Greywater
Greywater refers to domestic wastewater generated in households or office buildings from streams without fecal contamination, i.e., all streams except for the wastewater from toilets. Sources of greywater include sinks, showers, baths, washing machines or dishwashers. As greywater contains fewer pathogens than blackwater, it is generally safer to handle and easier to treat and reuse onsite for toilet flushing, landscape or crop irrigation, and other non-potable uses. Greywater may still have some pathogen content from laundering soiled clothing or cleaning the anal area in the shower or bath.
The application of greywater reuse in urban water systems provides substantial benefits for both the water supply subsystem, by reducing the demand for fresh clean water, and the wastewater subsystems by reducing the amount of conveyed and treated wastewater. Treated greywater has many uses, such as toilet flushing or irrigation.
Overview
Quality
Greywater usually contains some traces of human waste and is therefore not free of pathogens. The excreta come from washing the anal area in the bath and shower or from the laundry. The quality of greywater can deteriorate rapidly during storage because it is often warm and contains some nutrients and organic matter, as well as pathogens. Stored greywater also leads to odour nuisances for the same reason.Synthetic personal care products commonly rinsed into greywater may contain microbeads, a form of microplastics. Greywater originating from washing clothes made from synthetic fabrics is also likely to contain microfibers.
Quantity
In households with conventional flush toilets, greywater makes up about 65% of the total wastewater produced by that household. It may be a good source of water for reuse because there is a close relationship between the production of greywater and the potential demand for toilet flushing water.Practical aspects
Misconnections of pipes can cause greywater tanks to contain a percentage of blackwater.The small traces of feces that enter the greywater stream via effluent from the shower, sink, or washing machine do not pose practical hazards under normal conditions, as long as the greywater is used correctly.
Treatment processes
The separate treatment of greywater falls under the concept of source separation, which is one principle commonly applied in ecological sanitation approaches. The main advantage of keeping greywater separate from toilet wastewater is that the pathogen load is greatly reduced, and the greywater is therefore easier to treat and reuse.When greywater is mixed with toilet wastewater, it is called sewage or blackwater and should be treated in sewage treatment plants or an onsite sewage facility, which is often a septic system.
Greywater from kitchen sinks contains fats, oils and grease, and high loads of organic matter. It should undergo preliminary treatment to remove these substances before discharge into a greywater tank. If this is difficult to apply, it could be directed to the sewage system or to an existing sewer.
Most greywater is easier to treat and recycle than sewage because of lower levels of contaminants. If collected using a separate plumbing system from blackwater, domestic greywater can be recycled directly within the home, garden or company and used either immediately or processed and stored. If stored, it must be used within a very short time or it will begin to putrefy due to the organic solids in the water. Recycled greywater of this kind is never safe to drink, but a number of treatment steps can be used to provide water for washing or flushing toilets.
The treatment processes that can be used are in principle the same as those used for sewage treatment, except that they are usually installed on a smaller scale, often at household or building level:
- Biological systems such as constructed wetlands or living walls and more natural 'backyard' small scale systems, such as small ponds or biodiverse landscapes that naturally purify greywater.
- Bioreactors or more compact systems such as membrane bioreactors which are a variation of the activated sludge process and is also used to treat sewage.
- Mechanical systems
Reuse
Global water resource supplies are shrinking. According to a report from the United Nations, water shortages will affect 2.7 billion people by 2025, which means 1 out of every 3 people in the world will be affected by this problem. Reusing greywater has become a good way to solve this problem, and wastewater reuse is also called recycled or reclaimed water.Benefits
Demand on conventional water supplies and pressure on sewage treatment systems is reduced by the use of greywater. Re-using greywater also reduces the volume of sewage effluent entering watercourses which can be ecologically beneficial. In times of drought, especially in urban areas, greywater use in irrigation or toilet systems helps to achieve some of the goals of ecologically sustainable development.The potential ecological benefits of greywater recycling include:
- Reduced freshwater extraction from rivers and aquifers
- Less impact from septic tank and treatment plant infrastructure
- Reduced energy use and chemical pollution from treatment
- Groundwater recharge
- Reclamation of nutrients
- Greater quality of surface and ground water when preserved by the natural purification in the top layers of soil than generated water treatment processes
The potential economic benefits of greywater recycling include:
- Can reduce the demand for fresh water, and when people reduce the use of fresh water, the cost of domestic water consumption is significantly reduced, while alleviating the pressure of global water resources.
- Can reduce the amount of wastewater entering the sewer or on-site treatment system.
Safety
Greywater reuse in toilet flushing and garden irrigation may produce aerosols. These could transmit legionella disease and bring a potential health risk for people. However, the result of the research shows that the health risk due to reuse of greywater either for garden irrigation or toilet flushing was not significantly higher than the risk associated with using clear water for the same activities.
Irrigation
Most greywater should be assumed to have some blackwater-type components, including pathogens. Greywater should be applied below the surface where possible and not sprayed, as there is a danger of inhaling the water as an aerosol.In any greywater system, it is important to avoid toxic materials such as bleaches, bath salts, artificial dyes, chlorine-based cleansers, strong acids/alkali, solvents, and products containing boron, which is toxic to plants at high levels. Most cleaning agents contain sodium salts, which can cause excessive soil alkalinity, inhibit seed germination, and destroy the structure of soils by dispersing clay. Soils watered with greywater systems can be amended with gypsum to reduce pH. Cleaning products containing ammonia are safe to use, as plants can use it to obtain nitrogen. A 2010 study of greywater irrigation found no major health effects on plants, and suggests sodium buildup is largely dependent on the degree to which greywater migrates vertically through the soil.
Some greywater may be applied directly from the sink to the garden or container field, receiving further treatment from soil life and plant roots.
The use of non-toxic and low-sodium soap and personal care products is recommended to protect vegetation when reusing greywater for irrigation purposes.
Indoor reuse
Recycled greywater from showers and bathtubs can be used for flushing toilets in most European and Australian jurisdictions and in United States jurisdictions that have adopted the International Plumbing Code.Such a system could provide an estimated 30% reduction in water use for the average household. The danger of biological contamination is avoided by using:
- A cleaning tank, to eliminate floating and sinking items
- An intelligent control mechanism that flushes the collected water if it has been stored long enough to be hazardous; this completely avoids the problems of filtration and chemical treatment
Due to the limited treatment technology, the treated greywater still contains some chemicals and bacteria, so some safety issues should be observed when using the treated greywater around the home.
A clothes washer grey water system is sized to recycle the grey water of a one or two family home using the reclaimed water of a washing machine. It relies on either the pump from the washing machine or gravity to irrigate. This particular system is the most common and least restricted system. In most states with in the United States, this system does not require construction permits. This system is often characterized as Laundry to Landscape. The system relies on valves, draining to a mulch basin, or the area of irrigation for certain landscape features. The drip system must be calibrated to avoid uneven distribution of grey water or overloading.
Recycled grey water from domestic appliances also can be used to flush toilet. Its application is based on standards set by plumbing codes. Indoor grey water reuse requires an efficient cleaning tank for insoluble waste, as well as a well regulated control mechanism.
The Uniform Plumbing Code, adopted in some U.S. jurisdictions, prohibits greywater use indoors. However, the California Plumbing Code, derived from the UPC, permits it.