Floating Digester Covers

Floating Digester Covers

Plastic Fusion Fabricators designs and installs floating geomembrane covers for anaerobic digesters — flexible membrane systems that sit directly on the liquid surface, capture biogas, control odors, and rise and fall with changing sludge levels. Our HDPE and reinforced polypropylene floating digester covers are custom-fabricated for your tank or basin geometry and engineered for the corrosive gas environment inside an active digester.

A floating digester cover is fundamentally different from a fixed rigid cover. It rests on the liquid surface rather than spanning the tank opening, which means it accommodates variable liquid and gas volumes without mechanical lifting systems or gas holders. As gas production increases, the cover rises. As gas is drawn off to a flare, CHP unit, or RNG upgrading system, the cover settles. This passive gas storage function reduces pressure spikes, simplifies gas management, and eliminates the maintenance burden of steel truss covers, guide rails, and corbel systems.

Plastic Fusion has been installing floating cover systems for over 40 years and is recognized in the industry as a specialist in geomembrane digester cover and biogas collection cover systems. We handle the full scope — cover fabrication, deployment, gas-tight welding, pipe penetrations, gas collection piping integration, and QA/QC testing.

Applications

• Municipal wastewater treatment plant anaerobic digesters
• Agricultural manure digesters and covered lagoon digester systems
• Industrial wastewater anaerobic treatment basins
• Food waste and co-digestion facility digesters
• Dairy and livestock waste digestion systems
• Digestate storage covers for post-digestion holding tanks
• Renewable natural gas (RNG) production digesters
• Existing steel cover replacements — converting from rigid to flexible membrane

How Floating Digester Covers Work

The cover is fabricated from gas-resistant geomembrane material — typically HDPE or reinforced polypropylene — cut and welded to match the digester tank or basin dimensions. It’s deployed directly on the liquid surface with a perimeter seal system that creates a gas-tight enclosure between the cover and the liquid. Lateral float systems and ballast weighting maintain cover stability and prevent wind uplift.

As anaerobic digestion produces biogas (methane, CO₂, H₂S), the gas accumulates beneath the cover, lifting it. Gas collection piping routes the biogas from beneath the cover through headers to a central manifold, then to downstream utilization equipment. Condensate traps remove moisture from the gas stream, and pressure/vacuum relief devices maintain safe operating conditions. The entire system — cover, gas pickup, piping, condensate management — is designed and installed as an integrated unit.

Why Floating Covers vs. Fixed Steel Covers

Floating geomembrane digester covers offer several advantages over traditional steel truss, radial beam, or gas holder covers:

• Lower installed cost — geomembrane covers typically cost 30–50% less than steel alternatives for comparable tank sizes
• No guide rail or corbel maintenance — floating covers don’t require the mechanical systems that steel covers need for vertical travel
• Corrosion immunity — HDPE and reinforced polypropylene don’t corrode in the H₂S-rich digester gas environment that attacks steel
• Passive gas storage — the cover rises and falls with gas volume, providing buffer storage without a separate gas holder
• Simplified installation — no crane lifts, structural steel erection, or extended outage required
• Longer service life in corrosive environments — geomembrane covers in digester service routinely deliver 20+ years without the recoating and structural repairs that steel covers require

Why Engineers Specify Plastic Fusion

• 40+ years installing floating cover systems — digesters are a core application
• Certified thermoplastic welders trained in HDPE, RPP, and polypropylene
• Material selection matched to digester gas chemistry — H₂S concentration, temperature, and UV exposure
• Gas-tight seam welding with full destructive and non-destructive QA/QC testing
• Complete system integration: cover, gas piping, condensate, pressure control, and flare/energy tie-ins
• In-house HDPE fabrication for gas collection piping, manifolds, and penetration details
• Nationwide mobilization with experienced digester project crews
• Direct coordination with process engineers, facility operators, and equipment suppliers

Floating Digester Cover FAQs

What is a floating digester cover?

A floating digester cover is a flexible geomembrane membrane that sits directly on the liquid surface inside an anaerobic digester tank or basin. It captures biogas produced during digestion, controls odors, and rises and falls with changing liquid levels and gas volumes. Unlike fixed steel covers that span the tank opening on structural supports, floating covers rest on the liquid itself — eliminating guide rails, corbels, and mechanical lifting systems while providing passive gas storage as the cover inflates and deflates with gas production.

What is the difference between a floating digester cover and an anaerobic digester cover?

“Anaerobic digester cover” is the broad category — it includes fixed steel covers, gas holder covers, and floating geomembrane covers. A floating digester cover is one specific type: a flexible membrane system that floats on the liquid surface. Plastic Fusion specializes in the geomembrane floating cover type, which offers lower cost, corrosion immunity, and simpler maintenance compared to steel alternatives.

How long do floating digester covers last?

A properly installed HDPE floating digester cover typically delivers 20–30 years of service life in digester environments. The primary factors affecting longevity are H₂S concentration in the biogas (which can degrade certain materials), UV exposure on any cover area above the liquid line, and the quality of the original seam welding. Plastic Fusion’s QA/QC protocols maximize installed service life on every cover.

Can a floating cover replace an existing steel digester cover?

Yes. Replacing a deteriorated or corroded steel digester cover with a floating geomembrane cover is a common retrofit project. The steel cover is removed, and a custom-fabricated geomembrane cover is deployed on the liquid surface with a new perimeter seal and gas collection system. The conversion typically delivers significant cost savings compared to replacing steel with steel, plus eliminates ongoing corrosion maintenance. Plastic Fusion handles the full conversion scope.

What is a digestate cover?

A digestate cover is a floating geomembrane cover installed over a digestate storage tank or basin — where digested material is held after leaving the primary digester. Digestate continues to produce residual biogas and can generate odors. A floating cover captures this residual gas, controls emissions, and can integrate with the facility’s overall biogas collection system.

Request a Quote

If you’re specifying a floating digester cover for new construction, replacing a deteriorated steel cover, or adding gas capture to an existing digester, Plastic Fusion delivers custom-fabricated geomembrane cover systems built for decades of service. Contact us for project-specific pricing, material recommendations, and installation timelines.