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 waste-water. 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 center dot m- 2 center dot d-1 for attached and suspended) at loading rates of 1.2-1.5 gCOD center dot L-1 center dot 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.