The anaerobic consortium consists of several microorganism groups with different physiology that coexist syntrophically or antagonistically, resulting in a different response to environmental changes. As a consequence, when the activity of one of the microorganism groups is inhibited, the growth rates of other microorganisms are affected, changing the population balance, often causing a decrease in process efficiency or even failure. It has been recognised that the most important factors affecting the anaerobic digestion process are the pH, the temperature, the nature of the feedstock (composition, nutrients), the presence of toxic or inhibitory substances and the organic loading rate.

12.2.1 The pH

The pH affects the dissociation of weak acids and bases, and therefore, the formation of undissociated acids and bases which can easily penetrate the cellular membrane changing the internal pH of the cells. The pH also influences the function of the extracellular enzymes and has an impact on the hydrolysis rate. In most cases, the anaerobic transformation of organic matter is achieved most efficiently at a neutral pH. Many species though can grow at lower or higher pH values.

Low values of pH and the concomitant intermediate acid accumulation are more inhibitory to the methanogens than the acidogenic bacteria. Acidogens can grow and continue to produce acids at low pH values (5-6), intensifying the inhibitory conditions to the methanogens. However, it is known that methanogenesis can occur in extreme environments where very low or high pH values prevail such as swamps, hot springs, etc.

It is common to the acidogens that produce a mixture of metabolic products to switch their metabolism towards the formation alcohols to avert any further pH decrease (Huang et al, 1986; Gottschal and Morris, 1981; Lowe and Zeikus, 1991).