Volatile fatty acids up-concentration in fermentation liquids using Forward Osmosis

Abstract

In the face of the challenge posed by the current unsustainable situation, where large quantities of non-renewable resources, particularly petroleum, are being consumed to produce compounds of interest, there is a need to explore new production pathways based on biological processes. In this field, Volatile Fatty Acids (VFA) are of particular interest. These products have an increasingly high demand due to their wide range of applications, including their use as a molecular platform in biorefineries or as direct chemical products. Currently, 90 % of the demand is met by petrochemical processes, but they can also be produced through anaerobic digestion of biodegradable matter. One of the most attractive substrates is the Organic Fraction of Municipal Solid Waste (OFMSW) due to its abundance and high degradability, although the maximum achievable concentration is limited by product inhibition. This work focuses on obtaining a fermentation liquid from OFMSW and its subsequent treatment to broaden its field of application, specifically the recovery of ammoniacal nitrogen using a gas-permeable membrane and the up-concentration of VFA through direct osmosis. To conduct this research, several experiments were conducted using synthetic liquid with known concentrations of TAN (Total Ammoniacal Nitrogen) and acetic acid to study the permeability and selectivity of these compounds at a specific temperature (35°C) and different pH levels (5, 7, 9). Subsequently, two experiments were performed using fermentation liquid, first recovering nitrogen through Gas-Permeable Membrane (GPM), and then concentrating the VFA in the resulting solution using a direct osmosis module. The experiments with synthetic liquid showed very low rejection rates for TAN, regardless of the pH of the working solution. In contrast, acetic acid exhibited high rejection rates at pH levels above 7.Regarding the experiments with fermentation liquid, the nitrogen recovery stage resulted in high percentages of nitrogen removal from the feed solution. In the subsequent treatment using a direct osmosis module, concentrations were obtained that were much higher than the limit observed in the acidogenic fermentation stage. Therefore, from a technical standpoint, it is feasible to implement the following stages: fermenter, gas-permeable membrane, and direct osmosis. This approach yields a solution with a high concentration of (NH4)2SO4 and another solution with a very high concentration of VFA, exceeding 70 g COD/L.