Effluent Storage Volumes 

Answers to frequently asked questions about effluent storage volumes.

Why Do We Need Storage? 

Tasman District Council is required under the Resource Management Act to ensure that water resources are protected for the future. This includes water availability, protection of aquatic ecosystems, human drinking, recreational and irrigation needs.

If farms have insufficient or sub standard storage there is an increased risk of contamination of surface water, or (through a delayed impact) groundwater supplies. This contamination can either result by diffuse means from application or from point source via failing or leaking storage systems.

Adequate storage increases the flexibility of effluent systems and reduces the risk of non-compliance because it allows a farmer to store effluent instead of irrigating when weather and soil conditions are not suitable for irrigation.

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What are the Requirements for Storage? 

There are no current Tasman District Council rules for a required size or number-of-days-of storage. However, there must be sufficient storage so the storage facility never overflows, and so that irrigation can be deferred when the soil is saturated. You may not irrigate to saturated soil because it is likely to result in ponding or run off.

All stored dairy effluent must be contained on/within a surface that restricts the seepage of effluent contaminants into ground or surface water. Issues do not arise just at the dairy, wherever effluent is concentrated or stockpiled there can be contamination of water. Feed pad or stand-off pad effluent must be contained and spread appropriately. This requirement also includes effluent sludge removed from ponds, sandtraps and other holding facilities.

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What Size Storage is Needed? 

The size of storage required is unique to each farm and is influenced by several factors. These include the local rainfall and evapotranspiration rates, topography and soils, catchment and herd sizes and nutrient loading rates.

It is also influenced by how the farm chooses to manage effluent and water to fit with other aspects, such as calving, available labour and power supplies. The storage volume must be compatible with the type of technology selected, for example solid separation and irrigator capability.

Simply put, effluent storage must account for the volume of effluent generated less the rate of effluent irrigated and allow for the days when no effluent can be irrigated.

Using a water balance model is a method to help estimate the required volume. This can easily be calculated using the Dairy Effluent Storage Calculator developed by Horizons Regional Council and Massey University. This programme uses a 30 year historic data set and soils information for the region.

The Farm Dairy Effluent Design Code of Practice recommends using an accredited designer/installer to calculate specific storage requirements to meet individual farm circumstances. The system must also comply with current legislation.

It is important to remember that even if a system is compliant at inspection at a given point in time, if management factors are altered on the farm then the system may no longer be sufficient and could later be deemed non-compliant.

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Will All Industry Providers Give the Same Size Calculations? 

It is possible that industry providers may provide different sized storage volumes. This may be because the technology they are recommending has different performance parameters which will impact on management of the system. One of the benefits of using the FDE storage calculator is the ability to view different management scenarios for your farm.

It is critical that the measurements that are entered into the programme are accurate. It is also critical that if you agree to a system you are capable of managing it as designed, for example, the length of time spent irrigating.

It is also advisable to develop a contingency plan for effluent management should a problem occur with the system.

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How Many Litres of Effluent is Produced Per Cow Over 24 Hours? 

The standard figure for dung and urine production of an average dairy cow is 70 litres per day.

Ten per cent of this is generated during milking, i.e. seven litres per cow. When accounting for the effluent generated in the cow shed, the volume of wash down water must also be accounted for which ranges between 35-100 litres per cow depending on the efficiency of the system.

The use of a water meter on the water supply allowed calculation of the actual volume of wash down water being used.

Dung and urine on feed pads is accounted for on a pro rata basis per cow i.e. 70 litres divided by 24 multiplied by the number of hours on the feed pad. If water is used to wash the feed pad, this volume also needs to be accounted for.

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How Much Rainfall is Collected in the Pond During a 75mm Rainfall Event? 

The surface area of the pond influences how much rainfall is collected into the pond. The smaller footprint the pond has, the less rain is collected. For example in a 75mm rainfall event, a pond of 2000m² would collect 150m³ of rainfall compared to 45m³ in a 600m² pond.

It is therefore important to account for rainfall collection when designing and managing an effluent system.

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