*4.2.1 Hydrogen-ion (pH) concentration control*

The hydrogen-ion (pH) concentration in the digesting material affects the anaerobic digestion process. The hydrogen-ion concentration of the culture medium has an immediate effect on microbial growth due to the digestion is prevented by surplus acidity [100, 101]. Methanogens grow better under neutral and a bit alkaline environments. They are died by acidic conditions. Upon stabilization of the aerobic digestion process, the optimum values of pH in the system will be in the range of 7–8.5 with values near to 7 for optimal activity [21, 96, 102–105].
