Micro-aerobic treatment using purple bacteria enriched cultures for the treatment of gas-to-liquid (GTL) wastewater and PHA production

Gas-to-Liquid (GTL) wastewater is a carbon-rich industrial effluent characterized by elevated organic carbon concentrations that requires appropriate treatment before discharge into the environment. On the other hand, polyhydroxyalkanoates (PHA) production is receiving widespread attention due to their biodegradable characteristics. This study investigates a novel approach to simultaneously treat GTL wastewater and produce PHA using purple nonsulfur bacteria (PNSB) under micro-aerobic conditions. The treatments were carried out under light and dark conditions and compared with anaerobic and aerobic treatments. Under light conditions, micro-aerobic treatment showed the highest COD (1935 ± 4 mg/L) and TOC (512 ± 8 mg/L) removals. PHB accumulation was also the highest (18 ± 1 %) in micro-aerobic conditions. The light-anaerobic treatment showed the highest biomass productivity (0.46 ± 0.06 g-TSS/g-COD), and dark treatments showed lower biomass yields. Microbial analysis revealed the dominance of Pseudomonadaceae (64%) under micro-aerobic conditions, whereas Xanthobacteraceae dominated anaerobic (56%) and aerobic (44%) cultures. Carotenoids were the highest (2089 ± 7 μg/g) in dark anaerobic conditions, and bacteriochlorophylls in dark aerobic conditions (4566 ± 15 μg/g). Overall, the findings demonstrate that micro-aerobic operation creates favorable redox conditions for PNSB enrichment. This study provides a mechanistic insight into coupling GTL wastewater treatment with bioproduct recovery under micro-aerobic conditions.