The level of pH has an effect on the enzymatic activity in the micro-organisms, since each enzyme is in activity only in one specific range of pH, and it has its maximum activity with its optimal pH (Ahring, 1994). A stable pH indicates system equilibrium and digester stability. A falling pH decrease can point toward acid accumulation and digester instability. Gas production is the only parameter that shows digester instability faster than pH. The range of acceptable pH for the bacteria participating in digestion is from 5.5 to 8.5, though the closer to neutral, the greater the chance that the methanogenic bacteria will function (Golueke, 2002). Most methanogens function in a pH range between 6.7 and 7.4, and optimally between 7.0 and 7.2. The greatest potential for a digester failure is a result of acid accumulation. This would occur if the amount of volatile solids loaded into the digester as fresh waste increased sharply. Maintaining pH is especially delicate in the start-up because fresh waste must undergo acid forming stages before any methane forming can begin, which will lower the pH. To raise the pH during the early stages, operators must add a buffer to the system, such as calcium carbonate or lime.