Separation of volatile fatty acids from biogas plant hydrolysates

In this study the separation of volatile fatty acids (VFAs) from biogas plant hydrolysates and from each other was investigated. This creates the possibility to make the biogas process more profitable by selling these marketable volatile fatty acids (e.g. butyric acid, caproic acid, valeric acid) as isolates or as highly concentrated mixtures. Four kinds of screw-pressed liquid hydrolysates were used: sugar beet, grass cut and grass-corn hydrolysate from a bench-scale unit as well as a corn-triticale hydrolysate from a real biogas plant. The separation and the concentration of the VFAs were carried out by nanofiltration (NF) and reverse osmosis (RO). It was possible to separate acetic acid from the other acids in the permeate but only in low concentrations (0.25%). All VFAs were enriched in the retentate anyway. With the membrane SWC4 (RO) a total mass concentration enrichment of about factor 19 could be reached (from 7 g/l to 134.6 g/l). A further approach for concentration and separation of the investigated VFAs was their extraction with organic solvents (diethyl ether (DEE), methyl-isobutyl-ketone (MIBK)), followed by a consecutive distillation of the solvent. In this manner it was possible to extract the rather nonpolar butyric acid, valeric acid and caproic acid nearly completely, while the polar, shorter-chain VFAs remained in the aqueous phase. After distillation of the organic solvent, a highly concentrated mixture of butyric acid, valeric acid and caproic acid could be obtained (c(VFA;DEE) = 805 g/l). As the liquid hydrolysate still contained suspended particles, which possibly could clog the NF and RO membranes and disturb the extraction, tests were carried out to remove these particles by microfiltration (MF) and ultrafiltration (UF). UF membranes with separation limits from 10 kDa to 50 kDa proved to be the best. An assessment of the probably most valuable add product was carried out against market prices for the individual acids and against calculated possible extractable yields for a hydrolysate of a 250 kW biogas plant.