Integrated membrane contactor system using NaOH solution for biogas purification and polyhydroxyalkanoate synthesis via cyanobacteria

Abstract

This study develops and evaluates a membrane contactor system integrated with a photobioreactor for biogas purification and simultaneous CO2 utilization. The system uses NaOH solutions as chemical absorbents to capture CO2, which serves as a carbon source for polyhydroxybutyrate (PHB) synthesis via genetically engineered cyanobacteria, Synechocystis PPT1 (ΔpirC-REphaAB). The membrane contactor achieved high CH4 purity exceeding 97 % and CO2 absorption rates of up to 2 × 10−3 mol/s·m2 using a two-stage membrane configuration with 0.1 M NaOH. Long-term operation with 0.01 M NaOH maintained over 95 % CH4 recovery and <3 % performance decline over 180 h, indicating excellent membrane stability. Energy analysis revealed an 18.3 % reduction in energy consumption compared with that of conventional systems by eliminating the CO2 desorption step and reusing CO2-rich absorbents. The photobioreactor, fed with CO2-laden absorbents containing ∼0.014 mol/L of dissolved CO2, achieved a PHB yield of 63 % of dry cell weight, confirming effective carbon conversion. This integrated system provides a promising strategy to reduce greenhouse gas emissions and support a circular economy by converting biogas and captured CO2 into high-value biopolymers. © 2025 Elsevier B.V.