Slurry storage: circular storage tanks

Tom Ryan and Peter Varley look at the main points to consider and costs of building a circular slurry store.

Peter Varley

3 June 2018

Circular slurry stores are a good option for increasing the slurry storage capacity of existing farmyards. Below we have listed some of the advantages and disadvantages of such stores. Advantages

Circular slurry stores are a good option for increasing the slurry storage capacity of existing farmyards. Below we have listed some of the advantages and disadvantages of such stores.

Advantages

Ideal for increasing the slurry storage capacity in an existing farm without major construction work.

Speed and ease of construction (can take just one week to erect).

No major dig-out costs involved. Very little excess clay to take care of.

Can be a safer option compared with open slurry stores that require safety fencing.

Have the option of taking down the store again and moving to another site. This would be specifically appealing to farmers with leased land.

Disadvantages

Slurry usually needs to be pumped from a reception tank to the over-ground tank. This takes time and there is a cost even if you have your own slurry pump.

In a greenfield site, a reception tank would have to be built, adding significantly to the cost. The reception tank should have at least two weeks’ slurry storage to save the amount of times a farmer has to pump his slurry to the over-ground tank.

A propeller agitator usually needs to be purchased to fit inside the tank. Some farmers find even with a propeller agitator the slurry can develop a crust on top which can be difficult to break down.

Bunding may be required around the store.

The Department of Agriculture has outlined specifications for the construction of over-ground circular slurry stores. These specifications can be found on the Department’s website under S122. The main specifications include:

Hard core must be laid and compacted in the area where the slurry store is to be erected.

Hard core must be compacted in 150mm layers using a vibrating or heavy roller.

A 1,000 gauge polythene membrane must be laid on the finished hardcore with 600mm overlaps.

Steel mesh must be laid in all floors.

The concrete must be thoroughly compacted using a vibrating screed, and compaction around steel reinforcement must be carried out with a poker vibrator.

The sides of the tank must be constructed and placed by the store contractor.

An access ladder must be supplied to the side of the tank to enable viewing of the upper surface of the slurry in the tank. This ladder must be provided with a back safety cage.

If agitation is to be performed using a jetter affixed to the top of the tank, then a safety platform with rails must be provided for the jetter operation.

All points for emptying the tank through either the side or base of the tank must be controlled by at least two valves in series.

Costs

The Tables 1 and 2 outline the costs of circular slurry stores of different capacities based on the TAMS II reference costs. Storage periods of 16 and 20 weeks are shown for a range of cow numbers from 50 to 200. The net storage needed for the 16 and 20 weeks is shown. For a tank height of 4.23m the diameter of the tank is shown which will provide the total capacity required. In practice, the tank diameter and height will be chosen with the help of the supplier taking account of the commonly available diameters and heights as well as present and future storage needs.

The tables show the effect of economies of scale in reducing the cost per cow and per m3 as tank size increases. When the diameter is doubled the side wall length is doubled but the capacity is quadrupled for the same depth. The floor area of the tank is also quadrupled.

Table 3 shows costings for over-ground steel tanks based on information from a supplier. Prices exclude VAT. Prices don’t include pumps or through-the-wall propeller agitators. Tank price includes tank, jetters, steel ladder and platform, reinforcing mesh, valves, advice and all labour involved in erecting the tank, doing concrete work, etc. Price does not include any sumps/reception tanks and costs associated in getting the slurry to the reception tank. Suppliers ask that they are presented with a level site with hard core placed. TAMS II reference costs for tank number three are €53,659, which are a little behind this supplier’s costs.