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Digester Covers and Secrets of the Flexible Inflatable Membrane Cover

Digester Covers are an important part of any biogas plant, performing the triple function of sealing the digester from air, keeping it warm, and providing storage for a variable rate of pressurized gas, to balance the rate of biogas production with its use.

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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 in both directions as the water level in the digester tank varies. These are commonly seen in designs used in the hot-climate developing nations for domestic or community sludge (and other wastes) digestion
  2. Floating covers rise and fall according to volume of gas and sludge and their weight provides the gas pressure. They have an extended perimeter skirt which provides the necessary gas-to-air seal, and to allow storage of gas. This type is rather like the manner in which a gasometer operates, with the air-seal being made by the liquid within the tank perimeter.
  3. Flexible inflatable membrane covers are normally double skinned with the inner skin (membrane) held within a form of 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, and by varying the air pressure between the two skins the amount of biogas stored can be varied. The biogas tank’s dome remains inflated on the outside while the biogas volume is held at the chosen pressure by the air pressure between the inner and outer membranes.
Some Wiki articles refer to 4 types of covers, with the separate gas holder as a 4th type. This seems incorrect to me.

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 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 are the most commonly seen.

Both the floating gas holder types with an extended skirt, and 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.

Digester covers thumbnail imageOvivo 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

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 … via Digester Covers – Evoqua

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

Design, supply and field installation of waste water pollution control plant steel digester covers. via Digester Cover | Storage Tanks |Fabrication | Platecon …

Digester Cover, Wholesale Various High Quality Digester Cover Products from Global Digester Cover Suppliers and Digester Cover Factory,Importer,Exporter at Alibaba.com. via Digester Cover, Digester Cover Suppliers and Manufacturers …

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

The arch-rib digester cover from Evoqua is a radial beam steel digester cover for applications where sludge level variation is minimial. This… via Arch-Rib Digester Cover – Water Online

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.

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