Background
A rainwater harvesting system (sometimes wrongly called a grey water system) can be defined as a system which collects roof water for use as non-potable water. Much is written on the Internet and talked about with rainwater harvesting (recycling) systems about the use of ultra-violet sterilisation as a means of sterilising the water. Typically a rainwater harvesting system will have a physical "leaf and grit" type filter or filters. This is perfectly good enough to clean the water for uses such as WC flushing (accounts for 25% of daily water consumption), washing machines and outside taps. It is possible to upgrade the water to fully drinking (potable) standard. A good upgrade should also include a fine filter prior to the UV unit. The filter needs to be reducing particles smaller than 5 microns. This is because of the "shadowing" effect. This means that should the particles be greater than this, then bacteria and pathogens can be shielded from the UV light source. This finer filtration is often in the form of carbon filtering, and usually in sealed purpose made cartridges.
Potable upgrade or not?
So, it is possible to upgrade the collected water from a rainwater harvesting system, but is it actually worth it? From the paragraph above, you can extra equipment is needed to the rainwater recycling system. This can cost from around eight hundred pounds. With consumble carbon cartridges and a mains power operated ultra violet sterilisation unit, these have a running cost of around two hundred pounds per annum. Which in some circumstances is perfectly reasonable. For example, if a dwelling is off-grid by the remoteness of the site, then a private water supply is very desirable and a running cost like this is very affordable. But where mains water is freely available, and the householder is looking for savings or to be more environmentally responsible, then perhaps this type of upgrade is not worthwhile. As the manufacture of the UV bulbs, equipment and carbon filters contributes to industrial environmental damage. The extra power to use the system and the fact that mains water is already available, makes for a strong environmental case against upgrade, but using a rainwater harvesting system to supplement mains water and in most cases halve the amount used is a much more compelling case.
Quick calculation
Let's take a 150M2 roof in Surrey, 3 bedroom house with 4 people living in it.
The property might collect around 95M3 of water per year. The demand that could be fufilled by rainwater would be around 66M3 per year. If this project was to be upgraded to fully potable (drinking) standard, then the property would be using around 132M3. So, in this case there wouldn't be enough water to meet the demand. So, depending on which type of rainwater recycling system was installed mains water might be re-treated. This is because a direct pressure system tops up the external storage tank with mains water at times of low rainfall (or high demand). Making the idea of installing the potable upgrade not worthwhile.
Commercial rainwater harvesting system
Many commercial specifications include UV sterilisation units, this is not due regulations, but more because the client "decides" it's the thing to do. Often encouraged by industry professionals, using their experience of hot water systems. Legionnaires' disease is often mentioned, but this is only a problem where the water is held at temperatures over 25 oC. Rainwater stored underground is very unlikely to ever reach those levels. So it's not a problem, but in a £30M project a few hundred pounds is a very easy belt and braces approach for specifiers.
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