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ORIGINAL RESEARCH ARTICLE

On-site anaerobic treatment of mediumstrength wastewater under different hydraulic retention times

Sivcheng Lak1,2† ,  Borith Pang1† ,  Kimeng Sou1 ,  Rathborey Chan1 ,  Phaly Ham1 ,  Saret Bun1*
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1 Water Environment Laboratory, Faculty of Hydrology and Water Resources Engineering, Institute of Technology of Cambodia, Khan Tuol Kork, Phnom Penh , Cambodia
2 Water and Environmental Engineering, Graduate School, Institute of Technology of Cambodia, Khan Tuol Kork, Phnom Penh , Cambodia
†These authors contributed equally to this work.
EER, 026290028 https://doi.org/10.36922/EER026290028
Received: 18 July 2026 | Revised: 24 August 2026 | Accepted: 15 September 2026 | Published online: 13 October 2026
© 2026 by the Author(s). This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution 4.0 International License ( https://creativecommons.org/licenses/by/4.0/ )
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Abstract

Decentralized anaerobic wastewater treatment technologies have attracted growing attention because of their low energy requirements, simple operation, and suitability for developing countries. This study evaluated and compared the treatment performance of an anaerobic baffled reactor (ABR) and anaerobic filter (AF) treating medium-strength wastewater under hydraulic retention times (HRTs) of 12 h and 24 h. Laboratory-scale reactors were operated under continuous-flow conditions using domestic wastewater. Both systems were operated under natural sludge growth conditions and monitored through start-up and steady-state stages. Reactor performance was assessed based on pH, total suspended solids (TSS), volatile suspended solids, biochemical oxygen demand, chemical oxygen demand (COD), alkalinity, and volatile fatty acids. The results showed that increasing HRT improved treatment performance in both reactors. During steady-state operation, ABR achieved COD removal efficiencies of 39.34% and 50.14% at HRTs of 12 h and 24 h, respectively, while AF achieved higher COD removals of 55.74% and 73.56% under the same conditions. Similarly, TSS removal efficiencies increased from 76.45% to 78.57% in ABR and from 84.06% to 88.45% in AF when HRT was increased from 12 h to 24 h. Compartmental analysis showed progressive pollutant removal in both reactors, while AF consistently performed better due to enhanced biomass retention and attached microbial growth. Additional adsorption experiments demonstrated that sponge media contributed substantially to particle retention and turbidity removal, achieving approximately 90% turbidity reduction after 24 h of contact time. Biodegradation analysis further indicated that organic removal followed first-order kinetics with a degradation constant of 0.02 h−1. In conclusion, AF operated at 24 h HRT demonstrated the highest treatment efficiency and was identified as the optimum configuration for medium-strength wastewater treatment. The findings highlight the potential of attached-growth anaerobic systems as effective and sustainable alternatives for decentralized wastewater management.

Keywords
Anaerobic baffled reactor
Anaerobic filter
Hydraulic retention time
Medium-strength wastewater
Organic removal
Wastewater treatment
Funding
This work is funded by Cambodia - Second Higher Education Improvement Project (HEIP2) (Credit No. 76340-KH) through research project HEIP2-ITC-RGA#02 and by the Cambodia Higher Education Improvement Project (Credit No. 6221-KH).
Conflict of interest
The authors declare they have no competing interests.
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