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Greywater: treating and recycling wastewater

Reusing greywater (wastewater) can reduce household water use by up to 30%, and save you money on both supply and sewerage if your supply is metered. However, it’s important to understand that the savings are small compared with simple water-saving measures, such as using water butts and low flush toilets. For the biggest reduction in both bills and the household’s carbon footprint, focus on water efficiency first – specifically on reducing the amount of hot water used – and then consider using greywater to operate flush toilets or to water non-food plants in the garden.

What is greywater?

soapy greywater, imageGreywater is all the wastewater from a property, apart from toilet waste (which flows into the foul drainage system, or sewer). Greywater can be collected and reused from showers, baths and wash basins. More heavily contaminated water from kitchen sinks, dishwashers and washing machines is not normally reused.

Greywater deteriorates rapidly when it is stored because it is full of organic matter such as soap, detergents, hair and skin particles. It’s also often warm. If it’s kept for too long, bacterial growth turns greywater into a slimy, smelly soup.

What can you use greywater for?

Greywater is not safe to drink, but can be used for toilet flushing (which amounts to around a third of domestic water use in the UK) and, with some common sense limitations, for garden watering.

How do greywater systems work?

There are three groups of simple greywater systems, based on how they get round the problem of bacterial growth.

  1. Direct reuse systems avoid the need to treat the water by using it as quickly as possible. This option uses the least energy and is usually the cheapest to install and maintain. An example of a direct reuse system is the Watergreen device, which is used to syphon water out of a bathtub into a hosepipe through an open window – hardly rocket science, but very effective.
  2. Short retension systems perform simple mechanical water treatment (usually skimming and settlement) before storing the water for a limited time, often in or above the cistern of a flush toilet. If the treated greywater isn’t used within the time limit the system empties itself and refills with mains water. This type of system is relatively inexpensive to fit and maintain.
  3. Basic physical and chemical systems filter the greywater before treating it with chemical agents such as chlorine to improve its storage quality. Take great care if you are considering this type of system; the environmental benefits are far from clear, payback times are often longer than the projected life of the product; maintanance costs are high and reliability is questionable.

Other options include biological systems (such as reed beds), which use bacteria to remove organic material from wastewater, or biomechanical systems. Both these options are more suitable for large-scale or communal projects because of the setup costs and high maintenance involved.

Greywater systems which are integrated into a mains-fed domestic plumbing system in the UK must comply with the Water Supply (Water Fittings) Regulations 1999, and should also follow the good practice guidelines produced by the Water Regulations Advisory Scheme (WRAS).

The pros of using greywater

  • Reduces the amount of fresh water you use
  • Reduces bills for both supply and sewerage if your property is metered
  • Unmetered properties may still be able to claim a small reduction on sewerage bills: contact your supply company for details
  • Decreases the amount of wastewater produced
  • More reliable than rainwater for irrigation

The cons of using greywater

  • Less cost effective than simply reducing the amount of water you use in the first place. If you’re really serious about getting your water usage down, you’ll need to do both
  • Greywater systems can be expensive and space-hungry unless you’re able to include them during construction or major refurbishment
  • The more complicated the system, the more likely it is to be unreliable
  • Greywater can’t be used on food plants
  • Prolonged use in one area can cause long-term damage to the soil

outside toilet, image

Using greywater in the garden

Greywater is an obvious choice for garden watering because most people have a plentiful supply. Greywater watering doesn’t rely on rainfall or vary much with seasons, and it reduces your reliance on potable (drinking quality) water, so using it reduces your water bills and your carbon footprint at the same time. Most gardening experts do not recommend using greywater near food plants, for a variety of reasons. If you’re considering using greywater extensively in your garden, we recommend that you read The New Create an Oasis with Greywater: choosing, building, and using greywater systems by Art Ludwig.

Because of the potential danger of inhaling bacteria associated with greywater, it should ideally be applied using a soaker hose under mulch. Avoid concentrating greywater in one area of your garden, as one of the potential impacts of greywater is increasing salt content. This has a long term impact on the soil structure in your garden and is very difficult to fix later on. Avoid putting too much in one place, and monitor the health of your plants and the state of the soil where you use greywater.


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