The range of acceptable volatile acid-to-alkalinity ratios is 0.1-0.2. An acceptable ratio may be obtained by adjusting volatile acid concentration, alkalinity concentration, or both concentrations. Reducing or terminating feed sludge to the digester also helps to lower the volatile acid-to-alkalinity ratio. If feed sludge cannot be reduced or terminated, the use of chemicals for alkalinity adjustment is required.
An unacceptable volatile acid-to-alkalinity ratio is usually the first warning of an adverse operational condition. Significant deviations from an acceptable ratio may be caused by shock loading or excess sludge withdrawal
The volatile acid-to-alkalinity ratio is a key control parameter. Perhaps the best method of maintaining a properly operating anaerobic digester is to ensure an acceptable volatile acid-to-alkalinity ratio. A ratio of 0.07-0.08 is a good working ratio, whereas a ratio >0.5 is indicative of digester upset and possible failure.
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