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 CO2 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 (QL) and gas (QG) flow rates, gas-to-liquid ratio (G/L), and transmembrane pressure (TMP), were systematically evaluated to optimize separation performance. Results revealed CO2 removal efficiencies exceeding 95 %, with CH4 contents surpassing 95 % in the upgraded biogas under the optimal operational conditions: a G/L of 0.25, moderate QL ≤ 3.8 L min−1, low QG 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 CH4 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.