Grey Water Recycling – The Basics
Grey water is wastewater from showers, baths, washbasins, washing machines and kitchen sinks. You can collect it from some or all of these sources and, after treatment, use around the home for purposes that do not require drinking water quality such as toilet flushing or garden watering. There are two types of waste water produced in domestic homes; Sewage (black water) which contains human waste and other waste water (grey water). These are kept separate in the plumbing system of a house but are mixed together when entering the sewerage network.
Washing machines and kitchen sinks produce more contaminated grey water that is typically harder to reuse. It is not recommended that you use this grey water on vegetables, fruit trees, berry growing bushes or any plant in the food chain.
Despite the perception that it is always raining in this country, in some parts, such as the south east where the rainfall is lowest and the population highest, there is less water available per person than in many Mediterranean countries.
How Grey Water Recycling Systems Work
Grey water systems vary significantly in size and complexity. Most have the following common features:
- Storage tank for storing water once it has been treated
- Distribution system, including a pump for transporting the treated water to where it is needed
- A treatment process – grey water deteriorates in a couple of hours if untreated
Greywater contains organic matter such as skin particles, hair, soap and detergents. It is typically warm which allows bacteria to thrive which can cause bad odours and poor water quality, not to mention the potential health risk.
There are a several types of grey water recycling (GWR) systems that can be used depending on the situation and the amounts of water needed and available for recycling:
- Direct reuse systems (no treatment) – This type of system take the waste grey water, once cooled and uses it directly. Typically this will be for irrigating the garden or similar. They tend to be cheap and simple to use; some systems are siphons or hose that take water directly from a bath for use in the garden. Others use a valve to from the drainage pipe to collect grey water in a water butt so that it can bee cooled and used to irrigate the garden
- Short retention systems – These are systems that take the waste bath or shower water and conduct the bare minimum treatment on this, such as skimming debris off the surface and allowing larger particles to settle to the bottom of the tank . Being the simplest level of treatment they are relatively cheap to buy and run. The risk of equipment failure is less as expensive repairs (of more complex systems) is not needed. They can be located in the same room as the source of grey water, reducing the need for expensive, dual-network plumbing
- Basic physical and chemical systems – These systems use both a filter to remove debris from grey water before storing and chemical disinfectants (e.g. chlorine or bromine) to stop bacterial growth during storage
- Biological systems – These systems use bacteria to remove organic material. The bacteria digest the organic contamination, once oxygen is introduced to the system. There are different ways to introduce the oxygen; some use pumps to draw air through the water in storage tanks while others use plants to aerate the water. If there is a suitable amount of outside land, Reed beds can be created to treat the grey water
- Bio-mechanical systems – These use a combination of biological and physical treatment and are generally the most advanced. These tend to be complex ‘all-in-one’ units which treat and store water, and are very difficult to retrofit due to cost and the practical plumbing difficulties. Using both physical and biological treatment produces the highest quality water, but at a cost as it requires some energy to run. They are also expensive to purchase and operate although the maintenance costs are uncertain
- Integrated grey water/rainwater systems – These are used where either option cannot produce the sufficient amount of non-potable water needed. Careful calculation on the separate solutions is needed before opting for a combined system, and due to their complexity there are some things that need to be considered:
- Sizing is critical
- The guidelines for both grey water and rainwater need to be followed
- The overflow from an integrated system must be discharged into the foul sewer, from the point when grey water has been introduced, as only surface water is permitted to be discharged into water courses, surface water sewers or storm drains
The Grey Water Recycling Check List
This is a list of some of things that should be considered before embarking on a grey water project:
- Water Demand – simply how much water is needed for non-potable purposes and what it will be used for. This will determine the size and type of system that should be used
- The balance of supply and demand – The balance of shower use and toilet flushing must be right to makes it relatively effective. If for example, people are not at home during the day, they might create significant grey water from washing in the morning and evening, but there will not be the demand during the day from flushing the toilet as they are at work or similar
- Retro fitting – due to the complexity most systems are suited to installing in newly built homes and renovation projects, but maybe more difficult to retrofit
- Water quality – A high level of water quality may not be required if the treated grey water is restricted to use in an individual property for toilet flushing. Where stored grey water is treated to a high standard, there is potential for its use in other applications such as vehicle washing. A high standard of water quality may also be required in communal systems to overcome both real and perceived risks associated with using the treated water
The regulations that cover water quality depend on the water use. There are stringent standards that drinking water must meet, however there is no regulation of non-potable water. Even if there were regulations, enforcement would be very difficult.
Despite the absence of any formal regulation the British Standards Institute (BSI) has produced some guidelines for both grey water and rainwater reuse; the guidelines in BS 8525 have taken the standards included in the Bathing Water Directive.
There is also guidance which aims to prevent the cross contamination of drinking (potable) and non-potable water supplies in Guidance on Marking and Identification of Pipework for Reclaimed (Grey Water) Systems produced by the Water Regulations Advisory Scheme (WRAS) (WRAS 1999). Some key points include:
- Piping should be marked – with green and black banding
- There should be clear signage:
- label the incoming stop valve or other key points so that users are aware that a reused water system has been installed:
- Backflow prevention is required: ensure that a suitable fluid category 5 (air gap) backflow prevention has been incorporated into the appliance, to prevent contamination of the public mains water supply
Cost and Benefits of a Grey Water Recycling System
The costs of Grey Water recycling systems are highly variable which reflects the variety of systems that can be used. A simple short retention system will cost in the region of £1,000, where as more sophisticated bio-mechanical systems will cost from £3,000 upwards.
The size of the system and the non-potable water demand that they should satisfy will affect the design and therefore the cost.
Savings will be determined by the cost of the mains water that is being replaced and the amount. Typically these savings are not huge and as a result the payback period can be in excess of 50 years, which can be longer than the life of the system.
The key to running a grey water recycling system successfully is choosing the right system and making sure it is cheap to install and use. This is why generally the simplest systems are the most effective. Where there is a large non-potable water demand there is more likely a case for a more complex system.
In the future, domestic grey water recycling is likely to prove more viable because, firstly, more homes will move on to metered supplies, where water is charged by volume used as opposed to a flat rate. Secondly, it is likely that even in a country which is perceived as being as wet as this one, the pressure on water supplies will increase as both the population and per capita demand for water grow.
You might want to go to our video section on recycling to watch a video on various ways to recycle in the home.
All project content written and produced by Mike Edwards