Cylindrical digesters are covered to contain odors, maintain operating temperature, maintain anaerobic condition, and collect digester gas. Covers can be classified as either fixed or floating. Figure 5.11 shows several types of fixed and floating covers.
Fixed covers are either dome-shaped or flat and are fabricated from reinforced concrete, steel, or fiberglass-reinforced polyester. Concrete roofs are susceptible to cracking and therefore are sometimes lined with polyvinyl chloride (PVC) or steel plate to contain gas. Generally, fixed-cover digesters are operated so as to maintain a constant water level in the tank by compensating digested sludge withdrawals with the addition of raw sludge. This eliminates the possibility of air entering the tank and producing an explosive mixture of gas and oxygen.
Floating covers float on the surface of the digester contents and allow the volume of the digester to change without allowing air to enter the digester and mix with gas. Floating covers are normally used for single-stage digesters and in the second stage of two-stage digesters. A variation of the floating cover is the floating gas holder, which is a floating cover with an extended skirt (see Figure 5.11) to allow storage of gas during periods when gas production exceeds demand. A recent development in gas-holder covers is the membrane cover shown in Figure 5.11. The cover consists of flexible air and gas membranes under a support structure. As gas storage volume decreases or increases in the space between the liquid surface and the gas membrane, the space between the membranes is pressurized or depressurized accordingly, using an air-blower bleed-valve system.
In egg-shaped digesters, the volume available for gas storage is small. Therefore, supplemental external gas storage is required.
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