Anoxic Process Research Articles

Characterization of microbiome, resistome, mobilome, and virulome in anoxic and oxic wastewater treatment processes in Slovakia and Taiwan

This study presents a comprehensive analysis of samples from urban wastewater treatment plants using anoxic/oxic processes in Slovakia and Taiwan, focusing on microbiome, resistome, mobilome, and virulome, which were analyzed using a shotgun metagenomic approach. Distinct characteristics were observed; in Taiwan, a higher abundance and diversity of antibiotic resistance genes were found in both influent and effluent samples, while there was a higher prevalence of mobile genetic elements and virulence factor genes in Slovakia. Variations were noted in microbial community structures; influent samples in Taiwan were reflected from fecal and hospital sources, and those in Slovakia were derived from environmental elements. At the genus level, the samples from Taiwan's sewage treatment plants were dominated by Cloacibacterium and Bacteroides, while Acinetobacter was predominant in samples from Slovakia. Despite similar antibiotic usage patterns, distinct wastewater characteristics and operational disparities influenced microbiome, resistome, mobilome, and virulome compositions, with limited reduction of most resistance genes by the studied anoxic/oxic processes. These findings underscore the importance of region-specific insights into microbial communities for understanding the dynamics of antimicrobial resistance and pathogenicity in urban wastewater treatment systems. Such insights may lay the groundwork for optimizing treatment processes and reducing the dissemination of antibiotic resistance and pathogenicity genes for safeguarding public health.

Outstanding Potential for Treating Wastewater from Office Buildings Using Fixed Activated Sludge with Attached Growth Process

The application of fixed activated sludge with an attached growth process (FASAG) with optimal operating conditions (hydraulic retention time (HRT) of 7 h, dissolved oxygen (DO) of 6 mg/L, and alkalinity dosage of 7.14 mgCaCO3/mgN-NH4+) treats wastewater generated from office buildings to meet discharge requirements (as per the regulation in the nation where the study was conducted) with typical parameters such as pH of 6.87–7.56, chemical oxygen demand (COD) of 32–64 mg/L, suspended solids (SS) of 8–11 mg/L, N-NH4+ of 1–7 mg/L, and denitrification efficiency reaches 53%. In addition, the FASAG is an outstanding integration that makes both economic and environmental sense when applied in local wastewater treatment systems. In particular, this process combines aerobic and anoxic processes in a creation tank. This explains why this approach can save investment and operating costs, energy, and land funds. In office building regions, where land area is frequently limited, saving land funds presents numerous options to enhance the density of green cover. Furthermore, as a new aspect, investing in reusing wastewater after treatment to irrigate plants or flush toilets in office buildings contributes to a decrease in the quantity of wastewater released into the environment, saving water resources and supporting sustainable development.

SUITABILITY OF THE HOSPITAL WASTEWATER TREATMENT SYSTEM IN BINH DUONG PROVINCE TO THE ENVIRONMENTAL CARRYING CAPACITY

As the number of healthcare facilities increases, stringent management of wastewater treatment systems becomes imperative. This study investigates a wastewater treatment system designed for a general clinic in Bình Dương Province, Vietnam, which complies with Vietnamese environmental standards. The clinic, located in Bến Cát City, serves approximately 200 patients per day and generates an average wastewater volume of 1.7m³/day. The treatment system, with a capacity of 10m³/day, employs a combination of biological and chemical methods, including anoxic and aerobic processes, to efficiently reduce pollutants. Results indicate that the treated wastewater meets the QCVN 28:2010/BTNMT, column A standards, ensuring minimal environmental impact when discharged into the Thi Tinh River. The study demonstrates the system's effectiveness in managing hospital wastewater, contributing to environmental protection and public health.

Sediment porewaters serve as a transient organic carbon pool at the land-ocean interface

The organic carbon (OC) cycle at the land-ocean interface is an important component of the global carbon budget, yet the processes that control the transfer, transformation, and burial of OC in these regions remain poorly understood. In this work, we examined sedimentary OC (SOC) in short core sediments, dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and chromophoric dissolved organic matter (CDOM), as well as other solutes in sediment porewaters of the Changjiang Estuary and adjacent East China Sea (ECS) shelf. The main goal of this work is to investigate the variation of the sources and composition of different forms of carbon in estuarine sediments associated with different sedimentary regimes, to further understand the role of sediment porewater in carbon sequestration at the land-ocean interface. Concentrations of Fe2+ and Mn2+ in porewaters of the muddy sediments are much higher than those in the sandy sediments, and SO42− decreases with depth in the deep sediment layer, indicating the degradation of SOC in mobile muds is mainly driven by suboxic and/or anoxic diagenetic processes (e.g., iron-manganese reduction). The accumulation of DIC in the muddy sediment is higher compared to the sandy sediment, indicating relatively complete SOC remineralization. The DOC in porewaters of the muddy areas is mainly composed of highly degraded and low molecular weight humic-like substances (C1), whereas in the sandy area, porewater DOC is mainly composed of less degraded and high molecular weight protein-like substances (C2 and C3). The average DOC stock (28.5 t/km2) in the upper 30 cm sediment porewaters is significantly higher than that of DIC (12.5 t/km2) in sandy area, but less in muddy areas (17.0 t/km2 of DOC vs. 25.4 t/km2 of DIC). The total DOC stock in sediment porewaters of the sandy area accounted for ∼61 % of DOC stock in water column of the ECS, indicating that the porewater is an important DOC pool in the ECS. However, this DOC pool is rather transient due to its high reactivity and mobility, especially in sandy area. Nevertheless, compared with other marine environments, the carbon stock of DOC (average of 43.8 t/km2) in porewaters of stable sedimentary environments is much higher than that of DIC (average of 21.7 t/km2). This work further supports the notion that sedimentary regime plays an important role in OC cycling at the land-ocean interface and highlights the significance of sediment porewaters as a vast carbon pool in marine ecosystems.

Natural Attenuation of 2,4-Dichlorophenol in Fe-Rich Soil during Redox Oscillations: Anoxic-Oxic Coupling Mechanism.

Natural attenuation of organic contaminants can occur under anoxic or oxic conditions. However, the effect of the coupling anoxic-oxic process, which often happens in subsurface soil, on contaminant transformation remains poorly understood. Here, we investigated 2,4-dichlorophenol (2,4-DCP) transformation in Fe-rich soil under anoxic-oxic alternation. The anoxic and oxic periods in the alternating system showed faster 2,4-DCP transformation than the corresponding control single anoxic and oxic systems; therefore, a higher transformation rate (63.4%) was obtained in the alternating system relative to control systems (27.9-42.4%). Compared to stable pH in the alternating system, the control systems presented clear OH- accumulation, caused by more Fe(II) regeneration in the control anoxic system and longer oxygenation in the control oxic system. Since 2,4-DCP was transformed by ion exchangeable Fe(II) in soil via direct reduction in the anoxic process and induced ·OH oxidation in the oxic process, OH- accumulation was unbeneficial because it competed for proton with direct reduction and inhibited •OH generation via complexing with Fe(II). However, the alternating system exhibited OH--buffering capacity via anoxic-oxic coupling processes because the subsequent oxic periods intercepted Fe(II) regeneration in anoxic periods, while shorter exposure to O2 in oxic periods avoided excessive OH- generation. These findings highlight the significant role of anoxic-oxic alternation in contaminant attenuation persistently.

Importance of iron complexation and floc formation towards phosphonate removal with Fe–electrocoagulation

Phosphonates are widely used scale inhibitors, but the residual phosphonates in drainage are challenging to remove because of their chelating capacity and resistance to biodegradation. Here, we reported a highly efficient and robust Fe-electrocoagulation (Fe-EC) system for phosphonate removal. Surprisingly, we found for the first time that phosphonates like NTMP were more efficiently removed under anoxic conditions (80% of total soluble phosphorus (TSP) in 4 min) than oxic conditions (0% of TSP within 6 min) in NaCl solution. A similar phenomenon was observed when other phosphonates, such as EDTMP and DTPMP, were removed, highlighting the importance of iron complexation and floc formation toward phosphonate removal with Fe-EC. We also showed that the removal efficiency of NTMP by electrochemically in-situ formed flocs (97%) was much higher than post-adsorption systems (ex-situ, 40%), revealing that the growth of flocs consumed the active site for NTMP adsorption. Beyond the removal of TSP, 10 % of NTMP-P was also degraded after the electrolysis phase, evidenced by the evolution of phosphate-P. However, this did not happen in anoxic or chemical coagulation processes, which confirms the formation of reactive oxygen species via Fe(II) oxidation in the oxic Fe-EC system. The primary removal mechanism of phosphonates is due to their complexation with iron (hydr)oxide generated in the Fe-EC system by forming a Fe-O-P bond. Encouragingly, the Fe-EC system exhibits comparable or even better performance in treating phosphonate-laden wastewater (i.e., cooling water). Our preliminary cost calculation suggests the proposed system (€ 0.009/m3) has a much lower OPEX under oxic conditions than existing approaches. This study sheds light on the removal mechanism of phosphonate and the treatment of phosphonate-laden wastewater by playing with the iron complexion and flocs formation in classical Fe-EC systems.

Sulphate reduction and carbonate precipitation in a high-energy algal rim framework

Algal ridges are protective features for coral reefs that form through the accretion and encrustation of reef rubble and debris by crustose coralline algae (CCA) and processes of diagenetic cementation. Carbonate precipitation and dissolution dynamics on and within algal ridge frameworks are poorly understood. We studied the surface and subsurface geochemistry of the algal ridge framework at One Tree Island, Australia. Measurable quantities of hydrogen sulphide were detected in most porewater samples collected from bores, indicating a largely anoxic ridge framework. Total alkalinity (TA) and pH measurements indicate that the precipitation of carbonate minerals within the interior of the ridge framework occurs under largely anoxic conditions and is likely to be driven by TA changes associated with sulphate-reducing bacteria. Modelling of porewater hydrogen sulphide concentrations in combination with TA and dissolved inorganic carbon (DIC) indicates anoxic respiration processes produce alkalinity within the algal ridge framework. However, significantly more TA is removed via the precipitation of mineral carbonate, resulting in porewater TA concentrations falling below the open seawater values. The precipitation of mineral carbonate also lowers interstitial water pH, such that pH changes are not solely from organic carbon diagenesis. The simultaneous precipitation and dissolution of carbonate minerals within the algal ridge framework are key to forming and cementing algal ridges, which are important physical protective features for coral reefs.

Manganese reductive dissolution coupled to Sb mobilization in contaminated shooting range soil

A “redox-stat” RMnR bioreactor was employed to simulate moderately reducing conditions (+ 420 mV) in Sb-contaminated shooting range soils for approximately 3 months, thermodynamically favoring Mn(IV) reduction. The impact of moderately reducing conditions on elemental mobilization (Mn, Sb, Fe) and speciation [Sb(III) versus Sb(V); Fe2+/Fe3+] was compared to a control bioreactor RCTRL without a fixed redox potential. In both bioreactors, reducing conditions were accompanied by an increase in effluent Sb(V) and Mn(II) concentrations, suggesting that Sb(V) was released through microbial reduction of Mn oxyhydroxide minerals. This was underlined by multiple linear regression analysis showing a significant (p < 0.05) relationship between Mn and Sb effluent concentrations. Mn concentration was the sole variable exhibiting a statistically significant effect on Sb in RMnR, while under the more reducing conditions in RCTRL, pH and redox potential were also significant. Analysis of the bacterial community composition revealed an increase in the genera Azoarcus, Flavisolibacter, Luteimonas, and Mesorhizobium concerning the initial soil, some of which are possible key players in the process of Sb mobilization. The overall amount of Sb released in the RMnR (10.40%) was virtually the same as in the RCTRL (10.37%), which underlines a subordinate role of anoxic processes, such as Fe-reductive dissolution, in Sb mobilization. This research underscores the central role of relatively low concentrations of Mn oxyhydroxides in influencing the fate of trace elements. Our study also demonstrates that bioreactors operated as redox-stats represent versatile tools that allow quantifying the contribution of specific mechanisms determining the fate of trace elements in contaminated soils.Key points• “Redox-stat” reactors elucidate Sb mobilization mechanisms• Mn oxyhydroxides microbial reductive dissolution has a major role in Sb mobilization in soils under moderately reducing conditions• Despite aging the soil exhibited significant Sb mobilization potential, emphasizing persistent environmental effects

New insights into nitrogen removal by divalent iron-enhanced moving bed biofilm reactor: Performance, interfacial interaction and co-occurrence network

A divalent iron-mediated moving bed biofilm reactor with intermittent aeration was developed to enhance the nitrogen removal at low carbon-to-nitrogen ratios. The study demonstrated thatammonia removal increased from 51 ± 4 % to 79 ± 4 % and nitrate removal increased from 72 ± 5 % to 98 ± 4 % in phases I-IV, and 2–5 mg·L-1 of divalent iron significantly increased the anoxic denitrification process. Divalent iron stimulated the secretion of extracellular polymeric substances, which facilitated the formation of cross-linked network between microbial cells. Furthermore, the cycle between divalent and trivalent iron decreased the energy barrier between the biofilm and the pollutant. The microbial community further revealed that Proteobacteria (relative abundance: 40–48 %) andBacteroidota(relative abundance: 31–37 %) were the dominant phyla, supporting the synchronous nitrification and denitrification processes as well as the lower accumulation of nitrite. In conclusion, iron redox cycling significantly enhanced the nitrogen removal. This study proposes a viable strategy for the efficient treatment of nutrient wastewater.

Distinctive species interaction patterns under high nitrite stress shape inefficient denitrifying phosphorus removal performance

Denitrifying phosphorus removal using nitrite as an electron acceptor is an innovative, resource-efficient approach for nitrogen and phosphorus removal. However, the inhibitory effects of nitrite on anoxic phosphorus uptake and process stability are unclear. This study investigated the total phosphorus removal performance under nitrite stress and analyzed microbiome responses in 186 sludge samples. The results indicated that the total phosphorus removal rates and dominant taxon abundance were highly similar under nitrite stress. High nitrite stress induced a community-state shift, leading to unstable dynamics and decreased total phosphorus removal. This shift resulted from increased species cooperation. Notably, the shared genera OLB8 and Zoogloea under non-inhibitory nitrite stress, suggesting their vital roles in mitigating nitrite stress by enhancing carbon and energy metabolism. The response patterns of these bacterial communities to high nitrite stress can guide the design and optimization of high-nitrogen wastewater reactors.

Pre-anaerobic phase persists benefits for nitrogen removal in high carbon/nitrogen scenarios: Comparative study of aerobic–anoxic and anaerobic–aerobic–anoxic sequencing batch reactors

The pre-anaerobic phase in anaerobic–aerobic–anoxic (AOA) process is critical to intensifying nitrogen removal at low C/N ratios, yet its applicability to high C/N scenarios remains vague. Here, four bench-scale aerobic–anoxic sequencing batch reactors (OA-SBRs) were established at different C/N ratios (3–16.5) and compared with the parallel AOA-SBRs. The results showed that the NH4+–N and TN removal in the OA-SBRs increased by 24.7% and 112.2%, respectively, as the C/N ratio elevated from 3 to 16.5, but they remained lower than those in the AOA-SBRs till carbon leakage. Typical cycle tests indicated that the pre-anaerobic phase probably endowed the endogenous nitrification/denitrification in the AOA-SBRs with kinetic advantages over the canonical ones in the OA-SBRs, supported by the improved enzymatic activities. Further, microbial insights identified more intensive niche competition between heterotrophic nitrifiers/denitrifiers and non-functional heterotrophs in the OA-SBRs, which could explain the poorer nitrogen removal without the pre-anaerobic phase. This study underscores the implications of pre-anaerobic phase in nitrogen removal of AOA process in high C/N scenarios.

A simplified method for the recycling of spent lithium-ion batteries via manganese selective recovery by anoxic ammonia leaching and spontaneous precipitation

Due to the high cost of the manganese solvent extraction process in the conventional recycling of spent NCM-ternary lithium-ion batteries (LIBs), we employed an anoxic complexation ammonia leaching-spontaneous precipitation process to selectively recover manganese from the black mass of spent ternary LIBs. This innovation serves as a substitution for the conventional acid leaching-manganese solvent extraction, aiming to reduce costs and enhance the efficiency of the recycling process. Specifically, the controlled reduction roasting was performed to achieve the dissociation of LiNixCoyMnzO2 and the phase transition to MnO and metallic Ni and Co. Exploiting this difference, ammonia-ammonium carbonate were employed as leaching agents, which enabled the selective leaching of over 96 % of manganese, while those of nickel and cobalt were only 1.2 % and 2.6 %. After filtration, the MnCO3 product with high purity was precipitated by allowing the leachate to stand for 12 h without adding any reagent. The ammonia solution could be circulated to leaching the subsequent batch of spent LIBs. Thermodynamic calculations were used to elucidate the leaching and crystallization mechanisms. Comparative process analysis and economic assessments confirmed that the proposed new method is extremely appealing due to its high selectivity, considerable economic advantages, and environmental benefits.

Using the inherent elements in yeast biomass to produce Ni2P/N-doped biocarbon composites for efficient hexavalent chromium reduction.

The heterogeneous catalytic reduction of Cr(VI) to Cr(III) is an effective strategy for aqueous Cr(VI) contamination abatement, which requires the development of highly efficient, low-cost, and recyclable catalysts. Herein, Ni2P/N-doped biocarbon composites (Ni2P/N-BC) were fabricated through an anoxic pyrolysis process using NaCl and KCl as activators. A precursor of yeast biomass provided the essential C, N, and P elements for Ni2P/N-BC formation. When adopted for Cr(VI) reduction in the presence of oxalic acid as a reductant, the fabricated Ni2P/N-BC performed superior catalytic activity with a 100% Cr(VI) reduction efficiency within 10 min (Ni2P/N-BC-5 = 0.2 g L-1, oxalic acid = 0.4 g L-1, Cr(VI) = 20 mg L-1). Typical affecting parameters, e.g., catalyst dosage, oxalic acid loading, reaction temperature, initial solution pH, and water matrix, were investigated. Ni2P/N-BC exhibited good applicability in a broad pH range from 3.0 to 9.0 and in actual aquatic systems. Cr(VI) reduction efficiency remained 92.7% after five recycle runs. Such promising catalytic activity may originate from the well-crystallized Ni2P, N-doped biocarbon framework and high specific surface area of the materials. Preliminary reaction mechanism analysis indicated that the favorable charge state of Ni2P, fast hydrogen transfer, affinity of oxalic acid to Cr(VI), and inherent electron transfer in the biocarbon matrix contributed to effective Cr(VI) reduction. This work not only provides a facile and low-cost strategy to construct Ni2P/N-doped biocarbon nanosheet composite using environmentally benign biomass but also brings new insights for the remediation of Cr(VI) contamination.

Interaction between chromite and Mn(II/IV) under anoxic, oxic and anoxic-oxic conditions: Dissolution, oxidation and pH dependence

Chromite oxidative dissolution has been recognized as an important process leading to elevated Cr(VI) in soil and groundwater. Under natural conditions, direct oxidation of Cr(III) by O2 is very unfavorable, and a critical determinant of Cr(VI) generation in soil and groundwater is the interaction between chromite and Mn(II) or Mn(III/IV) oxides. Here, the effects of Mn(II) or Mn(IV) on the oxidative dissolution of chromite were investigated at pH values of 5, 7 and 9 during anoxic, oxic and anoxic-oxic processes. The results showed that the direct oxidation of Cr(III) by O2 was slow in aqueous-phase system, while the Mn oxides in chromite could oxidize dissolved Cr(III). The added Mn(II) can be catalytically oxidized to MnOOH on the chromite surface only under alkaline oxidation conditions, and the catalytic efficiency is slow, which has less effect on chromite oxidative dissolution. Compared with the direct oxidation of O2 and catalytic oxidation of Mn(II), the synthesized biogenic Mn oxides drove the oxidative dissolution of chromite to release more Cr(VI) and were the main threat to the long-term stability of chromite in the environment. Overall, both acidic and alkaline environments are favorable to the catalytic oxidation of chromite by O2, Mn(II) and δ-MnO2, while neutral conditions are favorable to the long-term stability of chromite. These above processes may occur in soils and sediments with redox fluctuations (e.g., rice paddies, river floodplains, wetlands, and peatlands), and the presence of Mn(II) and Mn(III/IV) may play an important role in the oxidation and mobilization of Cr(III), leading to elevated Cr(VI) levels in soils and groundwater.

Simultaneous denitrification and organics removal by denitrifying bacteria inoculum in a multistage biofilm process for treating desulfuration and denitration wastewater

This study aimed to treat real wastewater from the desulfuration and denitration process in a petrochemical plant with high-strength nitrogen (TN≈200 mg/L, > 90% nitrate), sulfate (2.7%) and extremely low-strength organics (CODCr < 30 mg/L). Heterotrophic denitrification of multistage anoxic and oxic biofilm (MAOB) process in three tanks using facultative denitrifying bacteria inoculum was developed to simultaneously achieve desirable effluent nitrogen and organics at different hydraulic retention time (HRT) and carbon to nitrogen (C/N) mass ratios. The optimum condition was recommended as a C/N ratio of 1.5 and a HRT of A (24 h)/O (12–24 h) to achieve > 90% of nitrogen and organics removal as well as no significant variation of sulfate. The denitrifying biofilm in various tanks was dominant by Hyphomicrobium (8.9%-25.7%), Methylophaga (18.6%-25.8%) and Azoarcus (3.3%-19.6%), etc., containing > 20% aerobic denitrifiers. This explained that oxic zone in MAOB process also exhibited simultaneous nitrogen and organics removal.

PENGOLAHAN LINDI (LEACHEATE) DENGAN METODE MOVING BED BIOFILM REACTOR (MBBR) DENGAN PROSES AEROBIK-ANOKSIK UNTUK MENURUNKAN KONSENTRASI COD, TSS, DAN AMONIA

Lindi mempunyai kandungan bahan organik, TSS, dan ammonia yang tinggi. Moving Bed Biofilm Reactor (MBBR) merupakan salah satu pengolahan biologi yang dapat menurunkan konsentrasi yang terkandung di air lindi. Tujuan dari penelitian ini untuk mengetahui kemampuan MBBR dalam menurunkan kandungan bahan organik, TSS, dan ammonia, dengan menggunakan variasi waktu detensi proses aerobik-anoksik dan media lekat. Reaktor di operasikan secara batch. Variasi media yang digunakan adalah kaldness K1 dan K3, sedangkan waktu detensi aerobik 14,5 jam - anoksik 10 jam dan aerobik 10 jam - anoksik 7 jam. Hasil yang didapatkan menunujukkan bahwa MBBR dapat digunakan untuk mengolah lindi. Penurunan konsentrasi COD, TSS, dan Ammonia paling baik terjadi pada media K1 waktu detensi aerobik 14,5- anoksik 10 jam dengan penyisihan mencapai 74,8%, 93,14%, dan 83,72%.

Nitrogen Removal Performance and Microbial Community Analysis of Anoxic and Oxic Process in Electrolytic Manganese Residue Leachate

Nitrogen Removal Performance and Microbial Community Analysis of Anoxic and Oxic Process in Electrolytic Manganese Residue Leachate

Non-uniform dissolved oxygen distribution and high sludge concentration enhance simultaneous nitrification and denitrification in a novel air-lifting reactor for municipal wastewater treatment: A pilot-scale study

Achieving simultaneous carbon and nitrogen removal with sludge-liquid separation in a single reactor offers a solution to land shortages and improves treatment efficiency in municipal wastewater treatment plants of megacities. This study proposes a novel air-lifting continuous-flow reactor configuration with an alternative-aeration strategy that creates multi-functional zones for anoxic, oxic, and settlement processes. The optimal operating conditions for the reactor include a long anoxic hydraulic retention time, low dissolved oxygen (DO) in the oxic zone, and no specific reflux for external nitrifying liquid, which exhibit a high nitrogen removal efficiency of over 90% in treating real sewage with C/N < 4 in the pilot-scale study. Results show that a high sludge concentration and a low DO concentration facilitate simultaneous nitrification and denitrification, and a well mixing of sludge and substrate in different reaction zones promotes mass transfer and microbial activity. The long-term operation enriches functional microbes for carbon storage and nutrient removal.

Pilot Study of Oxic–Anoxic Process under Low Dissolved Oxygen for Nitrogen Removal from Low COD/N Tropical Wastewater

Conventionally, nitrification in biological nitrogen removal (BNR) requires high dissolved oxygen (DO) concentrations (&gt;2 mg L−1), making the process energy intensive. Recent studies have shown that efficient ammonium removal and energy reduction can be realized by operating the nitrification at low DO concentrations (&lt;1 mg L−1). In this study, the low-DO oxic anoxic (low-DO OA) process was operated in a pilot-scale sequencing batch reactor (SBR) over 218 days to evaluate the feasibility of nitrogen removal from low chemical oxygen demand-to-nitrogen ratio (COD/N) tropical municipal wastewater. The results revealed that the low-DO OA process attained high removal efficiency for ammonium (97%) and total nitrogen (TN) (80%) under an average DO concentration of 0.6 mg L−1. The effective TN removal efficiency is attributed to the occurrence of simultaneous nitrification–denitrification (SND) under low DO conditions. Further batch tests revealed that slowly biodegradable COD (sbCOD) in tropical wastewater can support denitrification in the post-anoxic phase, resulting in a high TN removal rate. Compared with high DO concentrations (2 mg L−1), low DO conditions achieved 10% higher TN removal efficiency, with similar ammonium and COD removal efficiency. This study is crucial in promoting the energy efficiency and sustainability of wastewater treatment plants treating low COD/N wastewater.

Interrogating the performance and microbial ecology of an enhanced biological phosphorus removal/post-anoxic denitrification process at bench and pilot scales.

Research focused on interrogating post-anoxic enhanced biological phosphorus removal (EBPR) at bench and pilot scales. Average bench-scale effluent ranged from 0.33 to 1.4 mgP/L, 0.35 to 3.7 mgNH3 -N/L, and 1.1 to 3.9 mgNOx -N/L. Comparatively, the pilot achieved effluent (50th percentile/average) of 0.13/0.2 mgP/L, 9.7/8.2 mgNH3 -N/L, and 0.38/3.3 mgNOx -N/L under dynamic influent and environmental conditions. For EBPR process monitoring, P:C ratio data indicated that 0.2-0.4 molP/molC will result in stable EBPR; relatedly, a target design influent volatile fatty acid (VFA):P ratio would exceed 15 mgCOD/mgP. Post-anoxic EBPR was enriched for Nitrobacter spp. at 1.70%-20.27%, with Parcubacteria also dominating; the former is putatively associated with nitritation and the latter is a putative fermenting heterotrophic organism. Post-anoxic specific denitrification rates (SDNRs) (20°C) ranged from 0.70 to 3.10 mgN/gVSS/h; there was a strong correlation (R2 = 0.94) between the SDNR and %Parcubacteria for systems operated at a 20-day solids residence time (SRT). These results suggest that carbon substrate potentially generated by this putative fermenter may enhance post-anoxic EBPR. PRACTITIONER POINTS: Post-anoxic EBPR can achieve effluent of <0.2 mgP/L and <12 mgN/L. The P:C and VFA:P ratios can be predictive for EBPR process monitoring. Post-anoxic EBPR was enriched for Nitrobacter spp. over Nitrospira spp. and also for Parcubacteria, which is a putative fermenting heterotrophic organism. Post-anoxic specific denitrification rates (20°C) ranged from 0.70 to 3.10 mgN/gVSS/h. BLASTn analysis of 16S rDNA PAO primer set was shown to be improved to 93.8% for Ca. Accumulibacter phosphatis and 73.2%-94.0% for all potential PAOs.