Going grey — water that is

outdoor shower
Greywater to green water

Greywater consists of the used water produced at a site, except for water coming from the toilet, which is known as blackwater. When building a greywater system you separate the greywater from the blackwater and send it through a separate treatment system. Ideally, after the greywater passes through a purification process it will then be able to be reused. Greywater systems are particularly appropriate in off-grid locations. They reduce use of fresh water, place less stress on existing conventional septic tanks, have a highly effective purification process, and there is less chemical and energy use.

This article discusses the basics of a greywater system that could be used off-grid.

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Greywater, sometimes spelled graywater, grey water or gray water and also known as sullage, is non-industrial wastewater generated from domestic processes such as washing dishes, laundry and bathing. Greywater comprises 50-80% of residential wastewater. Greywater is distinct from blackwater in the amount and composition of its chemical and biological contaminants (from feces or toxic chemicals). Greywater gets its name from its cloudy appearance and from its status as being neither fresh (white water from groundwater or potable water), nor heavily polluted (blackwater). According to this definition wastewater containing significant food residues or high concentrations of toxic chemicals from household cleaners etc. may be considered “dark grey” or blackwater.

If collected using a separate plumbing system to blackwater, domestic greywater can be recycled directly within the home and garden. Recycled greywater of this kind is never clean enough to drink, but a number of stages of filtration and microbial digestion can be used to provide water for washing or flushing toilets; relatively clean greywater may be applied directly from the sink to the garden, as it receives high level treatment from soil and plant roots. Given that greywater may contain nutrients (e.g. from food), pathogens (e.g. from your skin), and is often discharged warm, it is very important not to store it before using it for irrigation purposes, unless it is treated first.

There are numerous “soft” processes based on natural biological principles such as using reedbed filter systems, the wetpark systems or the living wall that can be used to clean up greywater.

There are also “hard”, direct processes, such as distillation (evaporation) which need not necessarily be as energy intensive as they might initially appear.

Uses for recycled greywater:


Greywater typically breaks down faster than blackwater and has much less nitrogen and phosphorus. However, all greywater must be assumed to have some blackwater-type components, including pathogens of various sorts. When used for irrigation, greywater should be applied below the soil surface where possible (e.g. in mulch filled trenches) and not sprayed, as there is a danger of inhaling the water as an aerosol.

Indoor reuse

Recycled greywater from showers and bathtubs can be used for flushing toilets, which saves great amounts of water. The level of treatment required in this case requires the water to have low or nil biochemical oxygen demand (BOD), but it is not necessary for it to be treated to the same standards as potable water.

Extreme living conditions

Greywater use promotes the ability to build in areas unsuitable for conventional treatment, or where conventional treatment is costly. The Mars Desert Research Station utilizes greywater recycling for this use, and might be used on trips to Mars to reduce water consumption and increase oxygen generation. Talk about far off-grid!

Dangers associated with greywater use

When treated properly, greywatercan be used on gardens and landscapes but it is important to remember that greywater contains impurites and microorganisms that are capable of causing disease and illness. Uncontrolled release of greywater, and its associated nutrient load, could find its way into storm water drainage systems and streams causing algal blooms and disruption of ecosystems. Spraying greywater on vegetables eaten raw is not recommended.

Importantly, sensible precautions can almost entirely mitigate the risk associated with greywater use. In order for disease transmission, people have to contact (by aerosol or directly) the greywater. If the greywater is applied by dripper systems or by subsurface delivery, this cannot occur.

3 Responses

  1. I’ve lived with grey water systems most of my life, which is becoming quite a long time. I have never seen a problem from greywater systems.

    A significan benefit to reduced volume to the septic system gained by using a distinct and separate greywater system is increased retention time of sewage in the septic tank. This allows for more complete decomposition and lower stress on the final bacterial clean-up accomplished in the leach area.


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