It is usually assumed that solids reduction takes place only in the volatile portion of the sludge solids. Therefore, the common measure of the degree of stabilization is the percent volatile solids destroyed. Volatile solids reduction in high-rate anaerobic digesters usually ranges from 50 to 65%. Volatile solids reduction achieved in any particular application depends on both the characteristics of the sludge and the operating parameters of the digestion system. For a high-rate digestion system, the following empirical equation (Liptak, 1974) applies:
where Vd is the volatile solids reduction (%) and SRTd is the design SRT (days). Figure 5.9 shows volatile solids reduction as a function of SRT and is developed using equation (5.1).
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