Reuse of grey water - water drained from showers, baths, washing machines, and kitchen sinks - for garden irrigation, can reduce houshold demand for outside water supplies.
Grey Water. Sydney Water
Grey water is made up of the wastewater from our showers, baths, spas, hand basins, laundry tubs, washing machines, dishwashers and kitchen sinks. (It doesn't include water from toilets.) Used appropriately, you can water your garden with grey water and save around 400 litres of fresh water each day.
Grey water can be diverted to your garden through direct diversion or a domestic grey water treatment system (DGTS). A DGTS will produce higher quality grey water to use in the garden and possibly your toilet or washing machine.
Black water is made up of the wastewater from our toilets, urinals and bidets. It is grossly contaminated by human excrement and requires a detailed treatment process such as a composting toilet before it is suitable to use in the garden.
Sydney Water estimates that an average household (3.5 people) produces 586 litres of wastewater each day. Approximately 61 per cent of the total wastewater produced by an average household can be used as grey water. Kitchen wastewater is not usually included in this amount.
Direct diversion of Grey Water onto Gardens
A gravity diversion device diverts grey water from a plumbing fixture directly onto your garden, for example, grey water collected at your laundry tub outlet. The flow of grey water is usually activated through a tap or a switch and must be piped to your garden through sub-soil irrigation. During wet weather, the tap or switch directs grey water to the sewer.
A pump diversion device has a similar set up to a gravity persion device but includes a surge tank to control the flow of grey water to your garden during sudden surges.
Kitchen water should not be diverted through a surge tank (unless it passes through a DGTS or a grease arrestor) because fats, oils and food particles in the water can clog the system, create odours and cause the sub-soil irrigation system to malfunction. Never use a surge system as a storage tank.
Domestic grey water treatment systems
A domestic grey water treatment system collects, stores and treats grey water to a high standard. It includes components such as wetlands, intermittent sand filters, soil filters, grey water septic tanks and aerated wastewater treatment systems.
The treatment process varies according to how the grey water is used and includes settling of solids, floatation of lighter materials, anaerobic digestion in a septic tank, aeration, clarification and finally disinfection.
As primary treatment will only reduce the solids in the wastewater, secondary treatment is necessary to remove pollutants from the remaining liquid.
Disinfection of Grey Water
Disinfection is usually the last treatment process and is used to eliminate pathogenic micro-organisms. The efficiency of this process is measured by the amount of thermo-tolerant coliforms in the grey water.
Some micro-organisms such as cryptosporidium are resistant to disinfection. Additionally, the effectiveness of chlorine varies against different types of micro-organisms reinforcing the need for caution when using grey water.
Sunlight can also help disinfect grey water, although its effectiveness is neutralised by shade and irrigation at night.
You must use disinfection when it is likely that people, animals and insects will come into contact with the treated grey water. For example, gardens with above ground irrigation. If the treated grey water is not disinfected, you must create a 100 millimetre barrier of soil above the source.