Digester Covers are an important part of any biogas plant, performing the triple function of sealing the digester from the air, keeping it warm, and providing storage for a variable rate of pressurized gas, to balance the rate of biogas production with its use.
First of all, it almost goes without saying, that anaerobic sludge digesters must always be covered during operation with either a fixed roof, a rigid floating top, or a flexible inflatable cover:
- Fixed covers require a separate biogas holder that allows the gas to move freely into the gas holding vessel as the gas pressure in the digester tank rises. These are commonly seen in older digester designs used in WasteWater Treatment (Sewage) Works. A separate gas holder is necessary for fixed cover designs.
- Floating covers rise and fall according to the volume of gas and sludge and their weight provides the gas pressure. They have a perimeter-rim skirt that provides the necessary gas-to-air seal, which allows the storage of biogas. This type works in a similar manner to the way that natural-gas gasometers operate, with the air-seal being made by the liquid within the tank at the perimeter.
- Flexible inflatable membrane covers are normally double skinned with the inner skin (membrane) held within a form of a framework above the digester's rim. These are the most common for commercial anaerobic digestion plants in the developed nations. The double seal provides much-needed insulation from heat loss in temperate climates. By varying the air pressure between the two skins the amount of biogas stored is varied, to balance gas demand with the rate of gas production. The biogas tank's outer membrane “dome” remains inflated. At the same time, the changing biogas volume is held at the chosen pressure. This is accomplished by an air pressure control system which maintains the pressure balance between the inner and outer membranes.
Watch our video below to see examples of each type of digester cover:
This presentation contains images that were used under a Creative Commons License. Click here to see the full list of images and attributions: https://link.attribute.to/cc/591499
[box type=”info” style=”rounded” border=”full”]Some Wiki articles refer to 4 types of covers. They say that a separate gas holder is a 4th type. A “gas holder” is a stand-alone construction, for the storage of biogas, so I don't describe these as being covers.[/box]
Biogas pressure balancing is important to avoid a vacuum is being produced in a fixed roofed digester, whenever sludge is removed from the digester. Similarly, for membrane covered digesters, attention must be paid to the gas pressure between the double seals to allow for any rise and fall in the volume of the substrate materials in the digester, hour by hour.
- Fixed covers are either dome-shaped or flat and examples have been seen, fabricated from reinforced concrete, steel, or fiberglass reinforced polyester.
- Concrete roofs are susceptible to cracking, therefore they are sometimes lined with PVC or steel plate to ensure that they fully contain the generated gas.
- Floating covers, do as they are named and float on the surface of digester contents and allow the volume of digester to change without allowing air to enter the digester and mix with biogas.
- Flexible membrane covers are domed shaped and are the most commonly used. If welded in accordance with strict quality control requirements, and suitably tested for weld strength, these covers can be reliable and long-lasting. Couple this with a lower cost than steel, or concrete, covers and it is no surprise that the use of membrane biogas digester tank covers is the most commonly seen.
Both the floating gas holder types with an extended skirt, and double skinned membrane covers, allow storage of gas during periods when gas production exceeds demand, and for its drawdown, as needed when biogas production for a period of time, does not meet the demand.
We hope that this provides an easily understood summary of the available anaerobic digestion plant covers. Now take a look at what the leading digester cover manufacturers have been saying about digester covers, recently:
Digester Covers Overview:
Well Known Digester Cover Manufacturers/ Suppliers/ Installers
WesTech provides innovative designs for custom digester covers using the latest structural standards while integrating ease of erection and … via Digester Covers – WesTech Engineering
Buoyant » Used where sludge level is variable and/ no digester gas is required to be stored under the digester cover. Learn more about these covers » via Digester Covers | Olympus Technologies, Inc.
Ovivo provides a wide variety of steel digester covers. Whether its fixed or buoyant, each digester cover is constructed as a dome-shaped segment of a sphere … via Ovivo® Steel Digester Cover
WALKER PROCESS EQUIPMENT Division of McNish Corporation Anaerobic Digester Covers 1.0 … via Anaerobic Digester Covers – Walker Process
Diigester covers are used over the anaerobic digester concrete tank to seal the digested material and contain the gaseous byproducts. anaerobic digesters convert the … via DIGESTER COVERS | SPS Engineering
Recent Developments in Digester Covers
Equipment Overview: WesTech provides innovative designs for custom digester covers using the latest structural standards while integrating ease of erection and … via Digester Covers > Process Equipment > Spanish
Digester Covers: Floating. Back to Product Search. Floating covers float directly on the sludge, allowing variable sludge levels. However, practically no gas storage … via Digestor Covers: Floating – Walker Process
Dual membrane digester gas holder that maximizes storage and energy benefits. via Dystor® Double Membrane Digester Gas Holder Systems
An anaerobic digester for use in wastewater treatment systems having a fixed cover including a central tower for controlling gas and foam. A constant operating level … via Patent US7160457 – Digester cover – Google Patents
More Designers/ Installers
MPC designs, fabricates, and installs, floating covers for farms, food processing plants, paper mills, and other facilities producing organic waste worldwide. via Anaerobic Digester Covers for Organic Waste | MPC Containment
Shand & Jurs Biogas provides gas safety equipment on top of digester covers including pressure/vacuum vents, flame arresters, 3-way safety selector valves along with … via L&J Technologies – Typical Digester Cover Equipment System
REHABILITATION AND OPTIMIZATION OF ANAEROBIC DIGESTION MIXING SYSTEMS by … distance for floating covers … Digester Covers Gas Holder Covers via REHABILITATION AND OPTIMIZATION OF ANAEROBIC DIGESTION …
More About Double Membrane Cover Pressurisation
In double-membrane cover designs, the outer cover is usually pressurised with the low-oxygen content exhaust from an engine-generator or boiler, etc. The low oxygen level gas between the outer membrane and the inner reduces the explosion risk.
It also provides a durable and intrinsically safe, warm gas shell over the flexible, inner biogas collection cover and digester cell itself.
The outer cover is maintained at a preset pressure, to hold the outer membrane taught. Taught enough to withstand tornado strength winds.
Snow, sleet, and ice is no problem to these roofs because all are rapidly melted off of the outer cover due to its warmth.
The inner digester cover is, as a result, never directly exposed to wind or sunlight. That extends its useful life considerably.
The flexible inner cover is allowed to rise and fall based on the rates of biogas production and biogas fuel demand.
These covers, when fitted over digesters, tend to provide in the region of 48 hours of biogas storage.