology in a novel biological nitrogen removal process. Furthermore, to investigate deeply the mechanisms of microbial interactions in novel biological nitrogen removal process, this review also focuses on the influence of quorum sensing (QS) systems on nitrogen removal microbes, regulated by which bacteria secrete acyl homoserine lactones (AHLs) as signaling molecules to regulate microbial ecology in the novel biological nitrogen removal process. However, the mechanisms of action of AHLs on the regulation of functional bacteria have not been fully determined and the composition of QS system circuits requires further investigation. Meanwhile, it is necessary to further apply molecular analysis techniques and the theory of systems ecology in the future to enhance the exploration of microbial species and ecological niches, providing a deeper scientific basis for the development of a novel biological nitrogen removal process.

Response of the mainstream anammox process to the biodegradable carbon sources in the granule-based systems: The difference in self-stratification of the microbial community

The inevitable introduction of biodegradable carbon sources (such as monosaccharides and volatile fatty acids) originating from pretreatment units might affect the performance of the mainstream anaerobic ammonium oxidation (anammox) process. Two model carbon sources (glucose and acetate) were selected to investigate their effects on granule-based anammox systems under mainstream conditions (70 mg total nitrogen (TN) L^-1, 15 °C). At a nitrogen loading rate of 2.87 ± 0.80 kg N m^-3 d^-1, a satisfactory effluent quality (TN < 10 mg L^-1) was achieved in the presence of glucose or acetate at a chemical oxygen demand (COD/N) ratio of 0.5. The contribution of anammox to nitrogen removal decreased with increasing COD/N ratio to 1.0 because the expression of anammox functional genes was inhibited, whereas the expression of denitrifying functional genes was promoted. However, the nitrogen removal efficiency of the two considered reactors was maintained above 80 %. Self-stratification of the microbial community along the reactor height facilitated a functional balance through the retention of anammox bacteria in granules but resulted in washout of denitrifying bacteria in flocs under a high-flow pattern. These findings highlighted the advantages of granule-based systems in the mainstream anammox process due to their inherent biomass self-segregation property and the need for the development of targeted biomass retention strategies.

Double anammox process in the AOAO process of treating real low C/N sewage: Validation, enhancement, and quantification of the contribution of anammox in the oxic zone

Enhancement of anaerobic ammonium oxidation (anammox) process and enrichment of anammox bacteria in the oxic zone of mainstream sewage treatments are complex. Also, quantification of the anammox contribution for nitrogen removal in the oxic zone is hindered owing to the simultaneous occurrence of anammox and nitrification. An alternating anaerobic/oxic/anoxic/oxic bioreactor whose oxic zone boosted partial nitrification coupling anammox (PN/A) and anoxic zone boosted partial denitrification coupling anammox (PD/A), respectively, was operated to treat real sewage for >380 days. Desirable nitrogen removal (effluent total inorganic nitrogen (TIN) of 4.7 ± 1.9 mg N/L) was obtained from low carbon/nitrogen (3.6 ± 0.5) sewage with ammonium concentration of 52.5 ± 2.2 mg N/L in the influent. Under the condition of dissolved oxygen (DO) of 1.5-3 mg/L, anammox bacteria was still enriched within the aerobic biofilms, with the relative abundance increasing to 8.2 % (day 345) from 0 % (no biomass on day 1), which was higher than the value in the flocculent sludge (0.03 %) (P < 0.001). PN driven by flocculent sludge with high activity of ammonium oxidized bacteria (AOB) ensured sufficient nitrite supply for the anammox process with the existence of continuous DO. During the steady operation period, the maximum anammox contribution in the oxic zone was quantified to be 10.6 % by withdrawing aerobic biofilms from the system.

Biological nitrogen removal from low carbon wastewater

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

A-Stage process - Challenges and drawbacks from lab to full scale studies: A review

In response to the growing global resource scarcity, wastewater is increasingly seen as a valuable resource to recover and valorise for the benefit of the society rather than another waste that needs treatment before disposal. Conventional wastewater treatment plants (WWTPs) oxidise most of the organic matter present in wastewater, instead of recovering it as a feedstock for biomaterials or to produce energy in the form of biogas. In contrast, an A-Stage is capable of producing a concentrated stream of organic matter ready for valorisation, ideally suited to retrofit existing large plants. This technology is based on the principle of high-rate activated sludge process that favours biosorption and storage over oxidation. In this paper, we summarize peer-reviewed research of both pilot-scale and full-scale studies of A-Stage process under real conditions, highlighting key operational parameters. In the majority of published studies, the sludge retention time (SRT) was identified as a key operational parameter. An optimal SRT of 0.3 days seems to maximize the redirection of influent COD - up to 50% to the sludge flux, while simultaneously keeping mineralization under 25% of total influent COD. Other key optimal parameters are a hydraulic residence time of 30 min and dissolved oxygen levels of 0.5 mg⋅L^-1. In addition, nutrient removal efficiencies of 15-27% for total nitrogen and 13-38% for total phosphorus are observed. Influence of mixing on settling efficiencies remain largely underexplored, as well as impact of wet weather flow and temperature on overall recovery efficiencies, which hinders to provide recommendations on these aspects. Evolution of modelling efforts of A-Stage process are also critically reviewed. The role of extracellular polymeric substances remain unclear and measures differ greatly according to the different studies and protocols. Better understanding the settling processes by adding Limit of Stokesian and Threshold of Flocculation measures to Sludge Volume Index could help to reach a better understanding of the A-Stage process. Reliable modelling can help new unit processes find their place in the whole treatment chain and help the transition from WWTPs towards Wastewater Resource Recovery Facilities.

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.

Sludge minimization in mainstream wastewater treatment: Mechanisms, strategies, technologies, and current development

Excess sludge production in wastewater treatment plants has become an enormous environmental issue worldwide mainly due to the increased efforts towards wastewater purification. Researchers and plant operators are looking for technological solutions to reduce sludge production through the upgrading of existing technologies and configurations or by substituting them with alternative solutions. Several strategies have been identified to reduce sludge production, including the use of biological and physical-chemical methods (or a combination of them) and novel technologies, although many have not been sufficiently tested at full-scale. To select the most suitable system for sludge reduction, understanding the reduction mechanisms, advantages, disadvantages, and the economic and environmental impact of each technology is essential. This work offers a comprehensive and critical overview of mainstream sludge reduction technologies and underlying mechanisms from laboratory to full scale, and describes potential application, configuration, and integration with conventional systems. Research needs are highlighted, and a techno-economic-environmental comparison of the existing technologies is also proposed.

Deciphering the concurrence of comammox, partial denitrification and anammox in a single low-oxygen mainstream nitrogen removal reactor

One-stage anammox-based autotrophic nitrogen removal technology has attracted increasing interest to sustainable biological nitrogen removal for future wastewater treatment. However, its application in mainstream municipal wastewater treatment is still challenging due to low nitrogen and high organics of raw wastewater. Herein, a novel Simultaneous Carbon Oxidation, partial Comammox, Denitratation and Anammox (SCOCDA) was firstly developed in a single sequencing batch biofilm reactor operated at a dissolved oxygen concentration of ∼0.5 mg/L for treating synthetic municipal wastewater (50 mg/L NH₄⁺-N and 100-250 mg/L COD). The long-term operation showed that almost complete COD and nitrogen removal performance could be achieved at a carbon/nitrogen ratio (COD/NH₄⁺-N) of 3-5 with the corresponding effluent total nitrogen (TN)＜5 mg/L. Microbial community and amoA-targeting amplicon sequencing analysis further verified that comammox Nitrospira spp., denitrifier Thauera and other aerobic/facultative heterotrophs could work synergistically with anammox bacteria, Candidatus Kuenenia. Moreover, nitrogen metabolic and inorganic carbon fixation pathways through the interaction between comammox and anammox were also revealed with the aid of Kyoto Encyclopedia of Genes and Genomes (KEGG). Lastly, potential application of proposed SCOCDA process was illustrated. This research sheds new light on advanced nitrogen removal towards limit of technology via the synergy of comammox and anammox.

Effect of aerobic microbes' competition for oxygen on nitrogen removal in mainstream nitritation-anammox systems

The effects of C/N ratio in mainstream partial nitritation (PN)-anaerobic ammonia oxidation (ANAMMOX) considering competitive relationship of aerobic microbes competing for oxygen were investigated. Thy system was operated for 501 d with various C/N ratio. Competitive growth of aerobic heterotrophic bacteria (AHB) at ≥ 1 of C/N ratio acted effectively on the selective inhibition of nitrite-oxidizing bacteria (NOB) while contributing to stable PN-A. In-depth kinetic analysis indicated oxygen affinity of aerobic microbes was in the order of AHB > ammonia-oxidizing bacteria (AOB) > NOB. In addition, potential of denitritation by AHB could contributed to improving nitrogen removal up to 87.5 ± 4.3%. AHB was comparatively clustered into two groups with a C/N ratio of 1. Nitrosomonas sp. PY1 became predominant while Nitrospira spp. were the major NOB. The potential of AHB in establishing selective inhibition of NOB was identified, which could be a novel approach to stabilze the mainstream PN-A.

A novel process for anammox pretreatment of municipal wastewater: semi-partial nitrification, biological phosphorus removal and recovery

Achieving simultaneous semi-partial nitrification and deep phosphorus removal is a preferred process technology for Anammox pretreatment. In this study, semi-partial nitrification combined with in-situ phosphorus recovery (PNPR) was used to treat municipal wastewater. The SRT conflict between the nitrification and phosphorus removal was resolved by in-situ phosphorus recovery every 20 cycles of Anaerobic/Oxid, and a supernatant with more than 10 times the influent phosphorus concentration was obtained, thus achieving bio-enhanced phosphorus removal and recovery with satisfactory semi-partial-nitrification effluent. Interestingly, the results showed that phosphorus removal and recovery process could improve the activity of AOB. The PNPR system's nitrite accumulation rate (NAR) and phosphorus removal rate (PRR) were more than 90% each, whereas the relative abundance of AOB and PAOs increased from 0.04% to 0.74% and from 0.25% to 0.70%, respectively (P < 0.01). Furthermore, on average, the NO₂^--N_eff/NH₄⁺-N_eff value was 1.96, which laid the foundation for the subsequent anammox treatment.

Advanced organic recovery from municipal wastewater with an enhanced magnetic separation (EMS) system: Pilot-scale verification

The up-concentration process has been demonstrated as an attractive approach to carbon-neutral wastewater treatment. Innovation in the separation processes can help eliminate the current heavy dependence on gravity, and credible pilot-scale verification is crucial for application promotion. We hereby proposed a pilot-scale enhanced magnetic separation (EMS) system as an up-concentration step to maximize energy recovery from municipal wastewater. The design of EMS was based on the hypothesis that magnetic-driven separation could be a breakthrough in separation speed, and adsorption could further enhance the separation efficiency by capturing soluble substances. Jar tests confirmed the feasibility of activated carbon adsorption, which could also roughen the surface of aggregates. Further, over one-year operation of a 300 m³/d EMS equipment provided optimum operation strategies and evidence of system effectiveness. More than 80% of particulate organics and 60% of soluble organics were removed within 10 min at an energy consumption of only 0.036 kWh/m³. The characteristics of sludge were clarified in terms of organic concentration, extracellular polymeric substances composition, and micro-community analysis. The anaerobic experiments further demonstrated the potential value of the concentrated products. Surprisingly, the developed EMS system exhibited significant advantages in time consumption and space occupation, with competitive operating cost and energy consumption. Overall, the results of this study posed the EMS process for up-concentration as a potential approach to organics recovery from municipal wastewater.

Challenges, solutions and prospects of mainstream anammox-based process for municipal wastewater treatment

Anaerobic ammonia oxidation (anammox) process has a promising application prospect for the mainstream deammonification of municipal wastewater due to its high efficiency and low energy consumption. In this paper, challenges and solutions of mainstream anammox-based process are summarized by analyzing the literature of recent ten years. Slow growth rate of anammox bacteria is a main challenge for mainstream anammox-based process, and enhancement of bacteria retention has been recognized to be necessary. Compared with directly increasing sludge retention time (SRT) with membrane bioreactors or sequencing batch reactors, culturing anammox bacteria in the form of biofilm or granule sludge is more promising for its feasibility of eliminating nitrite oxidizing bacteria (NOB). Besides, adding external electron donors or conductive materials and enriching the concentration of ammonia with absorption materials have also been proved helpful to improve the activity of anammox bacteria. Other challenges include the elimination of NOB and achieving ideal ratio of NH₄⁺ and NO₂^-. To solve these problems and achieve stable partial nitrification, composite control strategies based on low SRT and limited aeration are needed based on the special characteristics of ammonia oxidizing bacteria (AOB) and NOB. When treating actual wastewater, interference of low temperature and components in the influent is another problem. Relatively high activity of anammox bacteria has been realized after artificial acclimation at low temperature and the mechanism was also preliminary explored. Different pre-treatment sections have been designed to reduce the concentration of COD and S^2- from the influent. As for the nitrate produced by the anammox reaction, coupling processes are useful to reduce the concentration of nitrate in the effluent. In brief, suitable reactor and coupling process should be selected according to the temperature, influent quality and discharge targets of different regions. The future prospects of the mainstream anammox-based process are also put forward.

Enhanced nutrient removal from mainstream sewage via denitrifying dephosphatation, endogenous denitrification and anammox in a novel continuous flow process

It is a challenging subject to realize nitrogen and phosphorus elimination synchronously from limited-carbon sewage through conventional biological processes. Herein, a novel continuous flow anaerobic/oxic/anoxic/oxic (AOA-O) process, which integrated denitrifying dephosphatation, endogenous denitrification and anammox in the anoxic zone, was developed to enhance nutrient elimination from low carbon/nitrogen sewage (3.4 in average). After the long-term operation (280 days), a satisfactory nutrient removal performance (effluent PO43--P: 0.2 mg P/L, total inorganic nitrogen (TIN):8.9 mg N/L) was obtained. Mass balance indicated that anammox contributed to 26.1% TIN removal and denitrifying dephosphatation contributed to 25.6% phosphorus removal, respectively. The cooperation of anammox bacteria retained in biofilms and endogenous denitrifying bacteria in flocculent sludge was responsible for the enhanced nutrient removal in the anoxic zone. Dechloromonas carried out phosphorus uptake both under oxic conditions and anoxic conditions. This study can broaden the application prospect of mainstream anammox.

Response of nitritation granules to anaerobically pre-treated municipal wastewater at low temperatures in a continuous-flow reactor

Achieving mainstream nitritation with aerobic granules is attractive based on increasing evidence but generally treating artificial low-ammonium wastewater. Real municipal wastewater is much more complex in composition, the behavior of the nitritation granules would be different when treating real municipal wastewater. Herein, the response of nitritation granules to influent shift from artificial low-ammonium (35-40 mg/L) wastewater to anaerobically pre-treated municipal wastewater (MWW_pre-treated) was investigated at low temperatures. Results showed that MWW_pre-treated caused the outgrowth of filamentous bacteria on the granule surface and developed into finger-like structures, which in turn resulted in the decrease of the overall granular sludge settleability. Batch-tests and microbial analysis indicated the functional and microbial differentiation between the newly formed fluffy exterior and the original compact granule. The fluffy exterior was dominated by genus Flavobacterium (66.6%) and primarily functioned as COD removal, whereas the nitrifiers (mainly Nitrosomonas) were still located in the compact core and performed nitritation. Moreover, the heterotrophs-dominated fluffy exterior hindered the oxygen transfer towards nitrifiers located in the compact granule and thereby facilitated the stable NOB repression in the granule particularly at low temperatures (<10 °C). Finally, gradual recovery of the granular sludge morphology and settleability occurred after the influent reverted to synthetic low-ammonium wastewater. Overall, this work demonstrated that the feeding of MWW_pre-treated only caused morphological changes of the nitritation granules, but its structural and functional stability could be maintained stably.

Rapid and stable achievement of mainstream nitritation at low temperature using the competitive inhibition caused by the organics

The unstability of mainstream nitritation at low temperature has always been a limiting factor for the nitrogen removal from domestic wastewater by anammox. In this study, the competitive inhibition caused by the organics has been demonstrated as an effective strategy to rapidly and stably achieve mainstream nitritation. This strategy was based on our novel discovery that nitrite-oxidizing bacteria (NOB) was more susceptible to the organics inhibition than ammonia-oxidizing bacteria (AOB). Batch tests showed while NOB activity gradually decreased by 66.5%, AOB activity only decreased by 31.6% with the initial C/N ratio raising from 0.0 to 3.0. Furthermore, treating the actual domestic wastewater at 14-18 °C, mainstream nitritation was rapidly established within 12 days and then the nitrite accumulation ratio was stable at ∼92.1% during a long-term operation. This strategy could be easily and effectively applied in urban WWTPs through modifying the operation mode.

Pilot-scale demonstration of a novel process integrating Partial Nitritation with simultaneous Anammox, Denitrification and Sludge Fermentation (PN + ADSF) for nitrogen removal and sludge reduction

Anammox process is a cost-effective solution for nitrogen removal, whereas unsatisfactory effluent with nitrate accumulation is usually achieved in treating domestic sewage, owning to the unwanted prevalence of nitrite-oxidizing bacteria (NOB) and the intrinsic nitrate production by anammox bacteria. Herein, a pilot-scale system integrating Partial Nitritation and simultaneous Anammox, Denitrification and Sludge Fermentation (PN + ADSF) process was developed to treat real municipal wastewater. In this process, PN was accomplished in a sequencing batch reactor (SBR) using the strategy of intermittent hydroxylamine addition, while ADSF coupling anammox and heterotrophic denitrification was conducted in an up-flow anaerobic sludge blanket reactor (UASB) to further remove nitrogen. The pilot-scale system achieved total inorganic nitrogen (TIN) concentrations of 10.0 mg N/L in effluent and sludge reduction efficiency of 42.3% simultaneously. The characterization on microbial communities revealed that Candidatus Kuenenia and Thauera were the dominant functional bacteria for anammox and denitrification, respectively. Supported by the slow-release carbon sources from sludge fermentation, heterotrophic denitrification contributed to about 28% of nitrogen removed from the UASB, while anammox played a more important role in nitrogen removal. The pilot-scale demonstration confirmed that the PN + ADSF process is technically feasible for enhanced nitrogen removal and sludge reduction.

Ni(II), Cr(VI), Cu(II) and nitrate removal by the co-system of Pseudomonas hibiscicola strain L1 immobilized on peanut shell biochar

At present, the improvement of nitrate and mixed heavy metals removal in wastewater by microorganism are urgently needed. Previous studies have shown that Pseudomonas hibiscicola strain L1 exhibited Ni(II) removal ability under aerobic denitrification. In this study, the characteristics of the free strain L1, peanut shell biochar (PBC) and further the co-system of strain L1 immobilized on PBC were investigated for the removal of Ni(II), Cr(VI), Cu(II) and nitrate in mix-wastewater. The results illustrated that strain L1 could remove 15.51% - 32.55% of Ni(II) (20-100 mg·L^-1), and removal ratios by co-system were ranked as Ni(II) (81.17%) > Cu(II) (45.84%) > Cr(VI) (38.21%). Scanning Electron Microscope (SEM), X-ray Diffractometer (XRD) and Fourier Transform Infrared Spectroscopy (FTIR) images indicated that the strain L1 immobilized well on PBC and had vigorous biological activity; the crystals of Ni(OH)₂, Cu(OH)₂ and CrO(OH) etc. were formed on surface of co-system with various functional groups participated in. In Sequential Batch Reactor (SBR), the pollutant removal ratios by co-system were higher than that by free strain L1. This study illustrated that the co-system of strain L1 immobilized on PBC was qualified to be applied for practical scenarios of effective heavy metal removal of electroplating mix-wastewater.

Bacterial biofilm and extracellular polymeric substances in the moving bed biofilm reactor for wastewater treatment: A review

Among the several biofilm-based bioreactors, moving bed biofilm reactors (MBBR) have been extensively used for wastewater treatment due to low operational costs, technical feasibility, and stability. Biofilm forming strains, e.g., Stenotrophomonas maltophila DQ01, achieved 94.21% simultaneous nitrification and denitrification (SND) and 94.43% removal of total nitrogen (TN) at a cycle time of 7 h, and a biofilm consortium consisting of Chryseobacteriumsp. andRhodobactersp. achieved 86.8% removal of total organic carbon (TOC) at hydraulic retention time (HRT) of 24 h using lab-scale MBBR. Modifications in the surface properties of the biocarrier materials achieved 99.5 ± 1.1% chemical oxygen demand (COD) and 93.6 ± 2.3% NH₄⁺-N removal, significantly higher than the conventional commercial carrier. This review article summarizes the application of MBBR technology for wastewater treatment. The importance of bacterial biofilm and extracellular polymeric substances (EPS), anammox-n-DAMO coupled processes, and carrier surface modifications in MBBR technology have also been discussed.

Long-term effect of fulvic acid amendment on the anammox biofilm system at 15 ℃: performance, microbial community and metagenomics analysis

The role of fulvic acid (FA) on the anammox system at 15 ℃ was investigated. After operation for 113 days, total inorganic nitrogen removal efficiency in FA amendment reactor achieved to 58.6% on average, higher than that of control group (42.1%). Anammox-related functional genes, i.e., hzo and hzs, also demonstrated higher expression level after introduction of FA. It was observed that Candidatus Kuenenia became more competitive than Candidatus Brocadia with the existence of FA at 15 ℃. Also, co-occurrence analysis showed that FA stimulated the complexity and interactive relationship of microbial communities in the anammox system. Metagenomics analysis revealed that FA introduction stimulated relative abundances of genes in central pathway of tricarboxylic acid cycle such as ACO, IDH, OGDH, SCS, FUM, and MDH. Meanwhile, metabolomics analysis revealed that metabolites related to amino sugar metabolic pathways (glucose 1-phosphate, UDP-D-glucuronate, UDP) and redox reactions (NAD⁺ and NADH) improved in the FA amendment reactor.

Mainstream double-anammox driven by nitritation and denitratation using a one-stage step-feed bioreactor with real municipal wastewater

A novel double-anammox process for advanced mainstream nitrogen removal was established using step-feed sequencing batch reactor (SBR) system with integration of suspend sludge and biofilms. Following optimization of influent distribution ratio, the effluent total inorganic nitrogen (TIN) was < 10.2 mg N/L, with influent TIN of 43.4 mg N/L, and anammox contributed 71.4% to TIN removal. Biological processes and batch tests revealed that gradient C/N reduction promoted denitratation/anammox in anoxic stage, and simultaneous nitritation and anammox were achieved in oxic stage. Specially, anammox maintained on biofilms with abundance over 10⁹ copies/ (g dry sludge). High-throughput sequencing revealed that Thauera and Nitrosomonas were enriched in flocs. Furthermore, metagenomic sequencing confirmed that Thauera owns narG and napA (NO₃^-→NO₂^-) and Nitrosomonas owns amoA (NH₄⁺→NO₂^-), support stable NO₂^- supply for double-anammox. This mainstream anammox-dominant process could potentially be used for stable nitrogen removal in municipal wastewater treatment plants.

Phosphate recovery from the P-enriched brine of AnMBR-RO-IE treating municipal wastewater via an innovated phosphorus recovery batch reactor with nano-sorbents

Municipal wastewater is a very unique pool full of energy and useful substances. Though the innovative integrated anaerobic membrane bioreactor and reverse osmosis-ion exchange (AnMBR-RO-IE) process can produce high-grade reclaimed water with high energy efficiency, phosphorus resources recovery in the WWTPs has been rarely reported thus far. This study evaluated the feasibility of a phosphorus recovery batch reactor (PRBR) as an approach for the phosphate production from the P-enriched brine from AnMBR-RO-IE. With operating PRBR for 162 cycles, high to 85% of P recovery rate was obtained for 145 cycles, leading to a P production rate of 6.17 g/m³ domestic wastewater with nano-sorbents (NSs) consumption rate of 10.2 g/m³. Acidification pretreatment efficiently improved the adsorption capacity and reduced the NSs renewing frequency. High adsorption selectivity of NSs contributed to low impurities (<0.3%) in the P-enriched reclaimed solution. Moreover, the integrated AnMBR-RO-IE-PRBR process saved 47% of energy consumption compared to the present NEWater production process in Singapore. The innovative PRBR reactor was competitive compared to the commonly-used chemical precipitation methods in conventional WWTPs in terms of phosphorus recovery/loss and energy balance. It is expected that the proposed integrated process can offer new insights into the direction of phosphorus reclamation in the future WWTPs.

Chemical oxygen demand and nitrogen transformation in a large pilot-scale plant with a combined submerged anaerobic membrane bioreactor and one-stage partial nitritation-anammox for treating mainstream wastewater at 25 °C

A novel municipal wastewater treatment process towards energy neutrality and reduced carbon emissions was established by combining a submerged anaerobic membrane bioreactor (SAnMBR) with a one-stage partial nitritation-anammox (PN/A), and was demonstrated at pilot-scale at 25 °C. The overall COD and BOD₅ removal efficiencies were 95.1% and 96.4%, respectively, with 20.3 mg L^-1 COD and 5.2 mg L^-1 BOD₅ remaining in the final effluent. The total nitrogen (TN) removal efficiency was 81.7%, resulting 7.3 mg L^-1 TN was discharged from the system. The biogas yield was 0.222 NL g^-1 COD _removed with a methane content range of 78-81%. Approximately 90% of influent COD was removed in the SAnMBR, and 70% of influent nitrogen was removed in the PN/A. The denitrification which occurred in the PN/A enhanced overall COD and nitrogen removal. The successful operation of this pilot-scale plant indicates the SAnMBR-PN/A process is suitable for treating real municipal wastewater.

Achieving high-rate partial nitritation with aerobic granular sludge at low temperatures

Partial nitritation is necessary for the implementation of the mainstream anammox (anaerobic ammonium oxidation) process in wastewater treatment plants. However, the difficulty in outcompeting nitrite-oxidizing bacteria (NOB) at mainstream conditions hinders the performance of partial nitritation. The present work aimed to develop a high-rate partial nitritation process for low-ammonium wastewater treatment at low temperatures by seeding aerobic granules. Experimental results suggested that both stratified structure of nitrifiers developed in the granules and sufficient residual ammonium concentration (18-35 mg N L^-1) in the bulk liquid contributed to efficient NOB repression. With the hydraulic retention time progressively shortened from 1.0 to 0.17 h, the influent nitrogen loading rate of the partial nitritation process reached 6.8 ± 0.4 kg N m^-3 d^-1 even at 10-15 °C. The high concentration (7.5 gVSS L^-1) and activity (0.48 g N g^-1 VSS d^-1 at 11 °C) of granular sludge made the reactor possess an overcapacity evaluated by the ratio between the actual ammonium oxidation rate of the granules and their maximum potential. The overcapacity helped the reactor to face the adverse effect of decreasing temperatures. Overall, this work indicated the great potential of applying aerobic granules to achieve high-rate partial nitritation at mainstream conditions. Moreover, anammox bacteria with a relative abundance of 2.8% was also identified in the partial nitritation granules at the end of this study, suggesting that the granules provided a habitable niche for anammox bacteria growth. Note that these results cannot fully relate to the treatment of real domestic/municipal wastewater, they are a source of important information increasing the knowledge about low temperature partial nitrification.

Achieving advanced nitrogen removal in a novel partial denitrification/anammox-nitrifying (PDA-N) biofilter process treating low C/N ratio municipal wastewater

For achieving mainstream anammox, a novel partial denitrification/anammox-nitrifying (PDA-N) biofilter process to treat municipal wastewater was developed. This process achieved a total inorganic nitrogen (TIN) removal efficiency of 81%, with an average effluent TIN of 7.31 mg·L^-1, when the ratio of influent chemical oxygen demand (COD) to TIN was 3.2. Approximately 97% of the TIN was removed by anammox in the PDA biofilter. Nitrite was provided by partial denitrification for anammox. Partial denitrification was driven by Thaurea in the middle and lower regions of the PDA biofilter, while anammox was mainly driven by Candidatus Brocadia in the middle and upper regions. When treating real municipal wastewater, the TIN was efficiently removed in the PDA-N biofilter, with the effluent TIN of 5.96 mg·L^-1. Anammox played a primary role, achieving approximately 98% of the TIN removal. Compared to the traditional nitrification/denitrification process, this process can economize organic carbon demand and oxygen consumption.

Superior nitrogen removal and sludge reduction in a suspended sludge system with in-situ enriching anammox bacteria for real sewage treatment

Mainstream partial anammox provides a cost- and energy-efficient alternative for wastewater treatment. This study provided a new strategy to achieve mainstream partial anammox in a single-stage suspended sludge system. The novel method coupling external excess sludge fermentation with simultaneous partial nitritation-anammox-denitrification process (SF-SPNAD) was established for 202 days in an anaerobic-aerobic-anoxic sequencing batch reactor (AOA-SBR) with real sewage and actual sludge fermentation products. Under the condition of low DO (0.6 ± 0.2 mg/L), short oxic and long anoxic hydraulic retention time (HRT_oxic = 6.5 h, HRT_anoxic = 8 h), the average total inorganic nitrogen (TIN) concentration in the influent and effluent during 110-day operation were 61.0 and 3.4 mg/L, respectively, and the TIN removal efficiency was 94.56%. Under the inhibitory effect of continuous sludge fermentation products addition, nitrite accumulation ratio reached 99.1% and the external sludge reduction ratio reached 38.75%. ¹⁵N-stable isotope tracing tests showed the great potential of nitrogen removal by anammox pathway in the system. High-throughput sequencing confirmed that CandidatusBrocadia (not detected to 0.50%) and CandidatusKuenenia (not detected to 0.06%) were successfully in-situ enriched. Nitrogen conversion pathways based on stoichiometry and cycle tests show that 34.69% of the TIN removal was obtained by simultaneous nitritation denitrification and anammox under oxic stage and 35.21% of the TIN removal was carried out by anammox under anoxic stage. Overall, the SF-SPNAD process provides a new possibility for coupling autotrophic and heterotrophic nitrogen removal with excess sludge utilization.

Potential of anammox process towards high-efficient nitrogen removal in wastewater treatment: Theoretical analysis and practical case with a SBR

Anammox process has been widely applied in the wastewater nitrogen removal for its high rate and low cost. However, few researches reported the process potential in treating low-strength nitrogen wastewater. In this study, a sequencing batch reactor (SBR) was taken to explore the feasibility of low-strength nitrogen wastewater treatment by anammox process in theory and practice. The practical operation indicated that the effluent with satisfactory quality (1.90 ± 0.70 mg-TN·L^-1) could be achieved, when the SBR was fed with low-strength nitrogen influent (6.20 ± 0.45 mg-NH₄⁺-N·L^-1 and 7.96 ± 0.59 mg-NO₂^--N·L^-1). The hydraulic retention time (HRT), nitrogen removal efficiency, nitrogen removal rate (NRR) and hydraulic loading rate of SBR were 5.42 h, 86.5%, 0.054 kg-N·m^-3·d^-1 and 4.43 m³·m^-3·d^-1 during the 79-day operation, respectively. The theoretical analysis revealed the potential of anammox SBR. When SBR is stably operated, the maximum NRR would be 0.062 kg-N·m^-3·d^-1 if the effluent nitrogen was required to be as low as 3 mg·L^-1. The NRR value is feasible for engineering. However, considering the lower specific substrates utilization rate in practice, the maximum stable NRR was calibrated and found inefficient afterwards. In order to improve the potential of anammox process, the reactors without back mixing and with periodic bioaugmentation should be taken in priority for the engineering applications. In particular, the bioaugmentation frequency and single addition amount were calculated as 7 d and 0.3 g-VSS·L^-1, respectively. The results may provide guidance for the development of high-efficient and stable nitrogen removal process under low-strength condition.

Recent advances in partial denitrification-anaerobic ammonium oxidation process for mainstream municipal wastewater treatment

To solve the bottleneck of the unstable accumulation of nitrite in the partial nitrification (PN)-anammox (AMX) in municipal wastewater treatment, a novel process called partial denitrification (PD)-AMX has been developed. PD-AMX, which is known for cost-efficiency and environmental friendliness, has currently exhibited a promising potential for the removal of biological nitrogen from municipal wastewater and has attracted much research interest regarding its process mechanisms, as well as its practical applications. Here, we review the recent advances in the PD process and its coupling to the anammox process, including the development, basic principles, main characteristics, and critical process parameters of the stable operation of the PD-AMX process. We also explore the microbial community and its characteristics in the system and summarize the knowledge of the dominant bacteria to clarify the key factors affecting PD-AMX. Then, we introduce the engineering feasibility and economic feasibility as well as the potential challenges of the process. The induction and implementation of partial denitrification and maintenance of mainstream anammox are critical issues to be urgently solved. Meanwhile, the implementation of a full mainstream anammox application remains burdensome, while the mechanism of partial denitrification coupled to anammox needs to be further studied. Additionally, stable operation performance and process control¹ methods need to be optimized or developed for the PD-AMX system for better engineering practice. This review can help to accelerate the research and application of the PD-AMX process for municipal wastewater treatment.

Novel insights into overcoming nitrite oxidation bacteria acclimatization problem in treatment of high-ammonia wastewater through partial nitrification

A partial nitrification sequencing batch reactor was operated to reveal mechanisms behind nitrite oxidation bacteria (NOB) acclimatization in high-ammonia wastewater treatment. The influent NH₄⁺-N increased stepwise from 499.7 ± 4.2 mg/L to 6994.5 ± 7.5 mg/L with initial free ammonia (FA) concentration rising from 37.9 ± 3.2 mg NH₃-N/L to 715.3 ± 47.3 mg NH₃-N/L, respectively. NOB acclimatized this FA range with NO₃^--N production increasing from 29.2 ± 2.6 mg/L to 144.1 ± 31.0 mg/L in a cycle, which was caused by the shift of dominant NOB genus from Nitrospira to Nitrolencea. Nitrosomonas as ammonia oxidation bacteria, could sustain its activity of 62.1 ± 0.1 mg NH₄⁺-N/(gVSS∙L∙h) under the same condition. Hydroxylamine addition could be implemented as an emergency measure to alleviate NOB acclimatization in short-term operation. The findings expanded knowledge about NOB acclimatization types and provided novel insights for addressing this problem in a targeted way.

Responses of anammox process to elevated Fe(III) stress: Reactor performance, microbial community and functional genes

The aim of present study was to re-evaluate the impacts of elevated Fe(III) stress on anaerobic ammonium oxidation (anammox) process. The results indicated that long-term low concentration Fe(III) (5 and 10 mg/L) exposure significantly improved the nitrogen removal efficiency of anammox process, while high concentration Fe(III) (50 and 100 mg/L) significantly deteriorated the reactor performance. Batch assays showed that the specific anammox activity, heme c content and hydrazine dehydrogenase activity were significantly increased and decreased under low and high concentration Fe(III) exposure, respectively, indicating an enhancement and inhibition of anammox activity. Moreover, the presence of high concentration Fe(III) significantly shifted the anammox community structure. Ca. Brocadia was the predominant anammox genus, whose abundance decreased from 14.26% to 8.13% as Fe(III) concentration increased from 0 to 100 mg/L. In comparison, the abundance of denitrifiers progressively increased from 3.70% to 6.68% with increasing Fe(III) concentration. These suggested that different functional bacteria differed in their responses to Fe(III) stress. Furthermore, long-term Fe(III) exposure significantly up-regulated the abundances of genes associated with nitrogen metabolism and Fe(III) reduction. Overall, the obtained findings are expected to advances our understanding of the responses of anammox process to elevated Fe(III) stress.

Study on the effect of new type of suspended carrier on oxygen mass transfer characteristics

Based on the characteristic that the air can form a vortex around the carrier after passing through the suspended carrier, a new layered fan-like carrier and a wave multi-channel carrier were prepared by designing the spatial structure of the suspension carrier and combining 3D printing technology. Compared with commercially available multi-faceted hollow sphere carrier, clear water oxygenation experiment was carried out in a transparent organic glass reactor, and the aeration volumetric flow rate was 0.28, 0.42, 0.56, 0.70, and 0.84 VVM, respectively. The results show that the growth rate of dissolved oxygen, the saturation value of dissolved oxygen, and the oxygen mass transfer coefficient in the new type of carrier reactor are higher than those of commercially available multi-faceted hollow spheres and blank reactors at various aeration intensities. Under the condition of aeration volumetric flow rate at 0.56 VVM, the reactor can reach dissolved oxygen saturation at a faster rate of dissolved oxygen growth. Combined with the FLUENT numerical simulation and sewage treatment experiments, it is proved that changing the spatial structure of the carrier can accelerate the reoxygenation of the water body, increase the value of dissolved oxygen saturated in the water, and create ideal mass transfer conditions for the MBBR (moving bed biofilm reactor).

A novel pilot-scale IFAS-MBR system with low aeration for municipal wastewater treatment: Linkages between nutrient removal and core functional microbiota

In this study, we proposed a novel IFAS-MBR with low aeration for the treatment of real municipal wastewater. With biocarriers packed in the anoxic tank, the pilot-scale IFAS-MBR operated with average dissolved oxygen concentrations of 0.56 mg/L in the oxic tank. Over 110 days of operation, highly efficient nutrient removal was achieved with the total nitrogen (TN) and phosphorus (TP) removal efficiencies of 78.1 ± 7.2% and 93.7 ± 5.8%, respectively. The average effluent concentrations of TN and TP reached 5.4 and 0.26 mg/L, respectively. Meanwhile, the removal efficiency of COD reached 95.3 ± 1.3% in the system, and the concentrations of COD decreased from 31.9 ± 3.7 (sludge supernatant) to 12.7 ± 1.6 mg/L (permeate) after membrane filtration. Microbial community analysis showed that Nitrosomonas (0.32%) and Nitrospira (1.85%) in activated sludge were the main drivers of the nitrification process, while various denitrifying bacteria in activated sludge and biofilms were responsible for nitrate reduction in the anoxic tank. Candidatus Accumulibacter (0.34%) and Dechloromonas (1.31%) primarily contributed to denitrifying phosphorus uptake in the anoxic tank. Furthermore, these organisms (i.e., core functional microbiota) exhibited stable levels over the entire operation. The highly enriched hydrolytic fermentation bacteria drove community succession, and the remarkable functional robustness of microbial communities in activated sludge and biofilms favored nutrient removal. Overall, the novel IFAS-MBR system provides an energy-efficient MBR alternative owing to its highly efficient performance and low operating costs enabled by low aeration rates and the absence of an external carbon source.

Rapid achieving partial nitrification in domestic wastewater: Controlling aeration time to selectively enrich ammonium oxidizing bacteria (AOB) after simultaneously eliminating AOB and nitrite oxidizing bacteria (NOB)

This study developed a novel strategy for rapidly achieving partial nitrification (PN) without additional chemical agents, and infrastructure costs, only by controlling aeration time to selectively enrich ammonium oxidizing bacteria (AOB) after simultaneously eliminating AOB and nitrite oxidizing bacteria (NOB). Shorter aeration time and sludge retention time (10 days) were implemented to simultaneously eliminate AOB and NOB, the bioactivities drastically decreased to 13 and 0%, respectively. Subsequently, a gradually prolonged aeration time selectively enriched AOB and resulted in PN. The amoA abundances increased to 1.9 × 10¹⁰ copies gVSS^-1, whereas Nitrospira and Nitrobacter abundances remained stable (3.2 × 10⁹ and 3.1 × 10⁹ copies gVSS^-1). A nitrite accumulation rate above 96% was achieved and maintained for 205 days over the entire temperature range (28.5-17.9 °C). The effluent contained 1.9 mg N L^-1 of ammonium, 25.3 mg N L^-1 of nitrite, and less than 1.0 mg N L^-1 of nitrate, facilitating mainstream wastewater anammox.