Benefits of covering slurry stores

Find information on potential legislation for covering slurry stores. Compare different cover types and the practical benefits they can provide.

Covering slurry stores can keep the rain out and odours and gasses like ammonia in. Ammonia and odorous gases are produced by microbial activity. These gases rise to the surface and are released at varying rates depending on the air speed over the surface.

Ammonia emissions from manure storage account for 9% of UK agricultural ammonia emissions. By covering your slurry store, you can significantly reduce emissions. Covers on conventional slurry stores are not airtight like those on anaerobic digesters. Therefore, gas can escape, but at a much-reduced rate compared to an open store.

Covering your store is a legal requirement if you operate a permitted pig farm. The Government’s Clean Air Strategy 2019 document states that, in England, all slurry and digestate stores must be covered by 2027. This was a proposal within the strategy which would be subject to consultation. The consultation did not materialise, but we expect to see a consultation early in 2027 about reformed environmental regulations.

If you operate a permitted pig farm, you must cover your slurry store and ensure all new slurry storage systems are built with a cover. Alternatively, you can use another approved technique, such as slurry acidification or slurry cooling, to achieve the same level of environmental protection. However, these options will cost significantly more.

Existing stores on permitted farms were required to have the correct measures for a cover in place by 21 February 2021 to reduce ammonia emissions.

The Environment Agency previously allowed an exemption to the requirement of covering your slurry store for permitted operators who could demonstrate that their livestock slurry has a DM content of less than 1%. This position was withdrawn in August 2020, meaning all slurry stores, including those with less than 1% DM, must be covered. This does not apply to wastewater or solid manure.

The Government also provides further guidance on slurry stores on permitted pig farms with less than 1% dry matter.

Slurry store covers can be impermeable or permeable, fixed (rigid) or floating. Impermeable covers keep rainwater out, significantly reducing the volume of slurry and increasing the effective storage capacity provided by the store. In moderate to high rainfall areas, these types of cover can be cost-effective.

• Applied in a layer 100–150 mm deep
• Moderate capital cost
• Easy to install on existing stores and lagoons regardless of shape (no problems reported with pumps, etc)
• Approximately 10% of the cover needs to be replaced annually
• These covers do not prevent rainfall from diluting the slurry

• Free-floating plastic plates, generally a hexagonal (six-sided) shape
• Moderate cost, can be recovered and reused
• Easy to install on existing stores and lagoons regardless of shape and is effective
• Up to 65% reduction in gas emissions can be achieved
• These covers do not prevent rainfall from diluting the slurry

• Large plastic sheet with integral floats and gas vents
• The edges of the cover are buried into the lagoon banks. Moderate to high cost
• Rainwater can be pumped off the top
• Stirring is possible if design allows
• Lagoon needs to be empty initially, and embankments must be suitable for fixing
• Access for de-sludging is difficult

• A plastic sheet is stretched over and tensioned around a plastic hoop, which floats on the surface
• Low to moderate cost
• It requires no structural alteration to the store
• Covers can be fitted with an agitation hatch
• Rainwater can be pumped off the top
• Suitable for retrofitting
• Access for de-sludging is difficult

• Cover coated in reinforced PVC polyester fabric
• These covers are usually attached to the sides of the tank with a central support pole and gas vents
• High cost
• Rainwater sheds from the surface naturally
• May not be suitable for retrofitting
• Requires store to be structurally suitable and may involve additional reinforcement

• Cover coated in reinforced PVC polyester fabric sat within an earth structure
• Restrained at sides, fitted with gas vents
• The cover forms part of the structural integrity of the store
• Moderate cost
• Smaller footprint than conventional lagoon cover
• Rainwater is kept separate from slurry
• Simpler earthworks than for lined and covered lagoons
• Agitation can be facilitated
• Site needs to be carefully selected
• Secure safety fencing is required