Greywater application, landscape and irrigation considerations
A number of variables need to be considered by the homeowner and greywater professional to ensure the system is designed to suit the specific site.
The local climate needs to be considered when designing and installing a greywater system, in particular evaporation and rainfall. For example Perth has a ‘Mediterranean’ climate with hot dry summers and mild winters with moderate rainfall. It is important that the greywater system is able to be switched off during winter with greywater diverted to the main sewerage system during this time. Regional areas of WA will have different climatic contexts to consider.
Soil and site assessment
A site assessment is required to ensure greywater is suitable for the property. Key considerations include: topography, geology (including original site soil and imported fill), flood potential and proximity to environmentally sensitive areas (including water bodies). Further detail can be found in the Code.
A soil assessment may also be required to ensure soil features are suitable for greywater. In particular: soil depth to groundwater/ bedrock; permeability, texture and porosity; and soil features such as pH, salt and dispersion.
There are different greywater application rates for different soil types. The Department of Health (2010) Code of Practice for the Reuse of Greywater in Western Australia, provides information on soil types and design irrigation rates, see Table 7: Recommended Design Irrigation Rate (DIR) for Irrigation Systems.
Calculation of application area
The amount of greywater generated by a household needs to be calculated to ensure there is adequate garden area for dispersal. A ‘simple calculation method’ is normally used as outlined in the Code, although this figure can vary depending on occupants and their water consumption.
If the area available on the property is greater than the application area required then a greywater system will generally be approved by the local government. However, the area intended to be irrigated may not be the same as the area approved as less greywater may be produced in modern (more water efficient) homes and in households with less occupants than bedrooms. Further, different plants have different irrigation needs.
Greywater can still be used where there is limited garden area, by diverting suitable volumes, but the benefits of greywater reuse may be reduced (i.e. the investment in a system may not be recovered). A greywater professional will be aware of these factors and is qualified to design a greywater system that will successfully irrigate any specific garden area or determine when greywater reuse may not be appropriate.
Application area from The Code
The Code requires a minimum application area to ensure the greywater is adequately dispersed across the garden area. The area is based on 3 factors:
1. The number of bedrooms in the property, and hence the maximum likely number of occupants. Bedroom 1=2 people; all other bedrooms = 1 person.
2. A fixed amount of greywater produced by each occupant daily in both the laundry (40L/person/ day) and bathroom (60L/person/day) (based on AS/NZS 1547:2000).
3. Soil type/ permeability (the ability of the wastewater to percolate downwards through the soil profile, and not pool or run off). This is expressed as a ‘Design Irrigation Rate’ (DIR) of x mm/day, where for example x=10 mm/day on the Swan Coastal Plain and x=2 on heavy clay soils.
Application Area required (m²)
Expected greywater volume (L/day) Design
Design Irrigation Rate (mm/day)
Example of general application rate calculation for sandy soils
• 4 bedroom 275m² house on a 550m² block located
on the Swan Coastal Plain.
• ‘Whole of house’ system (Laundry = 4oL & bathroom = 60L)
= 100L of water produced per person.
• Room 1 = 2 people & rooms 2/3/4 = 1 person each =
5 occupants in total.
• 5 people x 100L = 500L/person/day.
• 500L/10mm per day (sand irrigation rate) = 50m²
minimum area to be irrigated.
Example of general application rate calculation for loamy or gravelly soils
• 4 bedroom 226m² house on a 500m² block located
on the Darling Scarp on loamy soil.
• ‘Whole of house’ system (laundry = 4oL & bathroom = 60L)
= 100L of water produced per person.
• Room 1=2 people & rooms 2/3/4 = 1 person each
= 5 occupants in total.
• 5 people x 100L = 500L/person/day.
• 500L/5mm per day (loam irrigation rate) = 100m²
minimum area to be irrigated.
General Greywater irrigation system design: dripline, layout, setbacks Use of dripline
The Code requires greywater from GDDs to be dispersed into the garden via lilac (purple) coloured dripline, installed on the surface of the soil and covered with a 100mm layer of mulch to prevent human contact. The lilac colour coding of the dripline is required by Australian Standards to identify the pipe as carrying recycled water and reduce the likelihood of cross-connection with other water pipework.
Using dripline is the most efficient method of dispersing irrigation water, as the emitters pass water directly to where it is most needed – the root-zone. Emitters set into dripline used for scheme water (coloured brown or black) are generally smaller than those used for greywater (coloured lilac) and will clog. Protection strategies include:
• Using pre and/or post-pump filters to ensure solids don’t enter the dripline (follow individual manufacturer’s recommendations for what works best with their system).
• Incorporating flush valves to allow the system to be flushed out with clean water.
• Incorporating an air release or vacuum breaker valve to prevent the pipe sucking dirt back into the emitters.
Locking clamps should also be installed on barbed fitting designed for use with clamps (some barbed fittings are self-securing).
When using dripline with gravity GGD’s, at least two metres of vertical head pressure is required for the drippers to emit water. Frequent flushing with scheme water (at greater flow and pressure) will be needed to flush scum and debris from the insides of the pipes and drippers.
A mainline, generally 25mm lilac polyethylene, will run from the GDD to the garden. One or more supply pipes (supply manifold) of the same diameter will tee-off the mainline to service various garden areas. Driplines (laterals) in 13 or 16mm lilac polyethylene with a typical dripper spacing of 300mm are attached to this manifold and generally run in parallel 300mm apart on sandy soils and up to 600mm apart on less permeable soils. For trees, a spiral of dripline around the base of the tree delivers water evenly across the root zone.
Each length of dripline that is not attached to the supply manifold must be able to be opened for regular flushing of sediment accumulating in irrigation lines. Areas with several parallel rows of dripline can be connected to a second length of solid-wall mainline pipe, called the collector or flush manifold, and will end in a manual flush valve. Dripline should not loop back into the supply manifold, or connect back into itself to create a ‘closed circuit’ as this will result in sediment accumulating and clogging of drippers.
A vacuum breaker valve should also be included at the highest point of the irrigation line. This allows air to enter through the one-way valve as the water drains out of the line avoiding the potential for dirty water and soil being sucked back through the drippers and blocking them.
Greywater irrigation dripline must be setback from buildings, property boundaries, paths and driveways, retaining walls and pools to comply with the Code. Horizontal setback distances for drip irrigation areas are detailed in the Code and provided in the example below. Calculation of the area to be irrigated with greywater (the dispersal area) is defined by the actual borders or boundaries and not from the setback lines.
Using multiple irrigation zones within the garden
There is a limit to the length of dripline and hence irrigation area that can be supplied at any one time by the pump in a greywater system. This will be dependent on the model of greywater system (and pump type) installed.
Where there is an adequate supply of water available, multiple irrigation zones can beused with the water supply being switched sequentially between the different areas using an indexing valve. The indexing valve can be configured so that different areas can receive different amounts of water. For example: a four- outlet model could supply the same lawn area with two outlets, whilst the other two outlets each supplied a separate area of garden beds.
Indexing valves can either be pressure-activated, where the output is directed to a new zone each time the pump turns on (and hence a positive water pressure is detected); or volumetric, where the output will move onto a new zone after a certain number of litres have passed through the valve. A greywater professional can advise on the most appropriate valve type based on the application.
Plants selection and vegetation
Greywater can be used to irrigate above- ground food crops but should not come into direct contact with edible parts of fruit or vegetable crops for example potatoes, onions or strawberries. Fruit trees and vines are acceptable, as are shrub-type herbs that grow clear of where the greywater is being applied.
Using greywater under lawns
Greywater can also be used to irrigate lawn areas. Attention must be given to the following items:
1. Care must be taken to protect the emitters from root intrusion, either by:
• Using a brand of dripline where the emitters (drippers) are impregnated with copper oxide that will last the life of the dripline,
• Wrapping the dripline in geotextile material, or
• Using a disc filter before the dripline (a ‘techfilter’) where the discs are impregnated with a pre-emergence herbicide such as Trifluralin and need to be replaced annually.
2. Lawns require more water per square metre than trees and garden beds so an irrigation design including lawn must ensure that the supply of water will be enough by closer spacing of the dripline.
3. Installing dripline under lawn may limit the ability to practice certain lawn care activities such as coring for decompaction as this may result in physical damage of the dripline. Coring is particularly beneficial on high traffic lawn areas.
Western Australian native species have adapted to cope with the low nutrient levels in soils and some can be adversely impacted by greywater, in particular those from the Proteaceae family.
Greywater top up
A scheme water or bore water top-up is recommended for times when a dwelling may be unoccupied, for example when the occupants are away on holiday, or to increase the total amount of water irrigating an area during very hot periods with high evaporation rates. This will ensure that the plants still receive water and do not become stressed.
Connecting the scheme water supply directly to the GDD or greywater irrigation system is not permitted in order to avoid back-flow contamination of the scheme water supply. A dedicated irrigation line controlled by an irrigation controller with an adjustable valve located before the pipe discharge into the connection to the sanitary plumbing drainage is preferred. Alternatively, an external garden tap or separate pipe with adjustable gate valve should be installed for future connection to a battery operated irrigation timer, plumbed to discharge water above the connection to the greywater sanitary plumbing drainage. Greywater top up can also be achieved through direct top up via a Reduced Pressure Zone Device (RPZD) into the GDD, refer to AS/NZS 3500.1:2003 for more detail.
Water top up may be connected to the greywater sanitary plumbing drainage via a number of methods including a trapped tundish (preferred), overflow relief gully or other suitable waste trap/ gullies.
To ensure no contamination of the water source via backflow (refer to AS/NZS 3500.2:2003 for further details), scheme water top up must meet the following requirements:
• Connection shall be above the level of the water seal.
• The top of the tundish or waste pipe riser will be above the flood level rim of the fixture.
• Have an air gap of a size twice the internal diameter of the discharging pipe.
• Be located in an accessible location for maintenance.
• Pipework to have a nominal 55mm air gap (Refer to AS/NZS 3500.1:2003 – Table 4.3 for minimum air gap requirements).