Liquid gold or troublesome waste? Whether looking to increase herd size or simply maintain productivity, slurry storage is a significant investment and slurry management can be an onerous task.
Defra has set a target of 2027 to cover all slurry storage in England. The proposed Clean Air Strategy aims to reduce harmful emissions, especially those from agriculture.
While covering slurry is a seemingly rudimentary task, there are several factors to consider before making an investment.
John Tydeman, slurry storage specialist at Tramspread, says slurry management is a more holistic task than simply storage and application.
He says: “The type of storage, how often and thoroughly the slurry is agitated and whether any separation devices are used will make a significant difference to the quality and quantity of the slurry.”
Covering slurry can be costly because of the size, shape and location of the tank or lagoon. Creating new storage may require planning permission and greater capital outlay.
He says: “If herd size is a constant, maintenance is a good way to make the most of existing storage. Agitating slurry regularly to prevent solid build-up and covering slurry to keep out rainwater will enable accurate calculations to be made to ensure sufficient storage for the number of cows.”
The volume of a tank should be calculated based on Defra values per animal, the herd size and the land available to distribute the slurry to.
In nitrate vulnerable zones it is also important to account for additional rules which may prevent spreading at certain times.
Mr Tydeman says: “Allowing for growth is one consideration, but if a floating tile cover is being used the volume of the tank will increase with rainwater too.
“Canvas or other rigid covers which block rainwater will prevent the slurry from being diluted and also reserve the full volume of the store for slurry.
“However, the store must have the structural integrity to bear the weight of the cover and it is important to mix the slurry to prevent a build-up of solids.”
For a growing herd there may be the need to look at new storage options to expand the slurry provision available.
Mr Tydeman says: “Covering large lagoons can be costly and keeping slurry agitated to prevent solids building up is tricky with large square shaped lagoons because even the largest mixers will not be able to reach the middle of the lagoon.”
The cost and quality of materials for lagoon construction varies significantly and the concrete must be installed correctly to prevent slurry seeping into the ground.
While concrete has been a popular option, the environmental effects of slurry lagoons and stores which have failed are significant, especially if the store is large.
Detrimental effects to soil and ground water should be considered and above ground tanks offer a much better opportunity to monitor and maintain the integrity of a tank to prevent any leaks.
Mr Tydeman says: “A below ground slurry lagoon will require the retaining walls to support the external ground, as well as the slurry inside the store. Above ground slurry storage offers options to safely maximise storage volume for farms with limited space.”
Bag tanks are self-ventilating so require relatively little management.
Stainless steel tanks may require planning permission and are a higher cost solution. Covering existing steel tanks can also be costly.
However, there are floating covers available which can prevent 95 per cent of emissions escaping and is approximately a quarter of the cost of a tensioned cover. However, Mr Tydeman adds these do not keep rainwater out, so would be unsuitable for areas with heavy rainfall or farms close to their storage limit.
Bag tanks range in size from 200cu.m to 7,000cu.m and can be located on any soil type as only a shallow foundation is needed. Integral hydraulic or electric stirrers enable easy and consistent agitation of slurry. The tanks are self-venting, so relatively little management or farmer involvement is needed.
Bag tanks are manufactured in a UV-resistant grey-coloured material and, when installed, are low to the ground and have minimal impact on the surrounding landscape.
Mr Tydeman says: “Many farmers have installed bag tanks without requiring planning permission. However, those looking to install a bag tank are advised to seek guidance from their local authority as the location of an installation may require approval.”
The type of storage will dictate the type of stirrer used.
slurry should be regularly agitated to prevent the build-up of solids which make it more difficult to pump and reduces the overall capacity of the store.
A variety of pto and electric agitators are available, but the type of storage will be a big determining factor when choosing.
Mr Tydeman says: “Electric agitators often have timers so can be set to come on automatically. This is fine for storage close to or beneath buildings, but less easy for lagoons.
“A good method is to mix the slurry in the reception pit or channels before it is sent to the store.”
Tractor-mounted mixers are popular to reach into lagoons and over high tanks.
He says: “For those with large lagoons, the Reck Typhoon mixer can be mounted on a frame to offer a 16-metre long mixer. The Mammoth Titan can reach over a 5.9m tank. However, these options represent higher investments which may not be needed if a farmer were to use a separator.”
Separation is more common in Europe where farms use separators to reduce the overall slurry volume by up to 15 per cent. The nutrients which are so beneficial to the land are largely retained in the liquid, so removing solids simply reduces the overall amount of slurry.
Mr Tydeman says: “Separated slurry is easier to spread, especially when using umbilical applicators. It also requires less agitation which saves time and investment in larger, faster mixing machines.
“A further benefit is that separated solid material with 32% or higher dry matter can be used for bedding.
Stallkamp separators start at £12,000 and there are often grants and incentives available to help purchase them.”
Mr Tydeman adds that keeping slurry covered offers environmental and human health benefits worldwide. It also has significant agricultural benefits, offering the opportunity for farmers to make the most of slurry as a natural resource.
Well maintained, responsibly stored slurry is a cost-effective way to fertilise grassland and crops. Slurry which is covered will retain more nitrogen, making it a more valuable fertiliser.
Mr Tydeman says: “If farmers give this natural resource a little more time and thought it can be very valuable.
Responsible slurry management is a discipline which is likely to be targeted by the Government in years to come.
“Therefore, farmers who demonstrate best practice and a willingness to improve their processes will reap the benefits.”