Using impermeable slurry covers to reduce emissions

Nine per cent of the UK’s agricultural emissions of ammonia come from manure storage, a gas that is a key air pollutant. It has a significant effect on the environment and affects human health. It is also responsible for indirect emissions of nitrous oxide (N₂O), a powerful greenhouse gas.

In England, the 2019 Clean Air Strategy proposed that all slurry stores should be covered by 2027. This is also a requirement for all farms under environmental permitting regulations (EPR).

There are different types of impermeable covers. The main two are:

• A plastic tensioned cover, shaped like a tent and supported by a central column with a small vent in the top. Most suitable for above ground tanks
• A floating cover, also made from impermeable plastic, which rests directly on top of the slurry, rising and falling with the slurry level. Most suitable for lagoons

Covering slurry reduces the amount of ammonia it emits and mitigates the risk of indirect N₂O production. Ammonia emissions are a significant route to losing nitrogen (N) from slurry. Therefore, reducing emissions retains N in the slurry to be used by crops following land application.

Ammonia emissions from storage, which can vary according to environmental factors such as temperature and wind speed, can be mitigated by limiting airflow across the slurry surface. Fitting the store with an impermeable cover works as the barrier between the surface and the airflow above restricts diffusion.

A cover does not prevent the production of methane, but if airtight it can be captured. This can then be drawn off using a pump and either flared to convert to less potent CO₂ or processed into biomethane.

Unless designed to capture methane, slurry store covers should not be totally airtight. Emissions are therefore not eliminated but an impermeable cover offers mitigation.

An impermeable cover will keep rainwater out of the store, which means more space for slurry. This is particularly useful in regions that experience high levels of rainfall.

If constructing a new store, keeping out rain could significantly reduce the required capacity and cost of construction.

• Choose the right cover to suit the requirements of your farm
• Consider the size of the farming operation and the type of store in place, e.g. tent type design is only suitable for above-ground tanks that can support the weight
• Impermeable covers can be retrofitted to existing slurry stores but some covers are better suited to new builds than existing stores. Some stores may not be structurally capable of supporting a retrofitted cover. Check with the manufacturer
• Slurries with high dry matter can create challenges when emptying a covered store. Some slurries might require agitation and/or cover removal to remove solids. Installing a slurry separator might help
• Although covers don’t require planning consent, new stores do. Local Authority and Environment Agency requirements need to be considered if constructing a new store.
• Notify environmental regulators before making major changes to existing storage
• Use low-emission spreading methods (e.g. shallow injection or trailing shoe). Otherwise, the retained nitrogen will be lost

UK Agritech Centre’s GreenShed project has built a prototype that captures methane from housed beef cattle.

The captured methane is then converted for energy use, while the system also supports vertical farming and nutrient production, creating a more sustainable and efficient farming model.