Controlling anaerobic digester pH and neutralizing volatile fatty acids are commonly required during anaerobic digester starup. This is especially true when good quality seed sludge is not available. Saprophytic bacteria hydrolyze and convert complex compounds to low molecular weight ( simple ] compounds.When the pH slips to below 6.5, the methane formers are inhibited and the digester goes ' out of control " that is the pH goes down faster because the methane formers are no longer effectively using the volatile acids. When the pH reaches 5, the saprophytes are inhibited and the digester has gone sour and no digestion is taking place For more details consult https://www.dec.vermont.gov