The digester tanks used for this process may be up to 12,000-m3 volume and equipped with heating coils for accurate temperature control normally in the me-sophilic range (25°-38°, usually 28°-32° in anaerobic digestion) to increase the rate of digestion, which is also improved by mechanical agitation (Loll, 1977). Some digesters are operated thermophilically (50°-70°, usually 55°-63° in anaerobic digestion) to increase the rate of degradation and biogas production, though much of the extra gas produced will be consumed in maintaining the digester temperature. The use of thermophilic anaerobic digestion has been investigated for the treatment of high strength (16-25 g COD dm"3), low pH (~ 3.8) and high temperature winery effluents (Romero et al., 1988). Wiegant et al. (1985) reported the successful application of thermophilic anaerobic digestion of vinasse in completely mixed and UASB reactors, although methane production was found to decrease at high loading rates due to the presence of inhibitory compounds. Retention times are generally between 10 and 30 days, although with solids recycle retention times may be as low as 0.5 day. Most digesters operate on a semi-continuous basis with the appropriate volume of digested material being removed and replaced with 'fresh' material on a daily basis.
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