ARCHIVE: Nitrate Vulnerable Zones (NVZs) - Guidelines for Farmers (Scotland)
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Fertiliser and Manure Plans
Part 2 Manure Planning

Calculation guide - Is there enough storage for slurry?

Step 1: Calculate monthly slurry production

The quantity of slurry produced each month is calculated by completing Table F . If some of the excreta is collected as solid manure and stored separately estimate the proportion handled as slurry.

  • Enter the number of stock against each category
  • Enter the proportion of waste collected as slurry, if not 100%
  • Multiply the number of stock by the proportion of waste collected as slurry and then multiply by the monthly volume of slurry produced
  • Total the figures for the different stock categories

You will now have calculated the total volume of slurry produced each month.

Step 2: Calculate the volume of rain draining to the slurry store. Drainage from the following areas should be considered, if it drains to the slurry store:
  • The volume of rain falling directly into the slurry store
  • Drainage from all contaminated yards which drain to the slurry store
  • Drainage from uncovered silage clamps and dung stores
  • Runoff from roofs and yard drains
} Calculate the total surface area in m2

Exclude runoff from clean yard areas and roof areas if it is collected and discharged into a clean drain. Similarly, rainwater that has passed across a fouled concrete yard, but is collected separately from slurry, is not regarded as slurry under the NVZ rules and can be spread at any time, on any soil, subject to the requirements of the PEPFAA code.

Volume of rain draining to slurry = store per month (m3)

Area of slurry store and yards etc. (m 2) * Average monthly rainfall (mm)


Step 3: Calculate dirty water production from other sources e.g. milking parlour/dairy washings or washing down pig pens

Note: Dairy/parlour wash water, that is collected separately from slurry, is not regarded as slurry under the NVZ rules and can be spread at any time, subject to the requirements of the PEPFAA code.

If you know the actual amount of dirty water produced from cleaning operations, then use the actual figures. If you do not know the actual figures, use the following figures, which represent the typical daily quantity of water produced during cleaning operations.

  • For Dairy cows the typical daily production is 18 litres per animal/day.
    Monthly production (m3) = (number of milking cows x 18 x number of days in month) 1000.
  • For cleaning out pig pens after each batch of pigs (10 pigs per batch), the typical production is 18 litres/batch.
    Monthly production (m3) = (number pens cleaned per month x 18) 1000.
Step 4: Calculate the slurry storage requirement

For the purpose of the NVZ Regulations, this step is only applicable if you are affected by the closed periods for spreading slurry, poultry manure and liquid digested sewage sludge on Sandy/Shallow soils.

Slurry storage requirement =
(Total volume of slurry + Volume of rainfall into slurry store + Volume of dirty water) m3/month

The slurry storage capacity must be sufficient to store all of the slurry production that cannot be spread, due to the closed period restrictions.

A simplified approach to calculating your minimum closed period requirement is:

  1. Calculate the amount of slurry produced from the start of housing to the start of the closed period = a
    (this gives some allowance for a safety margin)
  2. Calculate the amount of slurry produced during the closed period = b
  3. The total minimum storage required to comply with the closed periods then equals a + b

This approach takes no account of the amount of land that is available for spreading during the closed period and you will have to consider this, if your calculations show that you have insufficient storage capacity. Similarly, you may have to consider whether additional storage will be required to cover the time from the end of the closed period to the end of winter housing.

Preparation of a Farm Waste Management Plan (FWMP) will help you to decide where, when and at what rates you can spread organic manures. This is not mandatory within the NVZs, but it will enable you to determine your storage requirements by balancing the amount of slurry that can be spread, against the amount of slurry produced on a monthly basis.

Step 5: Calculate the amount of storage available

To estimate the existing slurry storage capacity for a store which is:

  • Square or rectangular with vertical walls:

    • Multiply its length (m) by its width (m) by its height (m) (the height should be reduced by 0.3m to allow for freeboard as a safety margin against increased risks of pollution from storm events)

  • Circular:

    • Measure the circumference.
    • Calculate the radius by dividing the circumference by 3.142, then dividing by 2
    • Calculate the floor area by multiplying the radius by the radius, then multiply by 3.142
    • Multiply the floor area (m2) by the height (m) (reduce the height by 0.3m to allow for freeboard) to give the capacity of the slurry store (m3)

  • Earth banked lagoon:

    • Estimate the length (m) and width (m) and average depth (m), from the top of the bank to the base of the lagoon
    • Reduce the average depth by 0.75m to allow for freeboard and to give the watering depth
    • Reduce the measured length and width of the lagoon by enough to allow for the sloping sides
    • Multiply the adjusted length by the adjusted width by the watering depth to obtain the storage capacity (m3)
Step 6: Compare the amount of available slurry storage, with the actual amount likely to be produced during the closed period. If the amount of storage available is less than the amount of slurry produced, you will have to review your current storage.

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