Greywater Reuse Systems

Greywater is the wastewater stream from all sources other than the toilet (toilet water is often called blackwater or sewage). It is called greywater because it appears this colour, or turns grey after a short storage period.

Why recycle greywater?

The average Australian household uses about 800 L per day for both inside and outside use. Table 1 shows the approximate volume for each use. Typical values for the volume of greywater produced ranges from 100 to 200 litres per person per day.

Table 1 Approximate volume of greywater from household sources.

Greywater can be reused in a variety of ways, such as watering garden plants and for toilet flushing. Watering our gardens with greywater can be easily achieved. Water is too valuable a resource to waste, and any endeavour to reduce freshwater consumption or reduce wastewater disposal and treatment (and the energy this consumes) should be encouraged. More importantly, we should endeavour to reduce our water consumption by instigating a range of strategies which include installing reduced-flow shower heads, using water-efficient washing machines, turning off the tap when cleaning your teeth, and taking shorter showers.

Health concerns

Greywater contains a range of organisms which may cause disease (pathogens). Greywater is not allowed to be used in ponds or for above-surface irrigation systems due to the risk of mosquito breeding and contact with human skin and possible pathogen transfer. Many pathogens such as bacteria (e.g. faecal coliforms) and protozoans (e.g. Giardia) may be present in some greywater sources.

Greywater also contains bacteria and other microscopic organisms which feed on the nutrients in greywater, causing the wastewater to smell after a day or two. High levels of nitrate and phosphate may be beneficial to many plants, but can be detrimental to humans if ingested.


What greywater sources are best to reuse?

Greywater produced by a household can vary according to the number of occupants, and water usage patterns of the occupants.

Laundry water is probably the easiest to access as the plumbing often passes through the house wall, and a diverter can be installed.  On average it can contribute from anywhere between 30-40% of the total greywater volume.  Wastewater from the laundry varies in quality from each wash and can be contaminated with, lint, oils, greases, chemicals, soaps, nutrients and other compounds. 

Bathroom water is also suitable, but it may not be easily accessible as drainage normally occurs below the slab and may be close to the toilet water source. It may need to be collected and pumped to the infiltration area. On average it can contribute from anywhere between 50-60% of the total greywater volume.  Bathroom greywater can be contaminated with soaps, hair, shampoos, toothpaste, lint, body fats, oils and cleaning products. 

Kitchen greywater (and dishwasher water) generally should not be reused as this can contain oil, fats and food scraps, which do not break down easily and can clog irrigation pipes. On average it can contribute from anywherebetween 5-15% of the total greywater volume.   The detergents and cleaning products may be alkaline and contain chemicals that are harmful to soil structure, plants and groundwater. 

Greywater reuse strategies

Greywater from sources such as the laundry and bathroom is allowed to be used in a subsurface drain system for plant irrigation. This includes sewered blocks, provided the appropriate application (and fees paid) is submitted and approved by the Health Department (via the local council) and the Water Corporation or Sewerage Service Provider (who are responsible for sewerage).

Greywater cannot be used to irrigate a below ground (root) vegetable garden, but can be used on nearby/below fruit trees, lawn areas and on other plants (both exotic and native shrubs and trees). It is also possible to manually bucket greywater from the laundry trough to water plants. Placing greywater in the root zone of plants is the most effective way to ensure maximum uptake of both the water and the range of nutrients which are available in greywater. A word of caution: many native plants (e.g. the family Proteaceae such as grevillea, banksia and hakea) are susceptible to high levels of phosphate. Some introduced (exotic) plants, such as azaleas, camellias and gardenias, do not like the alkaline nature of some greywater sources. It is best not to use greywater on any of these types of plants.

A number of different subsurface drain configurations are shown on plans developed by Greywater Reuse Systems (GRS). GRS also sells a filter and a number of different greywater diverters.

GRS generally installs untreated greywater dispersal/diversion systems (Primary Treatment). Treated greywater systems (e.g. aerated wastewater treatment systems - Secondary Treatment) are available from other suppliers, and there are stringent monitoring and nutrient and pathogen content standards to be achieved. GRS may sell and install these systems in the near future. A licensed plumber is required for any changes to the sewer system.

Greywater should first pass through a filter and/or settling tank before dispersal in an infiltration area. This removes coarse material (e.g. hair, soap flakes, sand and lint) which could block the draincoil pipe or soil, or both. In sewered areas, a disused septic tank may be able to be converted to a settling or sedimentation tank.

An overflow safety device is required to divert greywater either back into a sewer line or into an existing wastewater treatment system (septic tank and leach drain) should problems arise. A vent, with a screened cowl, will need to be installed to prevent accumulation of foul air. Furthermore, there are restrictions on the location of greywater reuse systems within a property. For example, sedimentation tanks and trenches must be at least 1.2 m from boundaries and/or buildings. No greywater from one property is allowed to flow onto any neighbouring property.

Biological treatment by the use of aquatic macrophytes (reedbeds) is being investigated by GRS, and could provide other alternatives to greywater reuse in garden areas in the near future.