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Grey water treatment / recycling

Greywater is wastewater generated from domestic activities such as laundry, dishwashing, and bathing, which can be recycled on-site for uses such as landscape irrigation and constructed wetlands. This wastewater contains no fecal matter (human feces). Greywater differs from water from the toilets which is designated sewage or blackwater to indicate it contains human waste.
Greywater is any household wastewater with the exception of wastewater from toilets, which is known as blackwater. Typically, 50-80% of household wastewater is greywater from kitchen sinks, dishwashers, bathroom sinks, tubs and showers. Of course, if you use a composting toilet, 100% of your household wastewater is greywater.
Freshly generated greywater is not as nasty as blackwater, but if it's not handled properly it can soon become so. Greywater decomposes at a much faster rate than blackwater and if stored for as little as 24 hours, the bacteria in it use up all the oxygen and the greywater becomes anaerobic and turns septic. After this point it is more like blackwater - stinky and a health hazard. In fact, many jurisdictions have strict regulations about disposal of greywater, some even require it to be treated as blackwater.
Not all greywater is equally "grey". Kitchen sink water laden with food solids and laundry water that has been used to wash diapers are more heavily contaminated than greywater from showers and bathroom sinks. Although greywater from these sources contains less pathogens than blackwater, many regulatory bodies consider it as blackwater.
The safest way to handle greywater is to introduce it directly to the biologically active topsoil layer, where soil bacteria can quickly break it down, rendering the nutrients available to plants. This biological water purification is much more effective than any engineered treatment, thus protecting the quality of groundwater and surface waters.

Benefits of Greywater Recycling For Irrigation

Reduce fresh water use - When the weather is warm, about half of the water consumed by the average household in North America is for outdoor use. Capturing the indoor greywater for use outdoors can cut water usage in half.

Reduce strain on septic system or treatment plant - Greywater makes up the majority of the household wastewater stream, so diverting it from the septic system extends the life and capacity of the system. For municipal systems, decreased input means more effective treatment coupled with cost savings.

Develop otherwise unsuitable real estate - A greywater recycling system, along with the use of composting toilets, can enable the development of property that is unsuitable for a septic system.

Groundwater Recharge - Greywater recycling for irrigation replenishes groundwater, helping the natural hydrologic cycle to keep functioning.

Plant growth - Greywater can support plant growth in areas that might otherwise not have enough water.

Maintain soil fertility - The nutrients in the greywater are broken down by bacteria in the soil and made available to plants. This helps to maintain soil fertility.

Enhance water quality - The quality of groundwater and surface waters are much better preserved by the natural purification processes the greywater undergoes in the top layers of the soil than by any engineered water treatment.

Satisfaction - The greywater user gets the satisfaction of direct participation in the responsible management of global nutrient and water cycles.