Recycle pits

Recycle pits — Construction and operation

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• Overview
• Key Considerations
• Planning and design
• Construction and operation
• Links and references

Approvals

Approvals may be required for the construction of a treatment system. Contact your local government before any construction is undertaken to understand requirements.

For example:

• Recycle pits should be sited to avoid damage to vegetation, but if it is unavoidable and vegetation removal is required, approvals may be required.
• High impact earthworks within the vicinity of an area mapped as a wetland protection area may trigger an approval process.
• As recycle pits can capture stormwater run-off, approvals may be required for taking overland flow. There may be additional requirements or considerations in the lower Burdekin to ensure recycle pits do not lead to adverse impacts associated with existing elevated groundwater levels[3].
• Prior to construction, check for any existing infrastructure by contacting electricity, water and telecommunication providers.

It is also recommended that local Traditional Owners are engaged to ensure no sites or items of cultural significance are disturbed during excavation.

Engineering advice should be sought prior to construction to ensure the treatment system is sized and sited appropriately, taking into account soil suitability, groundwater and local hydrology. The stability and safety of the treatment system should also be considered.

Construction

Recycle pits will require earthworks to excavate a pit. The angle of the batters will depend on the soil type, but generally batters should be less than 2.5:1 and no steeper than 3:1.[2]. The upper batters should be stabilised with jute matting or non-invasive grass to prevent erosion. An inlet and outlet into a farm drain also needs to be constructed and can be either an open drain or pipe.

Spoil should be reused on the property or deposited away from natural wetlands and waterways and the spoil disposal site should be bunded to avoid sediment loss.

Time for establishment

Following construction, pipes and pumping infrastructure needs to be installed and connected to the existing irrigation system to enable water to be reused on the farm.

A recycle pit can be constructed and operational within a week (will depend on the size of the structure).

The batters of a recycle pit should be stabilised with grasses or groundcovers to prevent erosion. Use local guidelines if they are available to assist with plant selection or contact your local Landcare or NRM group. Natural waterways, wetlands and riparian zones are a good reference from which to create a possible species template. See Plant Guides.

The establishment of vegetation may require irrigation and weed management until it is fully established, which can take up to two years.

Operation

• Water collected in the recycle pit needs to be used on the farm as soon as possible to ensure that the recycle pit is less than 10% full and has the capacity to capture another irrigation (or rainfall) event[2].
• The tailwater captured during an irrigation cycle/event should be pumped back into the irrigation distribution system throughout the irrigation cycle.
• Stormwater run-off captured in the recycle pit should be used in the first irrigation sets, to empty out the recycle pit as soon as possible so that there is space/capacity to capture the next irrigation or stormwater run-off event.
• The irrigation tailwater from the first two irrigation events following fertiliser or pesticide application is the highest priority for capture and re-use, as these irrigation events pose the highest risk of pollutant transport[1] and have the highest value for re-use.

Monitoring and Maintenance

Recycle pits should be inspected every six months, or after every major storm event to ensure the structure and pumping infrastructure has not been compromised.

It is recommended that the water in the recycle pit be tested occasionally to ensure that the water being reused on the farm is suitable for the crop and does not have high herbicide concentrations or salinity that may impact crop productivity.

Regular water quality monitoring will provide information on nutrient and pesticide losses from the farm and help growers identify the nutrient concentration in the reuse water. Test strips are a quick and cheap way to check the presence of nitrogen. If nitrogen is present in the reuse water then laboratory testing is recommended to accurately determine the concentration. This will enable fertiliser rates (or timing, application etc.) to be modified accordingly to save input costs, without impacting productivity[2].

Maintenace of recycle pits include:

• Regular maintenance of the pumping infrastructure,
• Weed management, particularly around the inlet, outlet and pumping infrastructure,
• Slashing/mowing of upper batters,
• Occasional excavation (e.g. once every five years) to remove sediment buildup to maintain capacity, particularly where there is significant sediment runoff and slumping or erosion of batters

Lifespan/replacement time

With regular maintenance, particularly to the pumping infrastructure, recycle pits should last indefinitely.

References

1. ^ Devlin, M, Lewis, S, Davis, A, Smith, R, Negri, A, Thompson, M & Poggio, M (2015), Advancing our understanding of the source, management, transport and impacts of pesticides on the Great Barrier Reef, Tropical Water & Aquatic Ecosystem Research (TropWATER), James Cook University, Cairns.
2. ^ a b c NQ Dry Tropics (2009), Recycle Pit Guidelines: for cane farmers in the Burdekin Dry Tropics region, NQ Dry Tropics, North Queensland.
3. ^ Shannon, E & McShane, T (2013), Options for tailwater capture and reuse in the Barratta catchment: A scoping study, FarmAcist and BBIFMAC, Ayr.

Last updated: 10 June 2022