Grey water is wastewater that has been used in clothes washers, bathtubs and showers, etc. These can be collected using separate drainage pipes, after which they are filtered and temporarily stored without treatment. A further step is the distribution in subsurface outdoor irrigation. Furthermore, another usage of grey water is for toilet flushing after purifying with a specific treatment using disinfectant.  The grey water is stored inside a building and is not immediately discharged due to the necessity to have enough volume before distribution through irrigation pipes.

A grey water harvesting design system is formed by different components and phases.

–          Filtration: after the grey water is collected, two filtration steps are necessary in order to remove all the particles in the grey water.

–          Sterilisation: it is necessary to sterilise filtered water to destroy all algae, viruses, bacteria and other organic contaminants. Different methods can be used for this purpose.

–          Harvested water storage:  the storage of the treated grey water is determined by its demand and usage for different purposes.


1 The BREEAM Assessment Scheme aims to reduce the consumption of potable water through the use of water recycling systems and water efficient components. The BREEAM Wat 01 calculator related to water consumption assigns at a certain percentage of improvement in the water consumption a number of BREEAM credits up to a maximum of 5. A 65% improvement represents an exemplary performance. Furthermore, the BREEAM provides other detailed sections that regard water monitoring (Wat 02), water leak detection & prevention (Wat 03) and water efficient equipment (Wat 04).

2 Category 2 of the Code for Sustainable Homes establishes a scale of credits (from 1 to 5) which correspond to the water consumption (litres/person/day). A consumption of ≤ 80 l/p/day corresponds to 5 credits – the maximum number of credits achievable in order to have a better performance.

3 Part G of the Building Regulations 2010 establishes that rain water and grey water can be used for WC, washing machines and irrigation but an appropriate risk assessment has to be carried out in order to avoid waste, misuse, undue consumption or contamination of wholesome water. Furthermore, the new dwellings have to achieve a water efficiency standard of 125 litres use of wholesome water per person per day.

Syntegra Consulting is a leading energy consulting company in the UK – for further information about how design grey water harvesting systems, please telephone  +44 (0) 845 0091625 or e-mail: