The initial drop in pH is important since the activities of aerobes and facultative aerobes are essential to produce relevant acidic metabolites, which are later acted upon by methanogenic bacteria to bacteria to produce methane. Meth-anogenesis is known to occur best within a pH range of about 6.0 and 7.8. In the present study maximum biogas production corresponds with pH 6.4 of the 4th week (Fig. 3 and Fig. 4).