Anaerobic Digestion System Covers

TRUSS COVERS-Truss covers are rigid structures with a framework of steel trusses and purlins maintained in position by the peripheral side sheet (rim skirt) and a center compression ring. Truss covers generally include steel plate welded to both the top (roof plate) and bottom (ceiling plate) of the truss to form an attic space which protects the structural members from corrosion and provides an insulation barrier for the sludge.

BEAM COVERS-Beam covers include arched radial beams held in position with a peripheral tension ring and a center ring. A single layer of steel plate is welded to the top of the radial beams to form a strong dome-shaped structure to resist design loads. Since there are no ceiling plates, the material cost is less, and with almost half of the field welding of truss covers, beam covers are very economical. Beam covers may also be insulated to enhance process performance.

FIXED COVERS-Fixed covers can be utilized when sludge levels remain constant or only slightly vary. Fixed covers are typically used on primary digesters, but can be used on secondary digesters with sufficient gas storage capacity through other means. Fixed truss or beam covers are anchored to the top of the tank wall with tube supports which are integral to the side sheet. The cover supports are designed to compensate for thermal expansion and contraction of the cover.

GAS HOLDER COVERS-Gas Holder covers are designed to store varying volumes of digester gas and provide for compression of the gas for use downstream. They can be truss or beam construction and can be used as a primary or secondary digester. The gas holder cover "rides" directly on the digester gas rather than on the liquid surface like the floating cover. As gas is produced the side sheet lifts out of the sludge. Ballast blocks are used to maintain the required gas pressure.

FLOATING COVERS-Floating covers remain in contact with the sludge, minimizing the liquid surface and preventing a large scum blanket. This type of cover allows for variable sludge storage capacity and increased operator flexibility during sludge additions or withdrawals when compared to fixed covers. Floating covers can be either truss or beam design. Internal gas pressure, which is necessary to operate the gas handling equipment downstream, is created due to cover weight and additional ballast. The lower chord of the floating truss cover is sloped to convey gas to the center of the dome for withdrawal.

SLIDE GUIDE-WesTech's unique Slide Guide design prevents the cover from binding during upward or downward travel. The slide assemblies utilize UHMW (ultra-high molecular weight polyethylene) slide plates secured on three sides of a structural steel tube to resist wind, seismic, or unbalanced loads in both radial and tangential directions. The UHMW surfaces are wider than conventional rollers and are extended for increased load distribution. Lubrication is not needed with the WesTech slide guide resulting in a virtually maintenance-free installation. The chemical and physical characteristics of UHMW provide an ideal choice for the application. WesTech slide assemblies are mounted on both the top and bottom of the cover side sheet for added stability.