Greywater Use, Landscape, and Irrigation Considerations

Designing and installing greywater treatment plants is a delicate task. A greywater expert must consider several variables to ensure the system is tailored to the specific site. The versatility of greywater has proven its effectiveness and excellence. This image from Arabic National Geographic (2023) illustrates this in an article explaining the definition of greywater and its use in irrigation.

Local Climate

The local climate must be considered when designing and installing a greywater system, particularly evaporation and rainfall. It is important that the greywater system be capable of being shut down during the winter, with the greywater being diverted to the main sewer system during this period.

Soil and Site Assessment

A site assessment is required to ensure the greywater system is suitable for the property. Key considerations include topography, geology (including the original site soil and backfill), flooding potential, and proximity to environmentally sensitive areas (including water bodies).

A soil assessment may also be required to ensure its properties are suitable for greywater. Specifically: soil depth relative to groundwater/base water; permeability, texture, and porosity; and soil properties such as pH, salt, and dispersion.

Calculating Area Use

The amount of graywater produced by a home must be calculated to ensure sufficient garden space for dispersion. A “simple calculation” is usually used, as outlined in the law, although this figure may vary depending on the population and their water consumption.

If the available space on a property is larger than the required area use, a graywater system is usually approved by the local government. However, the area allocated for irrigation may not be the same as the approved area, as newer (more water-efficient) homes and homes with fewer occupants than bedrooms may produce less graywater. Furthermore, irrigation needs vary depending on the plant.

Greywater can be used in gardens with limited space by diverting appropriate amounts, but the benefits of reusing it may be reduced (i.e., the cost of the system may not be recovered). A graywater professional will be familiar with these factors and will be qualified to design a graywater system that will successfully irrigate any specific garden area, or identify when reuse may not be appropriate.

General Graywater Irrigation System Design: Dripline, Strip, and Setbacks

Dripline Use

Dripline use is the most efficient method of distributing irrigation water, as the emitters direct water directly to where it’s needed most—the root zone. Emitters installed in driplines used for strip irrigation (brown or black) are typically smaller than those used for graywater (purple), which can lead to clogging. Prevention strategies include:

Use pre- and/or post-pump filters to ensure solids don’t enter the dripline (follow each manufacturer’s recommendations for their system).
Install flow valves to allow the system to flow with clean water.
Install a vacuum or check valve to prevent the pipe from sucking dirt back into the emitters.

An important article explaining The types of valves (water valves) in water treatment plants.

Design

A main line, typically made of lilac polyethylene with a diameter of 25 mm, extends to the garden. One or more supply pipes (supply manifolds) of the same diameter are connected to the main line to serve different areas of the garden. Drip pipes (lateral pipes) made of 13 or 16 mm diameter polyethylene are connected at 300 mm intervals between each dripper. They typically run parallel, 300 mm apart in sandy soils, and up to 600 mm apart in less permeable soils. For trees, water is distributed evenly through the root zone via a spiral line around the base of the tree.

Each length of drip pipe not connected to the supply manifold must be openable. To flush out sediment accumulating in the irrigation lines. Areas with multiple parallel rows of drippers can be connected to a second length of solid-wall main line pipe. Called a manifold or flush manifold. That terminates with a manual flush valve. The dripper must not loop back to the supply manifold or reconnect to itself to form a “closed circuit”. This will lead to sediment buildup and clogging of the drippers.

A check valve must also be installed at the highest point of the irrigation line. This valve allows air to pass through the one-way valve as water drains from the line, preventing contaminated water and soil from being absorbed through the drippers and clogging them.

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Setbacks

Greywater irrigation drippers must be located away from buildings, property boundaries, driveways, entrances, retaining walls, and pools to comply with the code. Horizontal setback distances for drip irrigation areas are detailed in the code and are shown in the example below. The calculation of the area to be irrigated with graywater (dispersion area) is based on actual boundaries, not on setback lines.

Using Multiple Irrigation Zones Within a Garden

There is a limit to the length of the drip line, and therefore the area that can be irrigated at any one time by the pump in a graywater system. This depends on the model of graywater system (and the type of pump) installed.

If a sufficient water source is available, multiple irrigation zones can be used. The water source is alternated between the different zones using an indexing valve. The indexing valve can be adjusted so that different zones receive different amounts of water. For example, a four-outlet model can supply the same area of ​​the garden with two outlets. While the other two outlets supply each other separate area from the garden ponds.

Greywater Recharge

It is recommended to recharge the planned water or wells during times when the property may be unoccupied. For example, when residents are on vacation, or to increase the total amount of water used for irrigation during periods of extreme heat with high evaporation rates. This will ensure continuous water supply to the plants and avoid stress.

It is not permissible to connect the planned water supply directly to the general irrigation. Or greywater irrigation system to avoid contamination of the planned water supply by backflow. It is preferable to use a dedicated irrigation line controlled by an irrigation controller equipped with an adjustable valve. This should be located before the pipe drains into the sanitary plumbing connection. Alternatively, an external garden tap or separate pipe equipped with an adjustable gate valve should be installed for future connection to a battery-powered irrigation timer. A water outlet should be provided above the greywater plumbing connection.

The recharge can be connected to the greywater sewer network in several ways. This includes a confined sump, which is preferred, an overflow culvert, or another suitable trap/sewage outlet.

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Ensuring that the water source is not contaminated by backflow

To ensure that the water source is not contaminated by backflow, the water recharge scheme must meet the following requirements:

• The connection must be above the watertight level.
• The top of the sump or drain pipe must be higher than the flood level rim of the fixture.
• It must have an air gap twice the inner diameter of the discharge pipe.
• It must be located in an easily accessible location for maintenance.
• Pipes must have a nominal air gap of 55 mm.