The pH is another important environmental factor for the ADP. Different groups of methanogens have different ranges of optimum pH. The acidogens exhibit maxi­mum activity at pH 5.5-6.5 while the optimum for methanogens is pH 7.8-8.2 [68]. Since the methanogens are more sensitive to pH variation, the pH in anaerobic digesters is usually maintained in the range of 7-8. Rapid inhibition of methanogens at pH higher than 8 can be caused by dissociation of NH4+ to the neutral NH3 form [69]. The presence of alkalinity is an important marker of pH persistence in anaero­bic digesters. The bicarbonate alkalinity buffers the fluctuations in the generation of VFAs and carbon dioxide at pH close to neutral. A stable ADP is characterized by the bicarbonate alkalinity in the range from 1,000 to 5,000 mg/L as CaCO3 [70].

The ratio between VFAs to alkalinity should be in the range of 0.1-0.25. A further increase of the ratio of VFAs to alkalinity indicates possible process deterioration and requires the OLR to decrease in order to lower the VFA formation rate.