Search Results (59)

Constructed wetlands (CWs) are widely used for wastewater treatment but can also serve as significant sources of nitrous oxide (N₂O), a potent greenhouse gas. Balancing efficient nitrogen removal with N₂O mitigation remains a critical challenge for sustainable wastewater management. This review systematically elucidates the key microbial mechanisms underlying N₂O emissions in CWs and summarizes corresponding mitigation strategies. Mechanistically, N₂O production is primarily driven by hydroxylamine oxidation and nitrifier denitrification mediated by ammonia-oxidizing microorganisms, as well as incomplete heterotrophic denitrification resulting from electron-donor limitation. These pathways are tightly regulated by spatiotemporal redox gradients, carbon-to-nitrogen ratios, and influent strength conditions. To address these emissions, this review synthesizes mitigation strategies from an engineering perspective. Optimization of operational parameters, such as intermittent aeration and water-level regulation, together with the application of novel functional substrates, such as biochar and iron-carbon micro-electrolysis, can effectively facilitate electron transfer and improve micro-redox conditions. Furthermore, optimized plant species selection and community design, along with emerging low-carbon biological nitrogen removal processes, such as autotrophic denitrification and partial denitrification coupled with anammox, offer promising approaches for substantial emission reduction. Overall, this review provides practical guidance for designing efficient, low-carbon CWs toward carbon neutrality. Full article

Computational Fluid Dynamics (CFD) was applied to an intermittent anoxic/aerobic Moving Bed Biofilm Reactor (MBBR) operated under six different aeration intermittency cycles and dissolved oxygen concentration levels. Experimental results showed that most aeration cycles did not provide a sufficiently long anoxic phase to sustain effective denitrification, thereby limiting NOx removal efficiency. This behavior was not adequately captured by simulations performed using conventional biological models (BioWin), which rely on the assumption of complete mixing. In contrast, the CFD model implemented in ANSYS Fluent 2024 R2 enabled a detailed characterization of reactor hydrodynamics and the identification of several inefficiencies, including short-circuiting, back-mixing, and the presence of dead zones. Notably, the simulations revealed a pronounced asymmetric distribution of carriers within the reactor, with the majority accumulating along one side, leaving a significant fraction of the reactor volume largely unoccupied. Further analysis indicated that this phenomenon was caused by a design flaw—specifically, the asymmetric placement of the aerators—combined with an excessively high air injection flow rate. Full article

A pilot-scale moving-bed biofilm reactor (MBBR), operated under alternating intermittent aeration (IA) and non-aeration phases, was used for single-stage carbon (C) and nitrogen (N) removal from high-fluctuating municipal wastewater via simultaneous nitrification–denitrification. The reactor was operated under highly variable chemical oxygen demand to total nitrogen (COD/TN) ratios, low dissolved oxygen (DO) conditions, and progressively extended non-aerated periods to evaluate process robustness under real operational conditions. An active denitrifying biofilm developed on the carriers after 23 days of the anoxic start-up, as confirmed by batch activity tests. Under the most carbon-limited condition tested (COD/TN = 5.5), the application of 16 h·d⁻¹ of non-aerated phases at DO levels of 0–1.0 mg·L⁻¹ enabled simultaneous COD, N–NH₄⁺ and TN removal efficiencies of 70, 95 and 84%, respectively. These results confirm that transient IA is an effective strategy for simultaneous C and N removal at very low COD/TN ratios and real fluctuating influent concentrations. Energy assessment showed that extended non-aeration phases reduced blower energy demand by 67% and total plant energy consumption by 34%, improving the environmental sustainability of the single-stage process. The main novelty of this study lies in the pilot-scale validation of an IA-MBBR for SND using real municipal wastewater under naturally fluctuating C and N loadings, thereby bridging previous laboratory-scale evidence with realistic operating conditions. Full article

Wastewater treatment plants employ continuous aeration (CA) methods, during which nitrogen compounds including N₂O, a potent greenhouse gas, accumulate. Little research has focused on reducing N₂O emissions. Intermittent aeration (IA) suppresses NO₃⁻ accumulation, but its effectiveness in N₂O reduction remains unclear. Therefore, we investigated the relationship between N₂O emission and decreasing NO₃⁻ under different aeration conditions using swine wastewater in a 1-m³ aeration tank. Three test conditions were employed: CA, IA-1 (3 h aeration and 1 h of non-aeration), and IA-2 (ON/OFF aeration repeated every 2 h). IA suppressed N₂O emissions compared to CA, achieving decreases of 42% under IA-1 and 64% under IA-2. Microbial community analysis revealed a tendency for higher relative abundances of Nitrosomonas (ammonia-oxidizing bacteria), Nitrospira (nitrite-oxidizing bacteria), Zoogloea, Hydrogenophaga, and Dokdonella (among denitrifying bacteria) in activated sludge samples. This pilot-scale study demonstrated that changing the aeration conditions from continuous to intermittent in wastewater treatment plants may effectively reduce N₂O emissions. The mitigation of greenhouse gas emissions from wastewater treatment plants is expected to contribute to the realization of a sustainable society. Full article

Micro-nanobubbles (MNBs) aeration has been widely reported as an effective approach for membrane fouling mitigation. However, their optimal operation in polymeric hollow fiber membrane (HFM) systems remains unclear. In this study, the antifouling performance of MNB-assisted ultrafiltration was systematically investigated using a PVDF-HFM, with particular emphasis on release pressure and intermittent aeration strategy. Increasing the release pressure to 0.60 MPa produced smaller and more concentrated bubbles, significantly alleviating membrane fouling. A distinct intermittent-aeration window was observed, in which a 15 min interval achieved the best overall performance, with a rejection efficiency of 75% and a cleaning efficiency of 93%, approaching that of continuous aeration. Longer intervals resulted in rapid deterioration in fouling control, indicating insufficient bubble replenishment. Compared with continuous operation, the optimized intermittent mode maintained comparable membrane performance while reducing energy consumption by approximately 50%, demonstrating a clear advantage in energy efficiency. Importantly, the optimal intermittent interval for PVDF-HFM (15 min) differs from that reported for ceramic membranes (30 min), highlighting that the performance of intermittent MNB aeration is not universal but strongly dependent on membrane properties. This shift in optimal interval is attributed to differences in surface wettability, structural flexibility, and local hydrodynamic conditions, which collectively influence bubble retention, interfacial shielding, and foulant detachment. Full article

To address the problems of short-circuit flow and dead zones, complicated operation and control caused by intermittent influent, and the mismatch between aeration rate and oxygen demand in the Cyclic Activated Sludge System (CASS), a novel Multi-Compartment Fixed-Biofilm Cyclic Activated Sludge System (MCFCASS) was developed. This system operated in continuous-flow mode, and the aeration rate of each compartment was redistributed using a mathematical model. The results show that the plug flow ratio of the MCFCASS reactor increased from 18.75% to 31.25% compared with the CASS reactor. After aeration rate redistribution, the average total nitrogen (TN) removal efficiency of the MCFCASS reactor rose from 83.34% to 86.80%, and the effluent TN concentration consistently met the Grade I-A limit (15 mg/L) specified in the Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant (GB 18918-2002). The average removal efficiencies of chemical oxygen demand (COD) and ammonium nitrogen (NH₄⁺-N) increased from 91.58% and 93.39% to 92.98% and 94.57%, respectively. Microbial community analysis revealed that after aeration rate redistribution, the relative abundances of Pseudomonadota, Bacteroidota, and Bacillota in the pre-reaction zone of MCFCASS were 39.17%, 17.78%, and 10.33%, respectively. In addition, the abundances of some functional bacterial groups in the first and fourth compartments of the main reaction zone shifted adaptively in response to the aeration rate redistribution, consistent with the trends in pollutant removal contributions in these compartments. Hierarchical clustering and principal coordinate analysis (PCoA) further indicated that aeration rate redistribution influenced the microbial community structure. The above laboratory-scale optimization results may provide a preliminary reference for aeration control and improvement of denitrification performance in similar processes. Full article

Achieving stable partial nitritation (PN) in mainstream municipal wastewater treatment is critical for energy-efficient anammox-based nitrogen removal. However, selectively suppressing nitrite-oxidizing bacteria (NOB) while retaining ammonia-oxidizing bacteria (AOB) remains challenging. This study investigated the performance and microbial mechanisms of PN in a membrane-aerated biofilm reactor (MABR) under zero-pressure aeration. The results showed that zero-pressure aeration achieved a nitrite accumulation ratio (NAR) of 82.14%, significantly higher than that under constant aeration (13.2%) and intermittent aeration (53.5%). Zero-pressure aeration led to a significant increase in the fluorescence intensities of tyrosine/tryptophan protein in extracellular polymeric substances. 16S rRNA sequencing revealed that zero-pressure aeration achieved a modest reduction in the relative abundance of NOB Nitrospira from 3.39% to 2.74% while increasing the relative abundance of AOB Nitrosomonas from 0.04% to 1.09%. Enzyme activity assays further showed that zero-pressure aeration significantly decreased nitrite oxidoreductase (NXR) activity while maintaining ammonia monooxygenase (AMO) and hydroxylamine oxidoreductase (HAO) activities, providing direct functional evidence for NOB suppression. Zero-pressure operation required no external air supply, representing a passive aeration strategy for PN. These results suggest that zero-pressure aeration may reshape the competition between AOB and NOB by enriching AOB and suppressing NOB, providing a new energy-efficient pathway for mainstream nitrogen removal. Full article

Organic waste management remains a pressing environmental and economic challenge, particularly in small-scale or domestic contexts where access to industrial composting technologies is limited. This study investigates the performance of the BSG-2 fermenter, a low-cost aerobic system designed to convert brewery spent grain (BSG) and vegetable waste into nutrient-rich compost through solid-state fermentation. The fermenter, constructed from food-grade plastic, relied on intermittent forced aeration, and manual temperature and pH control to sustain microbial activity. Temperature, pH, and substrate degradation were monitored throughout a complete fermentation cycle. The system achieved consistent bio-thermal performance with peak temperatures of approximately 32 °C and a substrate volume reduction of 30–40%, confirming active microbial metabolism and substantial organic matter degradation. Minimal odor generation and low energy input highlighted the fermenter’s environmental suitability. While occasional anaerobic pockets and limited heat retention were observed, these limitations could be addressed through improved insulation and automated aeration. The sustained mesophilic heat generation observed in the system may also present opportunities for low-grade thermal recovery in small-scale applications, such as localized environmental conditioning, although the magnitude of heat produced is limited. Overall, the BSG-2 fermenter demonstrates a feasible, replicable approach to valorizing organic waste into compost and sustained mesophilic heat generation using simple, accessible materials, contributing to circular economy strategies and sustainable small-scale waste management. Full article

Thraustochytrids are promising alternatives for the production of docosahexaenoic acid (DHA; C22:6 n-3), a long-chain polyunsaturated fatty acid with health benefits. For practical application of this oleaginous microorganism, an efficient cultivation method to enhance DHA production is required, which relies on several factors that support cell growth, lipid accumulation, and lipid turnover. In this study, the robust submerged fermentation of an acid- and high-temperature-tolerant strain of Aurantiochytrium limacinum was investigated. Under controlled temperature and acidic conditions (pH 4.5 and 30 °C), glucose and peptone were the best carbon and nitrogen sources for enhancing biomass and DHA production, respectively, with a glucose concentration of 60 g/L and a C/N ratio of 24 being optimal for DHA production. Applying an aeration rate of 2 vvm and an agitation speed of 300 rpm using a combination of a ring sparger and pitch-blade impeller in a stirred-tank bioreactor improved DHA production using intermittent fed-batch fermentation. The highest DHA titer was obtained at 3.01 g/L, and the DHA content in biomass was 10.69% (w/w) after intermittent feeding of cassava starch hydrolysate as the substrate. Full article

The stable application of Partial Nitritation-Anammox (PN-A) in municipal wastewater treatment is primarily hindered by the ineffective suppression of Nitrite-Oxidizing Bacteria (NOB). This study systematically evaluated PN-A stability by comparing a Sequencing Batch Reactor (SBR) with two distinct Membrane Bioreactor (MBR) configurations. Results indicated that the SBR achieved superior performance through natural hydraulic selective washout, which efficiently eliminated NOB and fostered a robust AOB-AnAOB symbiotic biofilm. In contrast, MBRs were inherently susceptible to NOB proliferation due to their non-selective membrane retention. However, this study demonstrates that an intermittently aerated MBR (MBR-I) can effectively mitigate these disadvantages. By tailoring aeration control, the MBR-I successfully optimized the competitive kinetics for nitrite, suppressing NOB activity and achieving a robust total nitrogen removal rate (TNRR) of 76.38%. This work highlights that tailored aeration serves as a crucial synergistic strategy to bridge the inherent gap between membrane-based systems and conventional washout-driven reactors, providing a potential pathway for implementing PN-A within hybrid MBBR-MBR systems. Full article

This study investigated the impacts of diverse aeration processes (continuous aeration vs. intermittent aeration) and aeration rates on the aerobic composting process. The key properties examined include temperature, oxygen dynamics, lignocellulose degradation, humification, and the functional potential of carbohydrate-active enzymes (CAZymes) based on metagenomic analysis. Among all the treatments, continuous aeration at a low rate (CA_1.5) attained the highest level of lignocellulose degradation by balancing the thermophilic duration and oxygen supply. Conversely, intermittent aeration (IA_3) led to superior humus stabilization, with the ratio of humic acid to fulvic acid (H/F) increasing by 118.45% in comparison to the initial level. Low total ventilation in CA_1.5 and IA_3 facilitated an increase in the abundance of glycosyl transferases (GTs) genes. Notably, intermittent aeration (IA_3) synergistically augmented the activities of glycoside hydrolases (GHs) and GTs, propelling the efficient conversion of lignocellulose into stable humic substances. In conclusion, the aeration process influenced the functional potential of microbial CAZymes, thus exerting an influence on both the composting efficiency and the quality of the final product. Full article

Tire microplastics (TMPs) are a widespread pollutant with growing concern due to their diverse sources, persistence, and potential risks to the environment and human health. This study investigated the impact of TMPs (50–500 mg/L) on the sludge structure, activity, and extracellular polymeric substance (EPS) dynamics in granular sequencing batch reactors (GSBRs) under continuous aeration (CA) and intermittent aeration (IA) conditions. Increased TMP concentration reduced granule size and increased the specific surface area under CA, but under IA, it increased granule size and lowered specific surface area. Total EPS declined as TMP concentration increased in both aeration regimes, but the reduction was more pronounced under CA. Protein levels in the soluble EPS fraction were consistently higher during IA than CA across all GSBRs. Aeration regimes had contrasting effects on EPS polysaccharides, as TMP dose increased; polysaccharide content increased during IA and decreased during CA. During CA, TMP presence enhanced dehydrogenase activity to over five times that of the control, while during IA, activity remained stable despite TMP addition. Overall, biomass under IA showed greater tolerance to TMP stress than CA, as evidenced by enhanced granulation, stable dehydrogenase activity, and preserved EPS. Full article

Advanced control systems have been recently implemented in wastewater treatment plants (WWTPs) to optimize activated sludge processes, reduce operational costs, and decrease energy consumption, with the aim of moving toward sustainable operation. Real-time dynamic control of NH₄⁺-N and NO₃⁻-N concentrations is important for the optimization of biological nitrogen removal (BNR) processes. This study presents an advanced control strategy based on continuous monitoring of NH₄⁺-N and NO₃⁻-N concentrations at 22.8–25.1 °C to enhance nitrogen removal performance. Specifically, the control performance of an intermittently aerated and fed activated sludge (IAF-AS) system treated with domestic wastewater was evaluated using a controller under two different scenarios: (i) normal conditions at constant ammonium nitrogen loading rate (ALR) and (ii) varied conditions with a sudden increase in ALR. The effect of temperature changes on BNR efficiency was not analyzed. In both scenarios, the optimal duration ratio of the nitrification and denitrification phases was determined, which depended on the ALR. In the first scenario, at a constant ALR of 0.2 g L⁻¹ d⁻¹, the controller kept the duration of nitrification and denitrification at a low level, succeeding in complete nitrogen removal in less than 60 min. In the second scenario, when the ALR exceeded 0.3 g L⁻¹ d⁻¹, the controller dynamically extended these phases to achieve the effluent endpoints of 2 mg L⁻¹ NH₄⁺-N and 1 mg L⁻¹ NO₃⁻-N. The results show that the use of real-time dynamic control is of great importance, as the nitrogen removal efficiency is maximized by minimizing the anoxic/aerobic duration ratio, thus significantly reducing the aeration energy requirement and operating cost. Full article

To address the problems of eutrophication exacerbation in water bodies caused by low carbon-to-nitrogen ratio (C/N) wastewater and the limited nitrogen removal efficiency of conventional constructed wetlands, this study proposes the use of biochar (Corncob biochar YBC, Walnut shell biochar HBC, and Manure biochar FBC) coupled with intermittent aeration technology to enhance nitrogen removal in constructed wetlands. Through the construction of vertical flow wetland systems, hydraulic retention time (HRT = 1–3 d) and influent C/N ratios (1, 3, 5) were regulated, before being combined with material characterization (FTIR/XPS) and microbial analysis (16S rRNA) to reveal the synergistic nitrogen removal mechanisms. HBC achieved efficient NH₄⁺-N adsorption (32.44 mg/L, Langmuir R² = 0.990) through its high porosity (containing Si-O bonds) and acidic functional groups. Under optimal operating conditions (HRT = 3 d, C/N = 5), the CW-HBC system achieved removal efficiencies of 97.8%, 98.8%, and 79.6% for NH₄⁺-N, TN, and COD, respectively. The addition of biochar shifted the dominant bacterial phylum toward Actinobacteriota (29.79%), with its slow-release carbon source (TOC = 18.5 mg/g) alleviating carbon limitation. Mechanistically, HBC synergistically optimized nitrogen removal pathways through “adsorption-biofilm (bacterial enrichment)-microzone oxygen regulation (pore oxygen gradient).” Based on technical validation, a dual-track institutionalization pathway of “standards-legislation” is proposed: incorporating biochar physicochemical parameters and aeration strategies into multi-level water environment technical standards; converting common mechanisms (such as Si-O adsorption) into legal requirements through legislative amendments; and innovating legislative techniques to balance precision and universality. This study provides an efficient technical solution for low C/N wastewater treatment while constructing an innovative framework for the synergy between technical specifications and legislation, supporting the improvement of watershed ecological restoration systems. Full article

The Anaerobic–Swing Aerobic–Anoxic–Oxic (ASAO) process was developed to tackle problems such as temperature sensitivity during the Anaerobic–Oxic–Anoxic (AOA) process. By introducing a swing zone (S zone) with adjustable dissolved oxygen (DO), during the 112-day experimentation period, the ASAO system achieved removal rates of 88.18% for total inorganic nitrogen (TIN), 78.23% for total phosphorus (TP), and 99.78% for ammonia nitrogen. Intermittent aeration effectively suppressed nitrite-oxidizing bacteria (NOB), and the chemical oxygen demand (COD) removal rate exceeded 90%, with 60% being transformed into internal carbon sources like polyhydroxyalkanoates (PHAs) and glycogen (Gly). The key functional microorganisms encompassed Dechloromonas (denitrifying phosphorus-accumulating bacteria), Candidatus Competibacter, and Thauera, which facilitated simultaneous nitrification–denitrification (SND) and anaerobic ammonium oxidation (ANAMMOX). The enrichment of Candidatus Brocadia further enhanced the ANAMMOX activity. The flexibility of DO control in the swing zone optimized microbial activity and mitigated temperature dependence, thereby verifying the efficacy of the ASAO process in enhancing the removal rates of nutrients and COD in low-C/N wastewater. The intermittent aeration strategy and the continuous low-dissolved-oxygen (DO) operating conditions inhibited the activity of nitrite-oxidizing bacteria (NOB) and accomplished the elimination of NOB. Full article