Benefits of covering slurry stores

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

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Why you should cover your slurry store

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.

Environmental Permitting 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 ‘Best Available Techniques (BAT) Reference Document for the Intensive Rearing of Poultry or Pigs’ sets out the requirements for covering slurry stores and lagoons under BAT 16 and BAT 17.

Slurry stores with less than 1% Dry Matter (DM) content

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 This position was withdrawn in August 2020, meaning all slurry stores, including those with less than 1% DM, must be covered.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.

However, operators who had previously used this exemption do not have to comply with the BAT condition to cover their slurry store until 21 August 2022.

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

Find out more about Environmental Permitting Regulations

Different types of slurry store cover

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.

Permeable covers





Lightweight Expanded Clay Aggregate (LECA) or foam glass

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

Floating plastic plates

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 95% reduction in gas emissions can be achieved

These covers do not prevent rainfall from diluting the slurry

Impermeable covers





Lagoons – fixed floating plastic membrane

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

Tanks – free-floating plastic cover

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

Tanks – fixed cover

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

Integral store and cover (bag)

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

Useful links

Environmental Permitting Regulations

More information on slurry cooling