Recycle pits

Recycle pits — Planning and design

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

Site selection

Recycle pits should be located:

• Adjacent to existing drainage infrastructure to enable the irrigation tailwater from a large proportion of the production area to be captured in the pit and for larger storm flows to bypass the structure, once the recycle pit is full.
• Where there are impermeable (e.g. clay) soils[5]
• Where they will not intercept or leak to groundwater[1].

Sizing

The recycle pit needs to be large enough to capture all the tailwater of an irrigation event and/or a rainfall event (depending on the purpose of the pit). As it can be impractical to operate the recycle pit empty, an allowance for 10% additional capacity will minimise the risk of the recycle pit overflowing.

Capturing irrigation events

To calculate the minimum storage volume required for capturing irrigation tailwater requires an understanding of:

• the size of the farming catchment draining to the recycle pit (both the on-farm catchment area and area of neighbouring farm captured by the recycle pit)
• the volume of irrigation run-off entering the recycle pit per irrigation event
• how often the recycle pit will be emptied
• additional storage capacity to minimise the risk of the pit overflowing.

For example, a general rule of thumb used to calculate the size of a recycle pit in the Burdekin River Irrigation Area is to have a recycle pit of 5-10ML for each 100ha of irrigation lands. To make it cost-effective for the producer (i.e. sufficient water to offset the costs of storage, pumps and pipelines), a minimum area to supply the storage is around 200ha[6].

Capturing rainfall events

Recycle pits can also be designed to capture run-off from a specific sized rainfall event. Small to medium sized rainfall events during the dry season and early wet season can have the highest concentrations of pollutants and pose the highest risk to freshwater, estuarine and inshore marine environments[2]. Recycle pits can be sized and designed to capture and re-use run-off from these events. The minimum size required to capture stormwater run-off will depend on the characteristics of the catchment, the amount of stormwater run-off and pollutant load reductions required.

Compare the projected stormwater run-off in megalitres (ML) (as calculated below) with the projected irrigation tailwater run-off and choose the larger volume to ensure that the recycle pit can effectively capture irrigation and stormwater run-off. The volumetric run-off coefficient (C_v) for a single storm event can be used to estimate the catchment run-off volume for the design storm event. The C_vis defined as the ratio of the volume of stormwater run-off to the volume of rainfall that produced the run-off and is influenced by soil type and vegetation density[3].  Table 1 provides C_v for different soil types and if the soil texture is unknown a C_v of 0.5 should be used.

The net run-off (ML) = Catchment area (ha) x rainfall event (mm) x volumetric run-off coefficient (C_v) ÷ 100

Table 1 Volumetric run-off coefficient values (C_v) for different soil types. Source: NRW 2000 in DEEDI (2011)

The length, width and depth of the recycle pit will need to be tailored to the site, so that it avoids disturbing or leaking to groundwater or natural areas and also minimises impact on the production areas and farm operations.

Design

• High flow bypass: Recycle pits should be located adjacent to a farm drain, which can act as a high flow bypass to prevent mixing and export of pollutants during high flow events.
• The inlet and outlet should be close to each other (Figure 3): Having the inlet and outlet located close together minimises the chance of water flowing through the pit and out the other side, reducing the risk of resuspension and transport of pollutants out of the recycle pit[4].
• Impermeable base: The recycle pit should be built where the soil is impermeable (e.g. clay) to avoid losses to the groundwater through deep drainage. If the soil is permeable, a 0.5m layer of compacted clay is required, alternately a plastic lining or layer of sodium bentonite[4] can be used, however this will add significantly to the cost.

Macrophytes such as reeds and sedges can be used in recycle pits to enhance pollutant treatment and provide habitat. Guidelines for cotton and grain production areas, recommend separate cells to minimise evaporative loss and to enable different functions in each cell, i.e. one cell with macrophytes to help treat the water (e.g. removal of fine sediments and some pesticides), another cell with open water to enable sunlight to break down pesticides and the final cell (i.e. cleanest water) with habitat for fauna[5]. Unless designed specifically for habitat purposes, fish and wildlife should not be encouraged in recycle pits where water quality and dissolved oxygen levels are often poor as to cause fish kills.

References

1. ^ Canegrowers (2013), Smartcane BMP Irrigation and Drainage Management. [online], Canegrowers. Available at: https://smartcane.com.au/user/modules.aspx?id=e8b26cf7-33ea-4165-ba89-cef1ba4842f0&p_id=c78d4360-1d20-442e-8922-3b3e838d843c [Accessed 1 December 2021].
2. ^ Davis, AM, Pearson, RG, Brodie, JE & Butler, B (2016), 'Review and conceptual models of agricultural impacts and water quality in waterways of the Great Barrier Reef catchment area', Marine and Freshwater Research, vol. 68, pp. 1-19.
3. ^ Department of Employment, EDI (2011), Wetland Management Handbook: Farm Management Systems (FMS) guidelines for managing wetlands in intensive agriculture.. [online], Queensland Wetlands Program, Brisbane. Available at: https://wetlandinfo.des.qld.gov.au/resources/static/pdf/resources/reports/fms/fms_025_handbook_web.pdf.
4. ^ a b NQ Dry Tropics (2009), Recycle Pit Guidelines: for cane farmers in the Burdekin Dry Tropics region, NQ Dry Tropics, North Queensland.
5. ^ a b Rose, M, Crossan, A, Kennedy, I, Chapman, V & Spanswick, S (2006), Wetlands and water storages on cotton farms.
6. ^ Shannon, E, personal communication (2018).

Last updated: 10 June 2022