Control tools to selectively produce purple bacteria for microbial protein in raceway reactors

Abstract

Purple non-sulfur bacteria (PNSB) show potential for microbial protein production on wastewater as animal feed. They offer good selectivity (i.e. uneven community with high abundance of one species) when grown anaerobically in the light. However, the cost of a closed anaerobic photobioreactor (PBR) is prohibitive for protein production. While open raceway reactors are cheaper, their feasibility to selectively grow PNSB is thus far unexplored. This study developed tools to boost PNSB abundance in the biomass of a raceway reactor fed with volatile fatty acids as carbon source. For oxygen availability as tool, not stirring in the night (i.e. reduced oxygen supply) elevated the PNSB abundance from 8% to 20%. For light availability as tool, a 24-h illumination increased the PNSB abundance from 8% to 31% compared to a 12-h light/12-h dark regime. A reactor run at 2-d sludge retention time at the highest surface-to-volume ratio (10 m2 m-3 increased light availability) showed productivities up to 0.2 g protein L-1 d-1 and the highest PNSB abundance (78%). The estimated production cost is {euro}1.9 kg-1 dry weight (vs. PBR {euro}11.4 kg-1 dry weight). This study pioneered in PNSB-based microbial protein production in raceways, yielding cost efficiency along with high selectivity when avoiding the combined availability of oxygen, COD and darkness. Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=89 SRC="FIGDIR/small/912980v1_ufig1.gif" ALT="Figure 1"> View larger version (17K): org.highwire.dtl.DTLVardef@c5e3borg.highwire.dtl.DTLVardef@1fe8d28org.highwire.dtl.DTLVardef@e5630borg.highwire.dtl.DTLVardef@1c99bf3_HPS_FORMAT_FIGEXP M_FIG C_FIG