environment magazine

… going grey is not black or white …

About the Author

Terry Nash is Managing Director of Freerain Ltd, and Director of the UK-Sustainable Development Association; he also helped to found the UK Rainwater Harvesting Association, serving both as Chairman and as a Director.

Following the driest winter and spring for many years, with a long dry summer forecast, and water levels in many reservoirs at record low-levels for the time of year, the introduction of water restrictions and hosepipe bans is being widely predicted – even for the farming industry.

Added to the well-known existing structural stresses on national water supplies, the anticipated effect on patterns of rainfall due to climate change, and the inexorable rise in population (up another 20-million by 2050 forecast by the Environment Agency), the need to reduce personal levels of water consumption becomes ever more urgent

This is reflected in The Code for Sustainable Homes (CSH) which at levels 5&6, due to apply to all new-build dwellings from 2016 onwards, require domestic water consumption to be reduced from the current national average of 150-litres per person per day down to 80-litres. The underlying assumption is that this will be achieved in part by greater use of water-efficient fixtures and appliances, and in part by substituting water from other sources for mains-water.

The 2010 edition of Part-G of The Building Regulations pave the way for meeting this requirement by permitting two types of water in domestic dwellings, namely “wholesome” (ie mains) water for bathing, cooking, dish-washing and drinking, and “non-wholesome” water for toilet-flushing, clothes-washing and the outside tap. The revised Regulations also go on helpfully to identify possible sources of non-wholesome water, of which “greywater” – typically the outflow from baths and showers – and harvested rainwater are the two most commonly available.

Of these, harvested rainwater is the most straightforward to use, requiring only suitable filtration and storage arrangements to make it suitable for non-potable use; due to contaminants and chemicals picked-up in typical initial use, greywater must either be used straight-away for restricted purposes such as toilet-flushing, or be processed to make it suitable for storage for use when required, and wider applications such as clothes washing and garden irrigation.

On many projects, therefore, harvesting rainwater would be the most cost-effective way of providing non-wholesome water in order to meet sustainability codes; at levels 5&6 of the CSH, for example, perhaps around 30-litres per person per day of substitution water might be required to meet Code requirements once economising methods have been exhausted (subject to project-specific calculations to be performed according to the Government’s national calculation methodology). This would require a roof area of about 20-m2 per occupant in an area with an average annual rainfall of 600-mm. Where these broad parameters are not available, then the need to consider providing a supply of water to substitute for mains-water from a greywater system comes firmly into play.

Greywater systems can range from relatively simple temporary holding tank systems that retain bath or shower outflows for a day or two for flushing the adjacent toilet, to more sophisticated systems that bring the water up to a standard that enables it to be stored for longer-term use and wider applications.

The ways in which the greywater is processed can vary widely depending upon the system being used, but typically might involve passing it through membrane under air pressure to hold back solid matter, bacteria and potential viruses. Such water would usually be up to the standard of EU guidelines for bathing water and would be suitable, say, for storing in the rainwater harvesting tank (if also fitted).

Greywater recycling overcomes the two main constraints of rainwater harvesting, namely the requirements for an adequate ratio of roof-size to the number of inhabitants, and the need for reasonably predictable and reasonably regular rainfall; it also benefits greatly from the excellent match (broadly 1:1) between the amount of greywater we each produce each day, and the amount of water we use for non-potable applications. On the debit side, however, all but the relatively simple greywater systems are significantly more complicated and expensive than their rainwater harvesting counterparts, and need much more maintenance; the “simple” systems themselves can also be intrusive in a modern space-constrained modern bathroom.

From the foregoing, the decision on when or how to employ greywater recycling to meet modern building codes is not straightforward or clear-cut, and must be considered alongside other water-related aspects of a project such as the need to manage surface water in the most cost-effective way. Arrangements also need to be compatible with the near-future requirement for SUDS arrangements to be adopted to ensure their long-term maintainability and efficiency.

Factors that need to be taken into account from the outset is the good match on a typical 3 or 4-bed home, between the non-wholesome water requirements of the inhabitants, and the likely annual harvested rainwater yield of the roof. Individual rainwater harvesting systems therefore get the surface-water management challenge off to a good start by having in place a source control that will in any case need to be maintained for its own primary use purposes.

Assuming a mixed development that includes smaller/higher-density accommodation that perhaps do not justify individual rainwater harvesting systems, communal system can instead be considered; as by definition these will be short of water, these can be fed from both their own roofs and the overflows from nearby single-property systems. These communal systems effectively provide the solution to part of the surface-water management requirement and, like their single-property counterparts, will also be maintained for their own reasons.

Taking this incremental approach to both water supply and surface-water management leaves only two decisions to be taken: to what extent does greywater recycling also need to be considered to meet CSH requirement?; and what attenuation requirement remains on a whole-site basis to meet surface-water management requirements? Insofar as the second question is concerned, the future norm will be to provide this attenuation in the form of site and amenity enhancing water-features, open to visual inspection and thus easily maintainable by the local SUDS Adoption Board.

In short, all projects should aim from their outset to take a fully integrated approach to water-supply and surface-water management requirements, perhaps regarding rainwater harvesting as the default means of providing substitute water to meet building codes, combining with greywater recycling to provide any shortfall, with any surplus being fed through to the site amenity water features to provide the necessary levels of attenuation for SUDS purposes.

For further information on environmental technologies see www.uk-sda.org