Wastewater treatment facilities use both conventional and high-rate anaerobic digesters to reduce volatile solids in wastewater treatment sludge (see Figure 7.45.1).
Conventional digesters are loaded at a rate of 30 to 100 lb VSS per 1000 ft3 per day (commonly 30 to 70 lb per 1000 ft3 per day). Both feeding and sludge withdrawal are intermittent. Detention times of 30 to 60 days are common. Conventional primary digesters are heated to 85° to 95°F. Primary digesters are often followed by one or more secondary, unheated digesters, where primary digester detention time can be shorter. Although primary digesters are frequently equipped with fixed covers, secondary digester covers are often the floating type. Mixing is not used in primary digesters unless the digesters are followed by secondary, unmixed digester units.
High-rate digesters are loaded at a rate of 100 to 500 lb VSS per 1000 ft3 per day (commonly 100 to 400 lb per
FIG. 7.45.1 Types of anaerobic digesters. A, Conventional; B, High rate.
1000 ft3 per day). Feeding and sludge withdrawal can be intermittent or continuous. High-rate primary digesters are heated to 85° to 95°F. High-rate units can operate in series, or they can be followed by conventional, unheated digesters, sludge holding and separation tanks, or other means of solids-liquid separation. Mixing by either gas recirculation or mechanical means is used with high rate units.
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