Growth, hydrogen production, and community dynamics of Rhodopseudomonas palustris cultivated in forward osmosis-concentrated municipal sewage

Abstract Municipal sewage represents a sustainable resource for biohydrogen production; however, its direct application is hindered by low concentrations of soluble organic matter and elevated ammonium levels, which suppress nitrogenase activity. In this study, municipal sewage was concentrated approximately 20-fold by forward osmosis (FO) prior to cultivation experiments. While FO substantially increased organic matter availability, the resulting alkaline concentrate inhibited growth and hydrogen evolution. Adjustment of the medium to near-neutral pH restored robust growth and enabled detectable hydrogen production. Full-length 16S rRNA gene sequencing enabled improved taxonomic resolution of microbial dynamics. Anaerobic FO concentration enriched for fermentative taxa, specifically Campylobacteria (> 50%), while subsequent illuminated cultivation demonstrated that Rhodopseudomonas palustris was able to utilize dissolved organics (27.1–34.7% relative abundance), as confirmed by specific high-fidelity read counts. These results demonstrate that the integrated FO–PNSB process effectively transforms municipal sewage into a concentrated substrate capable of supporting biohydrogen production without external organic supplementation.