Greywater Recycling- What You Should Know

What Is Greywater?

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.

•    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.

Greywater Irrigation May Not Be A Good Choice If:

•    Soil is not suitable – If your soil is either too permeable or not permeable enough, you may not be able to recycle your greywater, or you may need a system with some modifications.

•    Area too small – You need enough soil to process the greywater and enough plants to use it.

•    Climate unsuitable – If it’s too wet to benefit from irrigating with greywater, there may be a better way to dispose of it. If it’s too cold, you will only be able to recycle in the warmer months. In cold climates, the heat in greywater may be more valuable than the water itself.

•    Permit hassles – Many jurisdictions in North America have no clear guidelines regarding greywater processing. With water shortages looming in the near future for many regions, this may change sooner than later. Health concerns are often cited as the reason for not allowing greywater recycling, although there has never been a documented case of somebody becoming sick as a result of exposure to greywater.

•    Low cost/benefit ratio – Where legal requirements dictate a complex system and there is only a small flow of water, greywater recycling is not economically feasible.

•    Inconvenience – If the greywater system you are considering is more expensive and requires more maintenance than a properly functioning septic or sewer system.

To recycle greywater safely, users must understand the nature of the grey water itself as well as the natural cycles and processes involved in the purification of it. Each set of circumstances requires its own unique recycling system for optimum results. For most residential purposes, low-tech, home made grey water systems tend to outperform and outlast expensive pre-made systems.

Greywater Recycling Health Concerns

Health risks are often cited by regulators as reasons for requiring high-tech expensive systems although there are no recorded instances of greywater–transmitted illness in the US. However, greywater may contain infectious organisms. Bear this in mind when designing and using a system. A poorly designed system could become a pathway for infecting people.

Two main principles for safety:

•    Greywater must pass slowly through healthy topsoil for natural purification to occur.

•    Design your greywater system so no greywater-to-human contact occurs before purification (ie: passing through the soil or mulch basin).


•    Prevent contact or consumption – Avoid accidental connections between freshwater and greywater plumbing.

– Label greywater plumbing, including garden hoses
– Use gloves when cleaning greywater filters
– Wash your hands after contact with greywater

•    Microorganisms on plants – Don’t apply untreated greywater onto lawns, or fruits and vegetables that are eaten raw (eg. strawberries, lettuce, carrots)

•    Breathing of microorganisms – Don’t recycle untreated greywater with sprinklers. Droplets can evaporate leaving harmful microorganisms in the air where they can be breathed in

•    Use only greywater that is fairly clean to start with – Greywater containing water used to launder diapers or generated by anyone with an infectious disease should be diverted to a sewer or septic system

•    Don’t store greywater – Use it within 24 hours before bacteria multiply. After 24 hours it is well on its way to becoming blackwater

•    Don’t overload your system – If you’re having company and your system is designed for 2 people, divert the greywater to the sewer or septic system for the evening

•    Chemical contamination – Don’t buy household cleaning products you wouldn’t want in your greywater system. Divert greywater containing harmful chemicals to the sewer or septic system

•    Prevent contamination of surface water – Discharge greywater underground or into a mulch filled basin

– Don’t apply greywater to saturated soils

– Apply greywater intermittently so it has a chance to soak in and the soil can aerate between waterings


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