Choosing a slurry store is a big decision and mistakes can be very costly.
On top of that, failure to get appropriate planning permission and meet environmental regulations could lead to fines and deductions to single farm payments.
AHDB Dairy technical officer David Ball explains the key steps farmers need to consider
Step 1: Calculating slurry storage capacity
The first issue when planning to install or replace a slurry store is the calculation of storage capacity. AHDB Dairy is in the process of updating its Slurry Wizard computer program and work sheets, which can be used to calculate requirements.
See also: Slurry pollution incidents
They will be revised once the new Silage Slurry and Agricultural Fuel Oil (SSAFO) regulations, which are under review, have been announced.
It may also be a good idea to factor in additional storage, if dairy numbers are expected to increase in the future.
Figures to have on hand include:
- Total excreta levels (with a mature, high-yielding dairy cow producing an average of 64 litres a day)
- Estimates of the contribution made by parlour washings and yard and roof water run-off
Step 2: Ways to reduce slurry storage requirements
Minimising the need for storage can give considerable savings. It is important to distinguish between slurry and dirty water, as the regulations treat them differently in relation to spreading.
Parlour washings using high-pressure/high-volume machines, for example, can add as much as 200-300 litres a cow each day.
The use of recycled rainwater collected from rooftops can work out cheaper, compared with metered mains water.
Producers can often be surprised at how much run-off water enters the system and the calculation can prompt changes.
Other options for reducing storage volume include roofing open feed or loafing yards. This water can then be diverted from the slurry storage system, as it is not considered “dirty”.
Using a separator and exporting slurry to a neighbouring farm are other ways to minimise storage needs.
Separating slurry reduces storage requirements and will make the residual liquid less liable to crusting, as well as limiting the volume of sediment.
It is also helpful when used in conjunction with an umbilical spreading system, as it reduces the risk of pipe blockages.
Two main options for separation are available:
The rotating screen – smears slurry over a mesh screen, so that the liquid passes through, leaving behind the solid material, which is moved away from the separator into alternative storage.
The screw press – squeezes the slurry using a screw-type plunger to divide the liquids and solids.
Separators usually reduce the volume of slurry by 15-20%. Both types of system are effective, but it is worth asking manufacturers about machine life and the cost and frequency of maintenance.
The separation process will affect the nutrient content of the material, with more available nitrogen and potash in the liquid portion and more slow-release nitrogen in the solids.
Step 3: Regulations and planning permission
Regulations are another major consideration when planning a slurry store. Mr Ball suggests waiting until details of the Environment Agency’s SSAFO rules have been released.
In the meantime, legislation on water pollution prevention has been linked to cross-compliance and any breach of the regulations may trigger a single farm payment penalty amounting to 3-5% of the total figure. Nitrate-vulnerable zone (NVZ)restrictions must also be followed.
The Environment Agency (EA) must be informed at least 14 days before starting the construction of a slurry store. This applies to all farms, not just holdings that fall into NVZs.
Prior notice must also be given for making structural changes to existing facilities and the EA must give consent before store capacity can be increased by more than 10%.
Planning permission must also be granted by the local authority. This can take time and full permission may be required, if the construction is to be sited within 400m of a dwelling.
Other slurry store projects that may necessitate planning are facilities with a storage area of more than 465sq m, or those within 25m of a metalled road.
Although the slurry store itself must be at least 10m clear of inland or coastal waters, drains and sealed pipes can be within 10m of these features, if the EA agrees an exception. Ask for an exception when you tell the EA about your project.
Slurry stores must be designed to last for at least 20 years, assuming they are correctly maintained. They must also meet performance standards. All farms within NVZs must have a minimum of 22 weeks of slurry storage.
Step 4: Deciding on the type of store
Deciding on the type of store suitable for an individual farm depends on a host of factors. The two broad choices are above-ground and below-ground. Below-ground stores are popular, because they tend to be cheaper.
Cost: £5/sq m or £75 a cow (slurry plus water for 22 weeks at 15 sq m a cow)
Positive: Cheapest option and they have an indefinite lifespan, when managed well
Negative: Not suitable for all farms and can become expensive if clay has to be brought on to the farm
The cheapest option is an unlined, clay-based lagoon. However, this will require soil testing, with a sample sent to a laboratory for a permeability test, to prove that the foundation is suitable.
A failed test could involve bringing clay on to the farm, but transport costs may be prohibitive. The ideal test result would show a clay content of 20-30%; higher levels may not be ideal, because the soil will be prone to cracking.
The layer of clay will usually have to be 0.5-0.75m thick to gain approval. Some producers prefer to lay a concrete base on top of the clay, to make the surface more durable, particularly if they plan to clean it out using machines at some point.
Cost: £17/sq m or £355 a cow (slurry plus water for 22 weeks at 15sq m a cow)
Positive: Reasonable in price
Negative: Can get damaged when stirring and emptying
The next option, in terms of price, is a lagoon with a high-density polyethylene lining. They are usually favoured where the soil has failed a test. The surface must be carefully prepared to ensure it is smooth before the liner is installed. This will enhance durability, as the material is normally only 2-3mm thick.
Lagoon liners can be subject to damage, so care must be taken when stirring and emptying the material. In certain circumstances the EA may insist on a double liner.
Cost: £29/sq m or £435 a cow (slurry plus water for 22 weeks at 15sq m a cow)
Positive: Can be relocated
Negative: Lifespan limited to about 20 years
Slurry bags are a relatively new concept in the UK, but they are used extensively in Holland. Their estimated lifespan is about 20 years and they are designed so they can be relocated relatively easily, if necessary.
Careful thought is needed when considering how to remove the slurry. Some versions come with electric or hydraulic stirrer and a couple of fill/empty pipes, with double sluice valves.
Cost: £34/sq m or £510 a cow (slurry plus water for 22 weeks at 15 sq m a cow)
Positive: Can be increased in size and takes up less ground space
Further up the price scale, a steel tower continues to be a popular choice with farmers. The ability to add rings, if storage requirements increase, offers flexibility, and vertical storage may take up less space on the ground. Slurry contained in steel towers will normally require frequent stirring, but this storage method is particularly suitable for use in conjunction with an umbilical spreading system. While there are a number of products on the market for covering steel towers, these will add to the overall expense.
Cost: £39/sq m or £585 a cow (slurry plus water for 22 weeks at 15sq m a cow)
Negative: Most expensive option
The concrete store is normally the most expensive option, says Mr Ball. Concrete stores are more expensive, but their shape is highly versatile and they can be sited under cover, if required, he explains. But as with all slurry stores, they must meet a set of stringent standards.
Note: Installation and running costs are based on 2010 figures provided by AHDB Dairy
Pipe or tanker?
It costs about four times as much to remove slurry using a standard tanker, compared with pipework, although the latter requires a sizeable investment.