Lactic acid was the dominant component
under acidic and alkaline conditions while volatile fatty acids (VFAs) became dominant under the neutral
condition. Furthermore, the neutral condition favored the chain elongation of carboxylic acids, especially
with ethanol as the electron donor. Ethanol addition enhanced valeric acid and caproic acid production by
44% and 167%, respectively. Lactic acid addition also had positive effects on VFAs production under the
neutral condition but limited to C2–C4 products. As a result, propionic acid and butyric acid production
was increased by 109% and 152%, respectively. These findings provide substantial evidence for regulating
carboxylic acid production during mixed culture fermentation of BSG by controlling pH and adding
electron donors.