Microbial community-based production of single cell protein from soybean-processing wastewater of variable chemical composition

Abstract

The use of food-processing wastewaters to produce microbial biomass-derived single cell protein (SCP) is a sustainable way to meet the global food demand. Yet, despite the potential benefits of lower costs and greater resource recovery compared to pure cultures, bioconversion processes relying on microbial community-based approaches to SCP production have received scarce attention. Here, we evaluated SCP production from soybean-processing wastewaters under controlled reactor conditions using the existent microbial communities in these wastewaters. Six sequencing batch reactors of 4.5-L working volume were operated at 30 {square} for 34 d in cycles consisting of 3-h anaerobic and 9-h aerobic phases. Four reactors received no microbial inoculum and the remaining two were amended with a 1.5 L of mixed culture from a prior microbial community-based SCP production. Microbial characterization was done via 16S rRNA gene metabarcoding. Influent wastewater batches had variable chemical characteristics but a similar microbial composition. Reactors produced more SCP when fed with wastewaters of higher soluble total Kjeldahl nitrogen (sTKN) content and a lower carbon-to-nitrogen ratio (sCOD:sTKN). The biomass protein yield ranged from 0.24 to 3.13 g protein/g sTKN, with a maximum protein content of 50%. An average of 92% of sCOD and 73% of sTN removal was achieved. Distinct microbial communities were enriched in all six bioreactors after 34 d, where the prevailing genera included Azospirillum, Rhodobacter, Lactococcus, Novosphingobium, and Acidipropionibacterium. In contrast, the microbial community of influent wastewaters was dominated by Lactococcus and Weissella. We showed that constituents in soybean wastewater can be converted to SCP through microbial community-based growth processes and demonstrated the effect of variable influent wastewater composition on SCP production.