Evaluation of liquid effluents from an urban wastewater treatment plant as CO2 absorbents in a membrane-based biogas upgrading system in an operational environment

This study investigates the biogas upgrading to biomethane using wastewater treatment plant (WWTP) effluents as absorbents in a membrane-based system treating real streams. A microporous polypropylene hollow fibre membrane contactor was applied at pilot scale to assess the CO 2 removal efficiency and biomethane quality using two effluents: anaerobic digestate centrate or liquid fraction (DLF) from an anaerobic reactor and final water effluent (FWE) from the WWTP. Key operational parameters, including liquid (Q L ) and gas (Q G ) flow rates, gas-to-liquid ratio (G/L), and transmembrane pressure (TMP), were systematically evaluated to optimize separation performance. Results revealed CO 2 removal efficiencies exceeding 95 %, with CH 4 contents surpassing 95 % in the upgraded biogas under the optimal operational conditions: a G/L of 0.25, moderate Q L ≤ 3.8 L min −1 , low Q G of 0.5–0.9 L min −1 , and a TMP at moderate values of ∼800 mbar. FWE emerged as a particularly promising absorbent due to the reduced CH 4 losses through the absorbent and its higher availability compared to DLF. Additionally, FWE requires lower filtration pretreatment than DLF and FWE allowed the direct discharge after absorption. This work highlights the potential of valorising WWTP effluents into biogas upgrading processes, offering a sustainable solution for renewable biomethane production with manageable capital costs. • Biomethane obtained by membrane absorption using liquid effluents at pilot scale. • CO₂ removal >95 % and CH₄ purity >95 % achieved under optimal conditions. • Final water effluent outperformed digestate centrate in minimising CH₄ split. • The optimal G/L ratio (0.25) and moderate TMP (800 mbar) improved biomethane quality. • Annual membrane cost estimated at 20000 € for full-scale setup treating 100 m 3 h −1 .