How Can We Help?

Rainwater and Greywater in Buildings (2001)

You are here:
← All Topics

D Brewer R Brown G Stanfield


Rainwater and Greywater Use In Buildings is a series of documents resulting from research projects undertaken by CIRIA and BSRIA, collectively known as Buildings That Save Water (BTSW). There are three main outputs to the series:

Decision-making for Water Conservation provides a background to the use of rainwater and greywater and identifies the barriers and benefits of water re-use and recycling. It discusses the issues surrounding rainwater and greywater use, including regulation, water quality, hazards of operation, economics, design and selection of systems and installation. The document is aimed at water utilities, local authorities and environmental regulators. In addition the guidance is aimed at private individuals and house builders with an interest in conserving water resources.

Best Practice Guidance provides practical guidance on the specification and implementation of rainwater and greywater systems. The Guidance is based on the findings from demonstration sites, monitored throughout the BTSW project, consultation with stakeholders and contact with other sites not directly monitored by the project. The Guidance expands further the issues discussed inDecision-making for Water Conservation, by providing further detail and specific best practice recommendations for rainwater and greywater re-use. The Guidance is aimed at developers, building owners and their consultants and contractors. The Guidance will also be of interest to water utilities, local authorities, environmental regulators and house builders wishing further detail than that provided in Decision-making for Water Conservation.

Project Report and Case Studies details the monitoring of demonstration sites, with rainwater or greywater systems, in the UK. It provides a background to the systems monitored, their operational and maintenance requirements, system reliability and user perception. Details of the monitoring results for water savings, financial analysis and microbiological data are also given. In addition to this the document reports on the experience from other sites using rainwater or greywater systems. Experience gained from this part of the project was used to develop the Best Practice Guidance. This Report will be of use to those responsible for the direct installation or monitoring of rainwater or greywater systems and those who wish to see the underlying data and information captured from the rainwater and greywater systems monitored.

Training Pack complements the Decision-making for Water Conservation andBest Practice Guidance as a training aid. The pack is in the form of a Microsoft PowerPoint presentation with separate slides to illustrate various points.


The increasing demand for mains water and threat from global warming has put increasing pressure on our water resources. This has led to the adoption of water conservation measures to reduce mains water use. These measures include rainwater and greywater use. To date there are few rainwater and greywater systems in the UK and little independent evaluation of their operation and the water savings achieved. The Buildings That Save Water Project set out to investigate these systems by obtaining in-depth information from seven research sites and general contact with 30 active sites which utilised either rainwater or greywater.

Three of the sites used rainwater for drinking but most used rainwater only for toilet flushing. Disinfection is not necessary on rainwater systems which are well designed and maintained where the water is used for toilet flushing. Acceptable water quality is maintained provided debris does not enter the collection tank. Where disinfection was believed necessary (e.g. drinking supplies) UV disinfection was found to be effective, although it increased the operating cost of a system.

There are fewer greywater systems installed in the UK than rainwater systems. The majority serve single houses with a very few examples of multi-residential or larger scale systems. The supply of greywater is not dependent on rainfall so savings can be made even in times of drought. Disinfection of greywater is required as the proliferation of harmful bacteria could potentially pose a health hazard. Systems required periodic topping up of disinfectant and cleaning or replacement of filters so operating and maintenance costs were higher than for similarly sized rainwater systems.

Several of the domestic greywater systems investigated suffered persistent reliability problems related to poor installation. Another issue was the lack of instruction and feedback provided to users. They often did not know whether the system was operating or had failed. Larger greywater systems were better installed and benefited from professional management and maintenance arrangements.

There are no specific regulations for rainwater or greywater reuse but systems must adhere to relevant provisions of the Water Supply (Water Fittings) Regulations 1999. Of particular importance is the prevention of backflow to the mains makeup supply. Where storage is installed at or below ground level, measures for the prevention of backflow from the foul drain are also strongly recommended.

Rainwater and greywater use has been demonstrated to save mains water. The proportion saved is very site dependent with each site requiring individual evaluation. Rainwater and greywater systems can also save money however the initial outlay and operating costs make achieving a sensible payback (within 10 years) impracticable for the majority of small scale systems using current technology. The majority of systems investigated were therefore installed by people or organisations who were environmentally committed, or for research purposes, with the direct economic benefits possibly less important than the environmental ones. In some situations however, extraneous factors e.g. local constraints on drainage, can provide a positive economic argument for rainwater or greywater use even with current technology.

Any major unplanned maintenance, such as a pump replacement, can outweigh the financial savings from reduced water charges so a high standard of reliability is essential to achieving a net saving on operating costs as well as safe operation of the system.