Digestion Cover: Floating
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Floating covers float directly on the sludge, allowing variable sludge levels. However, practically no gas storage is available and the gas volume is relatively constant. A floating cover provides for submergence of scum and again is often applied on the primary digester in a two-stage system. The floating cover floats on the liquid surface.
Floating covers have a minimum rim plate depth since they float directly on the sludge. This depth is generally about 3'0". A floating cover features both a ceiling plate and a roof plate, forming an attic space, in which are the trusses. Rollers are mounted on the cover roof to provide for travel.
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The following features are used with this type of cover:
A gas dome located at the center of the cover provides a gas reservoir from which gas can be withdrawn without also removing entrained liquid, scum, foam, etc.Sampling wells, usually one near the center, one at about 1/2 radius. These wells have quick-opening hatches, with seal pipe extending below liquid, so that samples can be withdrawn without losing gas.
Pressure relief and vacuum-breaker valve, mounted atop the gas dome for the purpose of relieving gas upon over-pressurization, and allowing atmospheric air to enter if vacuum exists when cover on corbels. Basically, this is a safety device, protecting the structure.
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The advantages of this cover are scum suppression, good insulation due to the attic space, support trusses not subject to digester interior corrosive products, and it is very stable. The disadvantages are it is the second most costly of the four types, and it cannot store gas in any appreciable amount.
When used with a gasholder, it should be ballasted at 2" water column pressure higher so it "pushes" gas to the gasholder.