Digester Covers

Gas storage solutions to effectively use produced biogas

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 utilization of biogas. Digester covers can provide storage space for generated biogas, contain odor and insulate the digester.

Evoqua offers leading solutions for three main types of digester covers: Membrane, Fixed and Floating.  We can help you select the right digester cover for your wastewater treatment application, whether you are looking to fully utilize digester gas or just contain odors.

 Membrane Covers

dystor coverThe Dystor® Double Membrane Digester Gas Holder System provides the highest return on investment for gas storage and odor prevention. The outer membrane remains inflated in a fixed position, while the inner membrane moves freely as it stores or releases biogas. 


  • Holds up to 6 times more gas than conventional steel covers
  • Maximizes energy benefits of the digestion process
  • Reduces operator and maintenance costs


 Fixed Covers

Evoqua Arch-Rib Covers are the most economical cover type. This radial beam shell type steel digester cover is best suited for applications where sludge level variation is minimal.


  • Custom designed based on customer specified loading
  • Steel weight is optimized using finite element analysis (FEA)
  • Ease of handling for on-site field-erection
  • Approximately 50% less field weld length than truss style cover

Arch-Rib Cover Datasheet

 Floating Covers

The floating arch-rib cover from Evoqua is the most economical solution for handling variable sludge levels.  This versatile cover can adjust to changing conditions when adding and removing solids.  Evoqua also continues to provide a robust and proven truss style steel digester cover. 


  • Robust and durable
  • Adjust to variable sludge levels
  • Prevent over or under-pressurization
  • Gas storage option for flexible utilization


Floating Arch-Rib Cover Datasheet

 Steel Cover Concrete Ballast Ring Form

Floating digester covers and gas-holders frequently utilize a concrete ballast pick up ring to achieve a desired gas operating pressure and for actuation of pressure relief valves. This ballast ring can be a continuous poured ring around the circumference of the tank or separate blocks placed on supporting structure. A poured ring requires formwork to make the ring, adding time and cost to the project. Evoqua now offers a steel ballast ring form that is integral to the digester cover side skirt. The form is erected along with the side skirt and then filled with concrete, providing the traditional continuous ballast ring. The steel form can then be left in place, saving time on dismantling and disposing of temporary concrete formwork usually required.

Benefits of continuous poured ballast ring vs separate ballast blocks:

  • Provides additional rigidity to the cover structure
  • Prevent chance of loose block falling into tank, damaging internal structure
  • Contractor safety, blind lifting separate blocks into digester tank is dangerous
  • Reduced cost, no need to pre-form blocks to specific dimensions and weights
  • Reduced cost, no additional crane time required for installing continuous ring
​Click on the tiles below to explore other leading Anaerobic Digestion technologies from Evoqua.