Pilot-scale purple phototrophic bacteria for wastewater treatment and resource recovery: an evidence-based framework for reactor selection and scale-up

Purple non-sulfur bacteria (PNSB), a major subgroup of purple phototrophic bacteria (PPB), have emerged as a promising platform for integrating wastewater treatment with resource recovery through the simultaneous production of value-added products, including single-cell protein (SCP), polyhydroxyalkanoates (PHAs), hydrogen, lipids, pigments, and other biomass-derived compounds. Although substantial progress has been achieved at laboratory scale, pilot-scale development remains fragmented across wastewater types, reactor configurations, operating conditions, and performance metrics, limiting cross-study comparison and practical technology selection. This review systematically synthesizes evidence from 15 pilot-scale studies ranging from 50 L to over 9,600 L, including flat-plate photobioreactors, tubular reactors, raceway ponds, membrane photobioreactors, integrated UASB-PPB systems, baffled reactors, and a stirred polypropylene-box photobioreactor operated under natural, filtered, or artificial illumination. Reported resource recovery outcomes include SCP contents of 42-65%, PHAs up to 36% of volatile suspended solids, hydrogen yields of 0.35-0.60 mol H₂ mol⁻¹ acetic acid, lipids up to 25%, and substantial pigment accumulation. Across these systems, Rhodopseudomonas, Rhodobacter, and Blastochloris consistently emerged as dominant genera supported by fermentative microbial consortia. The principal novelty of this review is the development of an evidence-based decision-support framework integrating wastewater characteristics, target products, reactor selection, technology readiness, and operational priorities. The framework identifies flat-plate photobioreactors as the preferred configuration for SCP production, integrated UASB-PPB systems for PHA recovery, and tubular and baffled photobioreactors for hydrogen production, while multi-criteria assessment ranks flat-plate photobioreactors, integrated UASB-PPB systems, and raceway ponds as the most promising configurations for pilot-scale implementation. The analysis further identifies light management, biomass harvesting, nutrient control, and process integration as the principal barriers to commercialization, providing practical guidance for reactor selection and accelerating the transition of PPB-based wastewater biorefineries toward commercial deployment.