Digester Covers and Secrets of the Flexible Inflatable Membrane Cover

Digester Covers, which also act as gasholders 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.

Digester covers are essential to the process of anaerobic digestion and the storage of biomethane. These covers provide a secure, protective layer that must keep the biogas contained and secure. They must protect the biogas from the elements, they must withstand corrosion and minimize fugitive methane emissions.

When selecting a digester cover, it is important to consider the environment in which it will be used. Depending on the climate and the location of the plant, different materials may be more suitable for the job. For example, membrane covers (or gasholders) may be more suitable in cold climates than rigid steel as they provide better insulation.

Finally, it is important to consider the overall cost of the cover. Different materials will have different prices, so it is important to consider the cost-effectiveness of the alternative cover types in order to get the most out of the investment.

3 Types of Anaerobic Digestion Tank Biogas Storage Covers

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:

1. 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.
2. 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.
3. 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 covers used in commercial anaerobic digestion plants in 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 that maintains the pressure balance between the inner and outer membranes.

Watch our video below to see examples of each type of digester cover:

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Maintaining A Suitable Biogas Storage Pressure

Biogas pressure balancing is important to avoid a vacuum being produced as it can be 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.

Important Features of the Most Common Digester Cover Types

• Fixed covers are either dome-shaped or flat and examples have been seen, fabricated from reinforced concrete, steel, or fibreglass 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 the 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. These covers can be reliable and long-lasting.
• Welding to all flexible membrane covers must be done in accordance with strict quality control requirements, and suitably tested for weld strength.
• Flexible membrane covers can be purchased at a lower cost than steel, or concrete, covers. So, it is no surprise that membrane biogas digester tank covers are the most common.

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:

There are several types of biogas storage membrane covers available on the market. These covers are made of HDPE, PVC, polyethylene, or other strong, durable materials that can effectively prevent the entry of pests, moisture, and other contaminants into the biogas storage.

Anaerobic digestion tanks are used for storing digester gas and are covered with a flexible membrane to provide insulation.

The flexible cover needed for an anaerobic digestion tank must provide insulation from outside temperatures and protection from wind, rain, snow, dust, etc.

The biogas storage covers come in a variety of shapes and sizes depending on the size of the anaerobic digestion tanks. This kind of cover is custom-made to meet the requirements of each AD facility. They are made in different shapes and sizes so that they fit well with the shape and size of anaerobic digestion tanks.

Biogas tank flexible covers can be used as a way to reduce costs by lowering heating bills.

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 it is 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

SPS Engineering Digester 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

Evoqua Digester covers are an important part of the anaerobic digestion process. Gas production is not always equal to consumption and gas storage is needed for effective running 24/7 … via Digester Covers – Evoqua

Recent Developments in Digester Covers

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 is a company that 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

Design, supply, and field installation of wastewater pollution control plant steel digester covers. via AD Covers. via AD Cover, Cover Suppliers, and Manufacturers …

Shand & Jurs Biogas provides gas safety equipment on top of digester covers including pressure/vacuum vents, flame arresters, and 3-way safety selector valves along with … via L&J Technologies – Typical Digester Cover Equipment System

The arch-rib digester cover from Evoqua is a radial beam steel digester cover for applications where sludge level variation is minimal. This… via Water Online

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, 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 are no problem to these roofs because they are rapidly melted off of the outer cover due to their 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.

[First published 26 August 2017. Updated January 2023.]