The anaerobic baffled reactor (ABR) is a decentralized treatment system that is commonly used for municipal wastewater treatment. Slower growth rate of anaerobic microorganisms requires extended hydraulic retention time (HRT), leading to a larger bioreactor volume. In this study, polyurethane sponge sheets were provided in a six-compartment ABR for retention and growth of biomass to improve its treatment performance at shorter HRTs. Polyurethane sponge was selected for its low cost, durability, availability, easy emplacement, and high voidage. The sponge anaerobic baffled reactor (SABR) was operated within a temperature range of 35 ± 1 °C at HRTs of 18, 12, 8, and 6 h to evaluate its treatment performance. Average removal efficiencies ranged from 60–77% for organics, 74–81% for total suspended solids (TSS), 50–66% for total nitrogen (TN), and 47–57% for total phosphorus (TP). The shortest HRT was 8 h with average removal efficiencies of 74, 63, 64, and 52% for organics, TSS, TN, and TP, respectively, to meet effluent discharge limits. With the shortest HRT of 8 h, the SABR demonstrated low volume requirements, thereby making it an efficient solution for decentralized wastewater treatment, particularly advantageous for developing countries with warm climates.

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.

The graphical abstract illustrates the performance of a municipal wastewater treatment system operated at different hydraulic retention times (18, 12, 8, and 6 h). It shows influent–effluent changes and removal efficiencies of COD, TSS, TN, and TP, with comparison against permissible effluent limits. The system operates at 35 ± 1 ºC, highlighting overall treatment performance and microbial contribution to pollutant removal. Conventional anaerobic baffled reactors often demonstrate inadequate performance in municipal wastewater treatment, necessitating innovations for improved efficiency. This study investigated sponge anaerobic baffled reactor (SABR) for municipal wastewater treatment and identified dominant bacterial strains using 16S rRNA sequencing. SABR, consisting of six compartments each provided with low-cost, porous polyurethane sponge sheet, facilitated biomass immobilization and microbial proliferation to enhance reactor treatment performance. The reactor underwent prolonged mesophilic operation for approximately four months, during which hydraulic retention times (HRTs) were sequentially adjusted between 18 and 6 h. The reactor performed most effectively under 8 h HRT, achieving removal efficiencies of 79% for chemical oxygen demand (COD), 96% for total suspended solids (TSS), 60% for total nitrogen (TN), and 53% for total phosphorus. At this operating conditions, the effluent quality satisfied the permissible discharge limits for COD, TSS, and TN. Microbial characterization identified eight dominant bacterial strains, including one potentially novel strain requiring further taxonomic validation. This study presents SABR as a promising bioreactor for decentralized wastewater treatment; however, further long-term and field-scale studies are needed to confirm its robustness and scalability. Additionally, it opens avenues for further research into the roles of diverse microbial consortia in optimizing anaerobic wastewater treatment processes.