A comparative study of the treatment of dark fermentation effluent by purple-phototrophic bacteria and microalgae with focus on substrate to biomass conversion

Abstract

The treatment of dark fermentation effluents from food waste was evaluated in two photobioreactor systems: a purple phototrophic bacteria (PPB) reactor and a microalgae-bacteria consortium (MBC) reactor. Experiments were performed at hydraulic retention times (HRT) of 5 and 10 days (P1 and P2, respectively) to maximize biomass yield for wastewater valorization. At the microbiological level, the PPB reactor exhibited a decrease in PPB abundance with longer HRTs, fa- voring other genera. In contrast, the MBC reactor showed a marked reduction in microalgae under both con- ditions, with PPBs predominating in P1 and a diverse microbial community in P2. The increase in HRT from 5 to 10 days improved pollutant removal but did not enhance biomass concentration, which stabilized at 0.61 ± 0.08 g/L (PPB) and 1.37 ± 0.16 g/L (MBC) at 5-day HRT. The highest biomass yield (1.03 ± 0.07 gCbiomass/gTOCremoved) was achieved in the MBC reactor at 5-day HRT, where preferential consumption of lactate and butyrate occurred, leaving acetate less assimilated. Despite the lower overall pollutant removal at 5-day HRT (TOC: 56.0 ± 3.5 %, TN: 60.3 ± 9.0 %, PO₄3 : 20.4 ± 7.4 %), this condition allowed for higher conversions of dissolved carbon into biomass rather than full mineralization. This trade-off is advantageous when targeting biomass valorization over complete pollutant removal, especially considering the commercial value of the residual organic acids. These results highlight the potential of short HRT operations in MBC systems for industrial application, enabling efficient resource recovery from fermentation effluents through selective assimilation, while maximizing biomass productivity and minimizing loss of valuable organics.