Jae Lester of Rainwater Harvesting Ltd examines the options for a Sustainable Water Supply for the future
With changing weather patterns and urban growth stress is being placed on UK water supplies. Future building projects should have design criteria which takes into account Sustainable Water Management.
So what are the options? Water Authorities are working to improve the ageing pipework of mains supply and losses have been reduced to 22 per cent. However it is likely that demand will continue to outstrip supply before 2020.
Are there other avenues worth exploring? Non-potable applications including toilet flushing, washing machines and outdoor usage account for 50 per cent of mains water in a domestic home. In business properties this can rise to 85 per cent. There is no requirement to use drinking quality water to flush a toilet.
Therefore other viable alternatives for non-potable applications include rainwater harvesting and grey water recycling. Water is collected and used locally forming a decentralised management system with responsibility shared between authorities, developers and end users.
BS 8515:2009 sets standards for the design, installation, quality of water, maintenance and risk management of rainwater harvesting systems. It covers ‘where the main body of water is stored’ which must be watertight and so designed to resist microbial growth. The filtration system must be water and weather resistant and easily accessible. In domestic properties it is often located within the tank body.
Rainwater harvesting is simple. Water off the roof is collected via standard guttering and down pipes, filtered and stored in an underground tank. With no light or temperature variance the water remains cool and fresh. The filtration is often a simple mesh where the flow of water flushes over it removing any leaves and other debris into the soak away or storm drain. The water entering the harvesting tank is released into a lower shoe known as the calmed inlet. This prevents any turbulence so small particulate is not drawn up into the system. The harvesting tank volume is calculated on a number of factors: Geographical location, average rainfall, collection roof area and intended applications. It may not be practicable (or required) to collect off every part of the roof. For ease of install, down pipes that are situated close to the harvesting tank maybe all that is required. A typical three bedroom house would require a storage capacity of between 3,000 to 5,000 litres.
The stored volume assumes rain replenishment within a 21 day period. Should a critical low level be reached a small amount of mains water is introduced into the system but will automatically switch back to rainwater when available. Systems are designed to prevent any cross contamination and many are now WRAS approved so accepted by all authorities.
There are two types of systems-direct and gravity fed. In a direct feed system the pump runs every time a toilet is flushed. In a gravity system the water is pumped up into a header tank located in the loft space and appliances are serviced from there. The latter method only requires the pump to be operated when the header tank needs replenishing. This can make running costs as low as 1p/person/day.
Grey water recycling is water that has been recovered from the shower, bath and hand wash basin. The water is passed through a filtration and sterilisation process. Less water requires storing as the system does not rely on rainfall but does require greater ongoing maintenance and chemical costs. All Installations must comply with the Water Supply (Water Fittings) Regulations and the Building Regulations (parts G & H).
The advantage of rainwater is that it is a relatively simple process and it also has a further benefit regarding Sustainable Urban Drainage. During heavy rainstorms water is collected within the tank first preventing localised flooding. New emerging systems including RainActiv allow a percentage of water to be used for non-potable applications but also have a SUDS element. Water is released back into the storm drain at a controlled rate so reducing the risk of downstream flooding. In periods of heavy rain the system chokes off the release of water until the water table has receded.
Both rainwater and grey water should become of interest to architects,engineers and planners as they offer Decentralised Water Management. Planning applications increasingly require consideration of a SUDS plan and rainwater harvesting with its further benefits should be considered.