Valorization of spent anaerobic digester effluents through production of platform chemicals using Clostridium butyricum

Abstract

Spent digester streams were reformulated into nutrient rich effluents, capable of supporting biotechnological production of platform chemicals. This was achieved, using a set of physical pre-treatments including, sedimentation, dilution and sieving, followed by cross-flow microfiltration processing to give particle and cell free nutrient streams with a C:N:P molar ratio of 1:1.35:0.03. These streams were inoculated on bench scale, with Clostridium butyricum NCIMB 7432, a well-known acids producer, giving good growth rates (μmax 0.24 h−1) and acids concentrations of 110 mM acetic acid and 18 mM per litre butyric acid. When the treated effluent was fortified with 2% w/v glucose there was a significant improvement with a 0.38 h−1 μmax and platform chemicals concentration of 279 mM acetic acid and 32 mM butyric acid per litre. These media gave comparable performance with the synthetic media, therefore demonstrating a valid alternative solution to commercial media preparations. The cost of treating the excess digestate by microfiltration within the scope of formulation to nutrient media has been calculated at 0.0033 $USD/kg, 3 times lower than the cost of production of nutrient media.