pH value and stability in anaerobic digesters are governed by the carbonic system. An expression is derived to determine the alkalinity requirement for maintaining a desired pH value in an anaerobic digester. The requirement is calculated from stoichiometric relationships and depends on measurable parameters of the influent (notably concentration of digestible material, influent VFA, alkalinity, acidity and organic nitrogen concentration) and operational conditions (temperature, recycle of effluent and/or biogas). The derived expression is applied to three cases of anaerobic digestion: dilute waste water (sewage), concentrated waste water (vinasse) and sludge. In each case it was calculated if the influent alkalinity was sufficient to maintain an adequate pH for digestion and when it was not, what was the alkalinity requirement to maintain the desired pH value. The calculated values of pH and alkalinity demand compared well with experimental data.

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