Aging geomembrane digester cover on a covered lagoon

Cover Lifecycle

Digester Cover Replacement

Assessment, removal, and full replacement of worn-out membrane covers on lagoons and tanks, without draining the basin.

500+

CLD Systems in the Field

50+

Repair & Replace Proposals in 2025

25 yrs

Cover Design Life

30+

Years of Failure Data

When a digester cover needs replacing

A geomembrane digester cover is engineered for 15 to 25 years. It reaches replacement when the membrane itself has degraded, which shows up as widespread chalking or stress cracking, seam failures in several unrelated places, coupon tests below 50 percent of the original tensile or elongation spec, or gas capture stuck under 70 percent of design. Before that point, almost everything that fails on a covered lagoon is a repair: vents, sump pumps, gas collection piping, anchor trenches.

Replacing the cover does not mean draining the lagoon. The work happens from the surface, which is what separates a cover replacement from a liner replacement. EFI has more than 500 covered lagoon systems in the field, processed over 50 repair and replacement proposals in 2025, and tests the membrane before recommending either path.

If your cover is still sound and you are here about a new one, start at anaerobic digester covers instead. For the engineering detail behind either, see the anaerobic cover specifications.

What Fails, and When

The Membrane Is Rarely First

Three decades of maintaining covered lagoon systems produces a predictable wear order. Knowing it keeps operators from replacing a cover when a $15,000 component repair would have bought another decade.

Component	Typical Service	What It Means
Vents and pressure relief	Years 5 to 10	Repair. H2S, UV, and thermal cycling take the vent assemblies first.
Sumps and rainwater pumps	Years 5 to 12	Repair. Continuous water, UV, and mechanical cycling wear them out.
Gas collection system	Years 8 to 15	Repair. Piping, condensate drains, and manifold connections develop leaks.
Anchor trench	Years 10 to 15	Repair. Settlement and erosion let the cover edge shift and open leak paths.
Cover membrane	Years 15 to 25	Replace. UV, H2S, and mechanical stress degrade the sheet itself.

Inspection

Four Signs the Cover Is Done

These are the findings that move a system from the repair column to the replacement column. Anything short of them usually points at a targeted fix.

Chalking and Cracking

A whitish, powdery surface is UV degradation. Once cracking spreads across large areas rather than isolated spots, patches stop holding.

Repeated Seam Failures

One failed seam is a repair. Failures in several unrelated locations point at systemic weld or material condition, not local stress.

Coupon Tests Under 50%

Lab testing of samples cut from the cover for tensile strength and elongation. Below half the original specification, the sheet is past service.

Capture Below 70%

Methane capture measured against the system's design capacity. A sustained shortfall means leaks the cover geometry can no longer close.

Inspection cadence, checklists, and the repair techniques behind each finding are in the cover inspection and maintenance guide.

Process

How a Replacement Runs

Condition Assessment

Visual survey, coupon sampling and lab testing, gas capture efficiency check, and a component-by-component rating of what is worth keeping.

Design and Fabrication

Panels engineered to the current lagoon geometry and fabricated at an EFI facility, with the gas collection layout updated to what the system actually needs today.

Purge and Removal

Gas collection safely purged, the old membrane cut, folded, and hauled out for disposal as industrial waste, and the anchor trench opened and inspected.

Install and Commission

New panels field-welded and seam-tested, ballast and rainwater systems rebuilt, gas collection reconnected, and the flare or energy equipment brought back online.

Two Different Jobs

Membrane Cover vs. Structural Cover

Search results for cover replacement are dominated by firms that rebuild fixed steel and concrete lids on tanks. That is a mechanical and structural job: demolition, crane picks, confined space entry, and the tank out of service while it happens.

Replacing a membrane cover is a geosynthetics job. Panels are fabricated in a shop to the current basin geometry, field-welded and seam-tested on site, and held down by ballast and an anchor trench. The basin stays full. EFI is a geosynthetics contractor, so this is the side of the work it does, on lagoons and on tanks.

The one case where the basin does come out of service is a liner replacement underneath the cover, which means draining, sludge removal, new liner, and then the cover system back on top. Below-grade HDPE liners are protected from sunlight and typically last 30 to 40 years, so that is a much rarer project than a cover swap. Details on the liner side are on lagoon liners.

EFI USA membrane digester cover system on a covered lagoon

Economics

Repair or Replace

Repair Is the Right Call When

The failure is localized to specific components, not system-wide
Coupon testing shows the membrane still meets specification
Repair cost is under 20 to 30 percent of full replacement
The cover has 5 or more years of useful life left afterward
Typical component repair runs $10,000 to $25,000

Replacement Is the Right Call When

Cracking or chalking covers large areas of the membrane
Seams are failing in several unrelated locations
Tensile strength or elongation is below 50 percent of spec
Gas capture sits under 70 percent of design capacity
Replacement returns a positive NPV over the new cover life

The full decision framework, including how EFI weighs cover condition, capture efficiency, present value, and regulatory status, is in when to repair vs. replace a covered lagoon system.

FAQ

Frequently Asked Questions

A geomembrane digester cover is engineered for 15 to 25 years. Where an operator runs a consistent preventive maintenance program, covers typically reach 20 to 25 years. Where maintenance is deferred, failures show up as early as 12 to 15 years. The membrane is rarely the first thing to go: vents and sump pumps wear out between years 5 and 12, gas collection piping between years 8 and 15, and anchor trenches between years 10 and 15, all of which are repairs rather than replacements.

Four signals point to replacement rather than repair. Widespread surface chalking or stress cracking across large areas of the membrane, instead of isolated damage. Seam failures appearing in several unrelated locations. Material coupon tests returning tensile strength or elongation below 50 percent of the original specification. And gas capture running under 70 percent of design capacity because of leaks the cover geometry can no longer hold. Any one of these on its own is worth an assessment. Two or more together usually means the cover is finished.

Repair is the right call when the damage is localized rather than system-wide, the membrane still tests within specification, the repair costs less than 20 to 30 percent of a full replacement, and the cover has 5 or more years of useful life left afterward. Most component-level repairs, such as a vent assembly, a clamping ring at a pipe penetration, or a patch on a punctured panel, run $10,000 to $25,000, with the full repair range at $5,000 to $50,000 depending on scope. EFI processed more than 50 repair and replacement proposals in 2025 alone, so the comparison is made against real recent numbers rather than a rule of thumb.

EFI runs a documented condition assessment: visual inspection of the membrane, seams, anchor trench, ballast, and gas collection components; laboratory testing of coupon samples cut from the cover for tensile strength, elongation, and thickness; a gas capture efficiency comparison against the system's design capacity; and a component-by-component condition rating. The economics are then run as a present value comparison of repair cost against replacement cost over the remaining credit or energy revenue. That process is designed to prevent the two expensive mistakes: paying for repairs that do not extend life, and replacing a cover that had years left in it.

No. A cover replacement works from the surface, so the lagoon stays full. That is the practical difference between a cover replacement and a liner replacement. Replacing the liner underneath requires draining or partially draining the lagoon and removing accumulated sludge first, then reinstalling the cover system on top. Below-grade HDPE liners are shielded from UV and typically last 30 to 40 years, which is why cover replacements are far more common than liner replacements.

A replacement is the whole cover system, not only the sheet. EFI purges and disconnects the gas collection system, cuts and removes the old membrane for disposal as industrial waste, inspects and restores the anchor trench, installs new factory-fabricated panels field-welded to the lagoon geometry, and rebuilds the gas collection header, condensate drains, ballast, rainwater management, and vents. Where the operation warrants it, a replacement is combined with other work in the same mobilization, as EFI did at Pilgrims Sanford by installing a sludge line alongside the new cover.

A structural cover, meaning a fixed steel or concrete lid on a tank, is replaced as a mechanical and structural project: demolition, crane work, confined space entry, and the tank out of service for the duration. A membrane cover replacement is a geosynthetics project. Panels are fabricated in a shop, welded in the field, and tensioned with ballast, and the basin does not have to be emptied. EFI is a geosynthetics contractor, so its replacement work is membrane cover systems on lagoons and tanks rather than structural lids.

Yes. EFI assesses and replaces covered lagoon and tank cover systems regardless of who built them originally, including systems where the original contractor is no longer in business. With more than 500 covered lagoon systems of its own in the field and 30 or more years of failure data behind them, EFI is generally looking at a wear pattern it has seen before.

Explore More

Trusted by industry leaders across agriculture, food processing, government, and renewable energy