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MMN May 2024 – Slurry Storage: What are the Options?

9 May 2024

Once considered a waste product that was scraped into a hole and forgotten about, slurry is now seen as a valuable resource, which if utilised effectively, can help reduce inorganic fertiliser costs, benefit soil health and support crop growth.

Recent slurry storage grants in both Scotland and England have prompted farmers to re-assess their current storage requirements and consider options for increasing capacity. Covering slurry stores has also become a hot topic (particularly in England where there’s talk of potential regulation changes). So, what are the options for storing, covering and agitating slurry? And what are the barriers?

There are numerous slurry storage options available; all have their advantages and disadvantages. When choosing the most appropriate store, considerations should be made regarding its location, the volume to be stored and current farming practices.

Earth-banked lagoons

These offer the most cost-effective, long-term option for storing large volumes of slurry. However, the large surface area will accumulate a significant amount of rainwater.

Pros

  • Cost-effective.
  • Versatile and easily extendable.
  • Long structural lifespan if properly maintained.

Cons

  • Accumulates large quantities of rainwater.
  • Large lagoons could be difficult to agitate.
  • 750mm of freeboard will need to be factored into storage calculations.
  • Covering lagoons could lead to challenges with crust formation.
  • Embankments are vulnerable to damage from machinery and vegetation.
  • Cannot be located in areas with a high water table.

Steel towers

More costly than an earth-banked lagoon, but circular steel towers take up less space and collect less rainwater.

Pros

  • Less space requirement and accumulates less rainwater.
  • Relatively simple to cover.
  • Possible to increase tower height/capacity.
  • Can be constructed on land with a high water table.

Cons

  • Requires slurry to be pumped and this may not be possible with high dry matter slurry.
  • More costly than an earth-banked lagoon.
  • Cannot be entered to remove bedding that settles (i.e. sand).
  • May not last as long as lagoons or concrete stores.

Concrete stores

These are constructed using pre-cast panels or poured concrete and can be built below ground or above ground and can be either circular or rectangular. Concrete stores vary in cost depending on the design, but above-ground circular stores are typically comparable in price to steel towers.

Pros

  • Below-ground stores can be gravity-fed.
  • Suitable for high water table areas.
  • Can occupy a small area.
  • Have a relatively long lifespan.
  • Have the potential to be extended.

Cons

  • Cost (depending on design).
  • Above-ground rectangular stores are difficult to cover.
  • May require a complex pumping/mixing system.

Slurry bags

Typically constructed using a reinforced PVC membrane, these are highly effective at stopping nitrogen losses and ammonia emissions but have a limited lifespan and can take up a large area of land. They are usually bunded to protect the area from leaks. Slurry bags could be moved once emptied, which may offer a solution for businesses looking to store slurry further away from the farm or on seasonal land.

Slurry store covers

Covering the store with an impermeable cover will keep out rainwater and could provide an additional 30% more storage in addition to minimising ammonia emissions by up to 90% (and retaining 10% more nitrogen). However, higher dry matter slurry can pose challenges when pumping, most notably when using umbilical systems.

Impermeable covers can be floating or self-supporting. Floating covers are more cost-effective but have a limited lifespan (5 to 10 years) and will require rainwater to be pumped from the surface. Floating covers are the most practical solution for covering lagoons, but crust build-up underneath will need to be addressed. Self-supporting covers are fixed around the rim of the store using tensioned ratchets and supported using a central pole, creating a dome-shaped roof that diverts rainwater away from the store. These cost 4 to 5 times that of floating covers but provide a longer-term solution and will allow the crust to be incorporated easily. Stores may need to be reinforced to support a fixed cover.

 Agitation

Mixing will homogenise the slurry and provide a more consistent product. The list of options for mixing slurry is endless. However most involve circulating the slurry by pumping and jetting or mixing using a submersible propellor. Other options include aeration systems and slurry additives/enzymes.

A Landia mixer for agitating slurry

Landia Slurry Mixer

 

What to consider:

  • The surface area of the store: will a standard reception pit/jetter set-up be effective at mixing stores with a large surface area?
  • The volume of slurry to be mixed: will multiple agitators be required?
  • T.O or electric: can a tractor be spared for frequent mixing? What is the cost of running an electric mixer? Can electricity be supplied? Will 3-phase be required?
  • Fixed or portable: can mixers fixed to the inside wall of a sealed store be serviced and maintained easily?
  • Is the store covered? Lagoons with a floating cover may require specialist mixing equipment.

The limitations of the options listed above should be discussed with the supplier and a suitably qualified engineer. New stores must be designed to SSAFO specifications and projects should be discussed with SEPA and the local planning authority.

marcus.fox@sac.co.uk; 01539 889990

 

Slurry Lagoon

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