A similar study found that a higher CH4 potential was obtained from the lactic acid fermentation residue of municipal organic solid (Dreschke et al., 2015). Coats et al. (2012) studied the biogas production from dairy manure with a two-stage AD process, and found that the additional prefermentation in the first stage with short solid retention time of 4 days increased CH4 content and enhanced the biogas production as compared to single stage AD process