8/20/2019 Anaerobic Digester Covers

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WALKER PROCESS EQUIPMENTDivision of McNish Corporation

Anaerobic Digester Covers

1.0 June 2004

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This paper deals with the various types of Digester covers, how they function and where they are used.There are four basic types of covers: Fixed Cover, Floating Cover, Gasholder Cover, and CombinationCover.

Fixed CoversFixed covers are used in digesters with novariation in total volume and are often used on theprimary digester in a two-stage system. Thesludge volume can be varied but only by varyingthe gas volume at the same time so as to keep thetotal volume constant. If gas is to be pressurizedfor use, it must also have a companion floatingcover connected by gas lines.

Fixed covers are anchored to the top of thedigester tank and normally there is a static orconstant sludge level. Thus the rim plate onlyneed be as deep as the tank freeboard plus thetank operating pressure and an appropriate sealdepth to contain the gas. A typical rim plate depthmight be approximately 2'-6" with 1'-0" freeboard. A fixed cover has only one steel membrane overits structural framework.

The advantage to this type of cover is lowest cost.The disadvantages are: it will not pressurize andstore gas by itself, will not depress scum, andprovides little insulating value.

Floating Cover

Floating Covers float directly on the sludge, allowingvariable sludge levels. However, practically no gas

storage is available and the gas volume is relativelyconstant. A floating cover provides for submergence ofscum and again is often applied on the primary digesterin a two-stage system. The floating cover floats on theliquid surface.

Floating covers have a minimum rim plate depth sincethey float directly on the sludge. This depth is generallyabout 3'-0". A floating cover embodies both a ceilingplate and a roof plate, forming an attic space, in whichare the trusses. Rollers are mounted on the cover roofto provide for travel.

The advantages of this cover are; scum suppression,good insulation due to the attic space, support trussesnot subject to digester interior corrosive products, andis very stable. The disadvantages are it is the secondmost costly of the four types, and it cannot store gas inany appreciable amount.

When used with a gasholder it should be ballasted at 2"water column pressure higher so it "pushes" gas to thegasholder.

8/20/2019 Anaerobic Digester Covers

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WALKER PROCESS EQUIPMENTDivision of McNish Corporation

Anaerobic Digester Covers

1.0 June 2004

Page 2 of 2

Gasholder CoverGasholder covers provide for gas storage by ridingon a constant pressure, variable volume gasbubble. Normally this type of cover is used with aconstant sludge level and this is the mosteconomical design, and would be applied on asecond stage digester.

 As a two-stage digestion system, it is desired that

all digestion occur in the first tank while the secondtank is used for supernatant separation and gasstorage.

Gas storage is a necessity since most gas burnerswill operate on an on-off cycle and in this mannergasholder serves as a "surge" tank.

The rim plate depth on a gasholder cover isdetermined by the required gas storage volume ortravel. Generally, a 5' to 7' deep rim plate issufficient for most applications. This cover alsohas one steel membrane over its structuralframework. Rollers and long columns areprovided for cover travel and to maintain stabilitythroughout its travel range. Note that the ballast islocated at the bottom of the skirt to lower itsmetacenter and increase its stability.

The advantages are; it is less costly than anycover other than the Fixed cover, and that it storesgas for usage by the boiler or other energyproducer. The disadvantage is that it has littleinsulating value.

Combination Gasholder/Cover

The combination cover is the least understood ofall, and is probably used for the wrong reason inmost instances. In reality, this cover combinesfeatures of the Floating Cover and the Gasholder.This can be visualized in two manners:

1. The combination Gasholder/Cover is a FloatingCover with a longer skirt, columns, and with theballast moved from the attic to the skirt bottom.2. It is a Gasholder with a ceiling plate added to

the bottom of the truss.

Its use may be the most misunderstood part.Some specifications require a Gasholder that willsuppress scum and therefore, require asubmerged ceiling plate. But, when gas is stored(which is the majority of the time), the ceiling plateis not submerged and therefore not "submergingscum".

The advantages are; it has better insulatingqualities than a Gasholder due to existence of anattic, and the trusses are not exposed to corrosiveinterior of digester.

The disadvantage is it is the most costly of allcovers.

Common Features: The above four sections dealtwith differences between cover types; followingare features commonly used on any digestercover.

 A. A gas dome located at the center of thecover provides a gas reservoir from which gascan be withdrawn without also removingentrained liquid, scum, foam, etc.

B. Sampling wells, usually one near thecenter, one at about 1/2 radius. These wellshave quick-opening hatches, with seal pipeextending below liquid, so that samples canbe withdrawn without losing gas.

C. Pressure relief and vacuum-breaker valve,mounted atop the gas dome for the purposeof relieving as upon over pressurization, andallowing atmospheric are to enter if vacuumexists when cover on corbels. Basically, thisis a safety device, protecting the structure.