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This study examined the effect of effluent recirculation on the performance of an anaerobic baffled reactor (ABR) in treating municipal wastewater under mesophilic steady-state conditions. Although effluent recirculation is proposed to enhance ABRs’ performance, its benefits remain inconclusive, highly dependent on wastewater characteristics, and inadequately understood during steady-state conditions. Utilizing the GPS-X computer application, an innovative modeling and simulation approach was employed to evaluate an ABR’s performance in removing chemical oxygen demand (COD) and total suspended solids (TSS). Sensitivity analysis was utilized to refine critical stoichiometric, kinetic, and operational parameters for precise model calibration and validation, thus enhancing the model accuracy. The average absolute relative error (ARE) and Thiel inequality coefficient (TIC) were employed for model calibration and validation. The effect of effluent recirculation on the treatment behavior of the reactor was investigated through model predictions, considering recirculation ratios of 10%, 20%, and 30%. The results showed that effluent recirculation had a limited effect on ABR performance at HRTs of 24, 18, and 12 h, with COD removal efficiency (RE) improving by up to 2.1%, and TSS RE by up to 5.7%. However, at an HRT of 8 h, COD RE declined from 65% (no recirculation) to 61.6% at a 30% recirculation ratio, while TSS RE slightly improved at 10% but decreased by 4.1% at 30%. This study concluded that, under mesophilic conditions, effluent recirculation may not be an effective strategy for improving ABR performance in municipal wastewater treatment, potentially increasing operational costs due to an increase in energy consumption for effluent recirculation.

High-ammonia-nitrogen organic wastewater poses significant challenges to traditional nitrogen removal processes due to their high energy consumption and carbon dependency, conflicting with global sustainability goals. Anammox presents a sustainable alternative with lower energy demands, yet its application is constrained by organic matter inhibition. This study aimed to optimize nitrogen and organic matter removal in Anammox systems by comparing two strategies: effluent recirculation and micro-aeration. Anammox reactors were operated under three conditions: (1) no recirculation (control group), (2) 100–300% effluent recirculation, (3) micro-aeration at 50–150 mL/min. The effects on total nitrogen (TN) and chemical oxygen demand (COD) removal were evaluated, alongside microbial community analysis via high-throughput sequencing. The results show that micro-aeration at 100 mL/min achieved 78.9% COD and 88.3% TN removal by creating micro-anaerobic conditions for metabolic synergy. Excessive aeration (150 mL/min) inhibited Anammox, dropping TN removal to 49.7%. Recirculation enriched Planctomycetota, while micro-aeration slightly increased Planctomycetota abundance at 45 cm and enhanced Proteobacteria and Chloroflexi for denitrification. Optimal conditions—200% recirculation and 100 mL/min aeration—improve efficiency via dilution and synergistic metabolism, providing a novel comparative framework for treating high-ammonia-nitrogen organic wastewater and filling a research gap in the parallel evaluation of Anammox enhancement strategies.

This study deals with the effect of effluent recirculation (ER) on the pollutant removal efficacy of a full-scale, hybrid treatment system composed of a macrophyte pond and a horizontal flow constructed wetland. The average removals of 5-day biochemical oxygen demand (BOD), chemical oxygen demand (COD), total suspended solids (TSS), turbidity, total N (TN), ammonium nitrogen (NH4-N), total phosphorus (TP), sulfates, E. coli and Total coliforms (TC) for the years 2017–2018 (no recirculation), 2019 (50% recirculation) and 2021 (100% recirculation) were compared. Results show a general improvement of the effluent with ER. Removals for 0%, 50% and 100% ER, respectively, were: 59%, 61% and 66% for COD; 90%, 96% and 96% for BOD; 94%, 94% and 99% for TSS; 33%, 40% and 67% for TN; 22%, 30% and 55% for NH4-N; 92%, 98% and 96% for sulfates; 99.6%, 99.7% and 99.9% for E. coli; and 99.5%, 99.7% and 9.9% for TC. No clear effect was observed on the removal of TP and dissolved PO4-P, which were very low. 50% ER improved turbidity removal from 88% to 91%, but 100% ER provided worse results. The removal of NH4-N and TN significantly improved with 100% ER. This indicates that ER can be a simple, economic, and feasible way to upgrade the performance of full-scale natural wastewater treatment systems.

The bleaching plant of a kraft pulp mill is the sector that consumes water and generates effluent with the highest volume. Water recycling is an attractive option to reduce water consumption and effluent generation. This study evaluated the technical feasibility of using treated effluent as washing water in the bleaching stages. The bleaching sequence was simulated in the laboratory using four types of washing water: deionized water, whitewater, low organic load effluent, and high organic load effluent. To achieve 90% ISO pulp brightness, the ClO2 consumption increased from 8.1 kg ClO2 odt-1 when using water to 13.8 and 16.3 kgClO2 odt-1 for the low and high organic effluents. Physical and optical tests of the hand-sheet papers did not show any statistical difference between various washing waters. The filtrates showed values that did not burden the efficiency of the effluent treatment plant. It was possible to use effluent in the bleaching stages, considering that the filtrates and the produced paper complied with the quality standards.

Waste stabilization ponds and constructed wetlands (CWs) are effective at eliminating pharmaceutical residues, but removals are not usually complete. Their combination is regarded as an efficient, robust wastewater treatment method, but their efficiency in the removal of pharmaceuticals and the effect of a mild effluent recirculation has not been sufficiently studied in full-scale systems. Effluent recirculation can help to improve performance by increasing hydraulic residence time and, eventually, dissolved oxygen concentration. In this work, the presence of pharmaceuticals in wastewater from a university campus, their removal in a macrophyte pond–CW system, and the effect of effluent recirculation on removal and ecological risk were evaluated. Stimulants (caffeine and nicotine) and non-steroidal anti-inflammatories (naproxen and ibuprofen) were the most detected compounds in the influent and showed the highest concentrations, ranging from 0.5 to 300 µg·L−1. The pond–CW combination showed notable elimination for these compounds, achieving 87% on average. The ecological risk was also reduced by between 5.5 and 12.4 times, but it was still over values that indicates high ecological risk, mainly because of the concentrations of nicotine and ibuprofen. The effect of effluent recirculation was not as high as expected since the removals of caffeine, paraxanthine and naproxen were significantly improved, but those of atenolol and ibuprofen were lower. These results suggest that a higher recirculation ratio should be tested.

In this study, a high organic loading rate of 58–146 g BOD 5 /m 2  day with a hydraulic loading rate (HLR) of 1.63 m 3 /m 2  day and retention time (RT) of 16 h was achieved to maximize the treatment capacity of a four-stage alum sludge-based constructed wetland (CW) system. An alternative operation strategy, i.e., the first stage anaerobic up-flow and the remaining stage tidal flow with effluent recirculation, was investigated to achieve the goal with good treatment performance of 82% COD, 91% BOD 5 , 92% SS, 94% NH 4 -N, and 82% TN removal. Two kinetic models, i.e., first-order model and Monod plus continuous stirred-tank reactor (CSTR) flow model, were employed for predicting the removal dynamics. The results showed that the tidal flow strategy enhances oxygen transport and diffusion, thus improving reduction of organics and NH 4 -N. Effluent recirculation could further increase elimination of organics by extending the interaction time and also benefit the denitrification process. In addition, denitrification could be further enhanced by anaerobic up-flow in the first stage.

Short hydraulic retention time (HRT) treatment operation offers promising techno-economic advantages; however, the treatment effect on extracellular polymeric substance (EPS), mass transfer, microbial structure, and metabolism still needs further understanding. We examined the treatment performance and microbial community structure using a spiral symmetry stream anaerobic bioreactor (SSSAB) at varied HRT conditions. After each operating period, water quality results indicated that phases I (HRT 12 h), II (HRT 6–2 h), and III (HRT 3.2–5 h) achieved chemical oxygen demand removal efficiencies of 89.7 ± 1.1%, 94.8 ± 0.3%, and 78.9 ± 1.6%, respectively. The NH 3 –N increasing rates observed in phases I, II, and III were 34.5 ± 5.5%, 33.4 ± 5.7%, and 18.2 ± 2.7%, indicating limited NH 3 –N removal efficiency in phases I and II. The EPS experimental results revealed that chitosan addition in SSSAB reduced the EPS secretion, notably polysaccharides. This reduction led to an increase in the protein/polysaccharide ratio within the microbial matrix. The altered EPS composition increased the mass transfer resistance between microbes and substrates. Our findings suggest that chitosan acts as a regulatory agent, influencing both EPS composition and microbial metabolism, potentially offering a strategy for controlling microbial activity in bioreactors. The analysis from high-throughput amplicon sequencing revealed that Geobacter and Methanosaeta dominated all bioreactor compartments. Chitosan addition did not inhibit Geobacter and Methanosaeta direct interspecies electron transfer for substrate digestion and methanogenesis.

Nitritation-anammox treatment can be a potentially energy-and resource-efficient technology for treating mainstream wastewater. However, the issue of nitrate residue from anammox treatment remains to be addressed. Herein, external recirculation of the anammox effluent to a hybrid anaerobic reactor (HAR), which was also to provide a continuous flow with low COD/N for the nitritation-anammox reactor, was employed to decrease the residue compounds. The recirculation ratio of 50% was observed to be the optimal to achieve the best overall performance with potential savings in energy demand. Specifically, in the operation scenario of R = 50%, the highest COD removal of ~90% by the HAR was achieved. Meanwhile, the lowest COD/NH₄⁺-N ratio of ~2.0 in the HAR effluent ensured the lowest observed NO₃⁻-N/ NH₄⁺-N ratio of ~14% in the nitritation-anammox reactor. These results have demonstrated the feasibility of applying external recirculation for nitrate residue removal via denitrification in the anaerobic pretreatment stage.

The objective of this study was to investigate the application of constructed wetlands as a mean to manage olive mill wastewater (OMW). Two free water surface (FWS) constructed wetlands, one without (CW1) and one with effluent recirculation (CW2), were operated for a two-year period with diluted OMW (1:10) and evaluated in terms of the removal of COD, TSS, TKN, NH4+-N, NO3−-N, TP and total phenols. The organic loading rate of CWs was adjusted to 925 kg BOD/ha·d. In CW1 the removal efficiency averaged 80%, 83%, 78%, 80%, and 74% for COD, TSS, TKN, TP, and total phenols, respectively, during the operation period. Effluent recirculation further improved the treatment efficiency which approached 90%, 98%, 87%, 85%, and 87% for COD, TSS, TKN, TP, and total phenols, respectively. Constructed wetlands also showed high removal efficiency for NH4+-N. Nitrate concentration maintained low in both CWs basins, probably due to the prevalence of high denitrification rates that efficiently removed the NO3--N produced by NH4+-N oxidation. Despite the increased removal percentages, pollutant concentration in effluent exceeded the allowable limits for discharge in water bodies, suggesting that additional practices, including enhanced pre-application treatment and/or higher dilution rates, are required to make this practice effective for OMW management.

The effects of two alternative operational techniques, i.e., effluent recirculation and step-feeding, on the removal of Biochemical Oxygen Demand (BOD) in Horizontal Subsurface Flow Constructed Wetlands (HSF CWs) are evaluated numerically. These include two feeding techniques: under the first technique, a percentage of the effluent is re-introduced to the wetland inlet; while under the second, wastewater is introduced to the HSF CW at several points along the flow path. For the numerical simulation, the Visual MODFLOW family code was used. This model has been calibrated using experimental data collected in five HSF CW pilot-scale units. Application of the model to study effluent recirculation showed that this operation mode does not improve the performance of the Constructed Wetlands (CWs), which seems to decrease as the effluent recirculating quantity increases. Although recirculation dilutes the influent pollutant concentration, the decrease in Hydraulic Residence Time ( HRT ), due to the additional flow entering the system, seems to act negatively on CW performance. For the step-feeding case, various values of the step-feeding percentage factors have been investigated at three inflow points along the CW. Again, the results showed no performance improvement. Thus, both operation modes are not recommended for HSF CWs.