Division 4.6.1 Techniques for reducing ammonia emissions from slurry stores and earth-banked storage

United Kingdom

Technique		Description
Reduce the ratio between the emitting surface area and the volume of the slurry store.		For rectangular slurry stores, the proportion of height and surface area is equivalent to 1:30-50. For circular stores, favourable container dimensions are obtained with a height — diameter ratio of 1:3 to 1:4. The side walls of the slurry store may be increased in height.
Reduce wind velocity and air exchange on the slurry surface by operating at a lower level of fill.		Increasing the freeboard (the length between the slurry surface and the upper rim of the slurry store) of the uncovered store provides a windshield effect.
Minimise stirring of slurry.		Keep the stirring of slurry to a minimum. This practice involves: filling the store below surface level; discharging as close as possible to the base of the store; avoiding unnecessary homogenisation and circulation of slurry (before emptying the slurry store).
Rigid cover.		A roof or a lid which can be made of concrete, fibreglass panels or polyester sheets with a flat deck or conical shape, applied to concrete or steel tanks and silos. It is well-sealed and 'tight' to minimise air exchange and to prevent rain and snow from entering.
Flexible covers.		Tent Cover: a cover with a central supporting pole and spokes radiating from the tip. A fabric membrane is spread over the spokes and tied to a rim brace. Non-covered openings are kept to a minimum. Dome-shaped cover: a cover with a curved structural frame installed over round stores with the use of steel components and bolted joints. Flat cover: a cover consisting of a flexible and self-supporting composite material held by plugs on a metal structure.
Floating covers.
	Natural crust.	A crust layer can be formed on the surface of slurry that has a sufficient dry matter (DM) content (at least 2 %) depending on the nature of the slurry solids. In order to be effective, the crust must be thick, not be disturbed and cover the whole slurry surface. The store is filled from below the surface once the cover is formed to avoid breaking it up.
	Straw.	Chopped straw is added to the slurry and a straw-induced crust is formed. This generally works well for DM higher than 4-5 %. A layer thickness of at least 10 cm is recommended. Air blowing can be reduced by adding straw at the time of slurry addition. Straw layers may need to be partially or completely renewed during the year. The store is filled from below the surface once the cover is formed to avoid breaking it up.
	Plastic pellets.	Polystyrene balls of 20 cm in diameter and 100 g in weight are used to cover the slurry surface. A regular replacement of deteriorated elements and a refill for uncovered spots are necessary.
	Light bulk materials.	Materials such as LECA (Light expanded clay aggregates), LECA based products, perlite or zeolite are added to the slurry surface to form a floating layer. A floating layer of 10-12 cm is recommended. A thinner layer can be effective for smaller LECA particles.
	Floating flexible covers.	Plastic floating covers (e.g. blankets, canvas, films) rest over the slurry surface. Floats and tubes are installed to keep the cover in place, while maintaining a void beneath the cover. This technique can be combined with stabilising elements and structures to allow vertical movements. Venting is needed as well as removal of rainwater that gathers on top.
	Geometrical plastic tiles.	Floating hexagonal plastic bodies are automatically distributed on the slurry surface. About 95 % of the surface can be covered.
	Air-inflated cover.	A cover made of PVC fabric supported by an inflatable pocket that floats over the slurry. The fabric is fixed by guy ropes to a peripheral metal structure.
	Flexible plastic sheets.	Impermeable UV-stabilised plastic sheets (e.g. HDPE) are secured at the bank tops and supported on floats. This prevents the cover from turning during manure mixing and being lifted off by wind. The covers can also be fitted with collection piping for removal of gases, other maintenance openings (e.g. for the use of homogenisation equipment) and a system for rainwater collection and removal.