Insights on the production of volatile fatty acids from saline wastewater

Abstract

This work investigates how several marine salt levels typical of canning industry wastewaters (0, 10, 15, 20, and 35 g salt/L), mainly composed of proteins and sugars, impact the production of volatile fatty acids (VFAs), their selectivity and the acidogenic microbiome. Mixed-culture fermenters were operated in continuous (25 °C, pH =7; HRT 1.5 d) and batch (25 °C, pH = 7, substrate-to-inoculum ratio 20 g COD/g VSS) regime. Low salinity levels (0–10 g/L) led to high VFA yields (74 %, COD basis), dropping to 46 % at higher salt concentrations. Acetic acid was always the main product (> 67 % molar basis), and butyric and valeric acid concentrations were negatively affected by high salinities (20–35 g/L). Salinity changes were also reflected in the microbial population: Clostridia was the dominant class in the whole salinity range although Actinobacteria increased its relative abundance as salinity increased. The experimental results were consistent with salt-in osmoregulation at low and medium salinities (0–15 g/L) while the compatible solutes strategy appears as more likely at high salinities (20–35 g salt/L). Given the needs for compatible solute strategy, these results enable the selection of co-substrates complementing wastewaters at the high salinity range to enhance the VFA production.