Methane production from ethanolic and acid fermentations of the organic fraction of municipal solid waste under different pH and reaction times

Abstract

Thermodynamically, lactic acid and ethanol allow the best energy recovery among all common metabolites for methane production. When faster methane production is required, previously fermented organic fraction of municipal solid waste (OFMSW) can be essential for methane production. This research deals with OFMSW fermentation in batch reactors at pH values of 4, 5, and 6 at 35 °C. Samples were taken at reaction times of 1, 3, and 6 days for every pH, characterized, and exposed to methane production at 35 °C. The results show that lower pH and shorter fermentation times allow better ethanol and lactic acid production. Without exception, the fermented liquid fractions produce more methane than the solid ones. Fermented OFMSW produced 19% more methane than unfermented ones. The first methanization stage of the liquid fractions required 1–2 days and the solid fractions from 4 to 8 days. According to Michaelis & Menten, the reaction rate Vmax values are significantly higher for the liquid fractions than for the solid ones. Vmax shows that the lowest tested pH (4) during fermentation delivers the best substrate for methane production. The liquid digestates show significantly lower Km values than the solid ones, indicating a good affinity of the methanogens for the substrate. These results show the possibility of producing methane fast and efficiently using liquid fermented digestates. The solid fraction produces methane, but only slower than the liquid fraction.