As part of the changes to the Nitrates Action Programme and in an effort to retain our derogation, closed periods for spreading soiled water will come into play this year, with a closed period for spreading soiled water between 21 December and 31 December.

As farmers should already have a minimum 15 days of soiled water storage, this should not affect them.

The closed period for spreading soiled water extends from 10 December to 31 December for 2023 for all milk producers and from 1 December to 31 of December in 2024 for creamery milk producers, with those on a liquid milk contract having until 2025 to secure one month’s storage.

Volume of soiled water

The volume of soiled water produced varies from farm to farm, with an increase in cow numbers or cows per unit of the milking parlour resulting in a lower volume of dirty water.

However, 30l/cow/day is being taken as the figure for the storage requirements by the Department. Across a 100-cow herd requiring 30 days of storage, 90,000l of storage is required, or 90m³.

Two other factors need to be taken in to consideration however – rainfall and freeboard. Where soiled water storage is uncovered, the volume of rainfall must be calculated and added to the tank capacity.

For Co Cork, 98mm of rainfall occurs on average between 1-31 December, resulting in an additional requirement of 0.098m³ of storage per m² of tank floor area. Where tanks are uncovered, 300mm of freeboard is required and can not be calculated as part of the storage capacity, with 200mm required for covered or roofed tanks.

With most tanks being 2.4m (8ft) deep, 0.398m, nearly 20% of the storage, will have to be designated to freeboard and rainfall. In the above example of a 100-cow farmer in Cork, soiled water storage requirement increases to 108,000l or 108m³.


For the above 108m³ tank, 16m³ of concrete will be required for the tank walls, with an additional 14m³ required for the tank floor. At €110/m³, concrete costs will be €3,300 plus VAT.

Labour will be €3,500 plus VAT, with an additional €800 plus VAT for digging out and backfilling and €1,500 plus VAT for steel rebar reinforcement in the walls and floor (non-grant spec).

A 16ft 6in slat across the length of the tank will cost around €2,500 plus VAT.

The total cost of the tank is €11,600 plus VAT, €107.40/m³ of storage or €116/cow.

Larger tanks will see a reduced cost per m³, but this is a good baseline to go by.

Where farmers are planning on placing the tank in a collecting yard or a high-traffic area, the dry period might offer an opportunity to carry out works without impeding milking.

The accelerated capital allowance scheme is in place from January 2023 and allows you to write off slurry storage costs over a two-year period.