Resource recovery using enriched purple phototrophic bacteria in an outdoor flat plate photobioreactor: Suspended vs. attached growth

[Display omitted] •PPB can be efficiently grown as biofilm in outdoor flat plate reactors.•Attached growth did not compromise treatment performance nor productivities.•The product quality in the biofilm was much higher than in the suspended biomass.•Biomass productivities of 16 ± 2.2 gCOD·m−2·d−1 were achieved during attached growth.•The harvested biofilm could be used as feed. Purple phototrophic bacteria (PPB) can produce single-cell protein from wastewater at high yields. Growing in a biofilm vs suspended can improve product quality and consistency. This study compares suspended and attached growths of enriched PPB cultures in an outdoor flat plate photobioreactor treating poultry-processing wastewater. Attached growth had lower VFA removal efficiencies (95 ± 2.7 vs 84 ± 6.4 %) due to light limitations and low substrate diffusion rates. Nevertheless, similar overall treatment performances and productivities were achieved (16 ± 2.2 and 18 ± 2.4 gCOD·m−2·d−1 for attached and suspended) at loading rates of 1.2–1.5 gCOD·L−1·d−1. Biofilms had higher quality than suspended biomass, with lower ash contents (6.9(0.6)% vs 57(16)%) and higher PPB abundances (0.45–0.67 vs 0.30–0.45). The biofilm (20–50 % of the total biomass) might be used as feed and the suspended fraction as fertiliser, improving the economics of the process. Semi-continuous PPB growth outdoors as biofilm is technically feasible, obtaining a superior product without jeopardising performance.