Covering Digestate and Biogas Lagoons: Reducing Crust, Emissions, and Rainwater Dilution
Anaerobic digestion turns organic waste into renewable energy and a nutrient-rich byproduct, digestate, that closes the loop back to the land as fertilizer. But the lagoons and stores that hold digestate and slurry between production and field application are a quiet source of operational cost and environmental exposure. Surface crust, ammonia and methane emissions, and rainwater dilution all happen at the open surface. A floating cover changes that.
This article explains the specific problems digestate and biogas operators face at the lagoon surface, and how a modular floating cover addresses each.
The crust problem
Digestate and slurry tend to form a surfae crust as fibrous solids rise and dry in contact with air and sun. A heavy crust is more than a nuisance: it complicates mixing and pumping, creates uneven conditions in the process, can harbor its own odor and emission dynamics, and often requires mechanical breaking before the digestate can be homogenized and applied to fields. Managing crust is labor and equipment time that adds nothing to the value of the product and in a tight agronomic window, time spent breaking crust is time not spent spreading.
A floating cover changes the surface conditions that drive crust formation. By shading the surface and reducing the drying and oxidation at the air interface, a modular cover helps keep the stored material more homogeneous and easier to handle when it is time to mix and pump. Less crust means less mechanical intervention, faster turnaround at emptying, and amore predictable product.
Emissions: ammonia and methane
Open digestate and slurry stores emit, and those emissions matter on two fronts. Ammonia volatilizes from the surface, representing both a loss of fertilizer value, nitrogen that should reach the crop instead escapes to the air and a regional air-quality concern that regulators across many jurisdictions increasingly scrutinize and limit. Methane, a potent greenhouse gas with far more warming power than CO2 over the short term, also escapes from open stores, directly undercutting the climate benefit that anaerobic digestion is meant to deliver.
Covering the surface reduces the interface through which these gases escape. For ammonia, that means more nitrogen retained in the digestate, preserving its value as a fertilizer and reducing the need to supplement with purchased nutrients, a direct financial return as well as an environmental one. For methane and odor, surface coverage reduces fugitive emissions and the odor footprint that makes digestate and biogas operations unpopular with neighbors. In a sector whose entire premise is environmental benefit, reducing fugitive emissions from storage is not a side issue, it is central to the credibility of the value proposition and, increasingly, to maintaining the operation's permits and subsidies.
Rainwater dilution
Every liter of rain that falls into an open digestate lagoon is a liter you now have to store, transport, and spread, diluting the nutrient concentration and inflating the volume that must be handled. In wet climates, rainwater ingress can substantially increase the volume of material to be managed without adding any fertilizer value, raising transport and application costs, consuming spreading capacity, and potentially forcing additional storage capacity to be built or rented during wet periods.
A floating cover sheds or limits rain water ingress into the stored material, keeping the digestate more concentrated andthe managed volume closer to what the process actually produces. Over a wet season, the reduction in diluted volume to transport and spread can be significant, both in direct haulage cost and in the storage headroom it preserves for the material that actually has agronomic value.
Why modular HDPE for digestate process
Digestate is a demanding environment: biologically active, variable in solids content, and subject to crust and scum. Modular HDPE floating covers suit it for several reasons:
• Chemical and biological resistance: HDPE tolerates the aggressive, variable chemistry of digestate and slurry over a long service life.
• Level tracking: as the store fills and empties through the application cycle, the modular field floats with the surface without re-anchoring.
• Serviceability: modules can be removed for mixing or pumping access and replaced afterward, fitting the operating rhythm.
• Geometry tolerance: modular fields conform to existing lagoon shapes without custom fabrication.
• No structural support required: unlike tensioned membranes or rigidgas-collection covers, a floating modular field needs no perimeter anchoring structure, which lowers installation cost and complexity.
Fitting covers into the operating cycle
Digestate stores are not static, they are filled, mixed, pumped, and emptied on an agronomic calendar tied to crop and weather windows. A practical cover design accounts for that rhythm, allowing the access needed for mixing and pumping while maintaining coverage the rest ofthe time. The goal is to reduce crust, emissions, and dilution without making the store harder to operate or slowing the critical spreading windows. A supplier familiar with agricultural and biogas operations will design around your cycle rather than against it.
Building the case
For digestate operators, the business case usually combines several lines: retained fertilizer value from reduced ammonia loss, lower handling costs from reduced crust, reduced transport and spreading volume from less rainwater dilution, and a smaller odor and emissions footprint that protects the operation's standing with regulators and neighbors and supports its environmental credentials. Individually, each is meaningful; together they make a strong case for covering open stores, particularly as emissions regulation tightens around the sector.
Putting numbers to the decision
The digestate case is built by adding up several distinct savings on the same facility. Retained ammonia is valued as the equivalent nitrogen fertilizer no longer lost to the air. Reduced crust is valued as the labor and equipment hours saved at mixing and emptying. Reduced rainwater dilution is valued as the haulage and spreading cost avoided on waterthat carries no nutrient value, plus any storage capacity that no longer has tobe built or rented during wet periods. Reduced methane and odor carry a compliance and reputational value that is harder to quantify but increasingly tied to permits and subsidies.
Take the next step
If you manage digestate or slurry lagoons and want to reduce surface crust, retain fertilizer value, cut fugitive emissions and keep rainwater out of your stored material, learn more about how modular floating covers are applied to digestate and biogas storage.
→ Learn more about floating covers for digestate: https://www.covex-cover.com/digestate