Current construction costs have caused many to press pause on farm developments, among other factors. But for some, a necessity to construct slurry accommodation now amid pressures in springtime will mean that they will have to bite the bullet. Current ready-mix prices are floating around the €130-€140/m³ mark, and with it being the primary component of underground slatted tanks, careful planning must be undertaken to get maximum bang for buck.

Deep tanks

Standard tank size is generally 2.4m (8ft) in depth, with a 200mm freeboard for covered tanks and 300mm for outdoors tanks. This results in a usable capacity of 91.6% and 87.5% respectively when freeboard is subtracted from gross tank capacity.

If we were to increase the tank depth to 2.7m (9ft), usable capacity increases to 92.5% and 88.8%. While these changes may seem insignificant, in a standard five bay tank using a 16ft 6in slat, the usable capacity increases from 276.21m³ to 318.88m³, a difference of 42.67m³ or 9,386 gallons. If we were to stock this shed with 45 suckler cows at 0.26m³/cow/week, it will give us an added three and a half weeks of storage.

The instance of where a deep tank does not work is where bed rock is present. Breaking out with a rock breaker is a costly process and widening or lengthening a 2.4m tank will likely be a better option. Where ground conditions are poor and significant slippage of the banks on a tank occurs, or where a tank is located tight to an existing building, again the standard 2.4m tank will be a wiser option.

Length versus width

Wider, shorter tanks will generally have an increased cost per m³ of storage than narrow, long tanks. While other factors come into play regarding tank width, such as the building layout and position of feeding passages and cubicles, a 16ft 6in or 14ft6in slat should be used as opposed to smaller slat sizes.

Taking our same tank five bay tank as above and using a 16ft6in slat, we have a usable capacity of 276.21m³.

However, if we narrow in our tank and use a 12ft6in slat, our usable capacity goes down to 204.93, equating to 71.28m³ or 15,679 gallons, a much greater difference than the example of deepening the tank above.

It’s also important to remember that a reduced slat size will more times than not result in reduced lying space, unless an increased layback is added.

On a per m3 basis, using the wider tank above results in a cost of €93.69/m³, according to TAMS III reference costs, with the overall tank costing €28,215 + VAT.

The narrower tank has a reference cost of €25,306 + VAT, meaning our additional tank capacity from widening the tank (71.28m³) is only costing an extra €2,909, or €40/m³. While the TAMS reference costs are generally out of kilter, the price differential is likely the same if we were to calculate actual costs.

Separating soiled water from slurry

Once the dry matter of soiled water rises above 1%, it is classed as slurry with the appropriate storage rules applying to it (16-20 weeks). Minimising soiled water entering slurry tanks is a must.

One way of doing this is to insert a dividing wall in a slatted tank, which will allow for soiled water and slurry to be stored separately. Cows should not be fed on, or have continual access to slatted areas above soiled water tanks, as even small volumes of dung can raise the dry matter of the tank contents above the miniscule 1% threshold.