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 produce methane. Methanogenesis 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 (Figs. 3 and 4).