After rainwater is collected from a roof, it should be filtered before it is stored in a tank. This is to ensure that it is kept in the best possible condition, to avoid degradation of biological material, development of odours etc. The roof and gutters of a building are not a sterile environment – therefore rainwater will pick up moss, dirt, leaves, bird faeces etc. on its way to the tank.
It is therefore essential to install an adequate filtration system, and in some cases additional treatment, to ensure that the stored rainwater can be used effectively and safely.
Mechanical Pre-tank Filters
The most effective way of keeping rainwater clean is to prevent dirt and debris from entering the storage tank in the first place. To do this, most systems use a pre- or in-tank filter. These generally consist of a stainless steel element housed within a plastic body, though some inferior models are entirely plastic. The element is usually a stainless mesh or grill. The fineness of the mesh is measured in microns – the lower the micron
rating, the finer the degree of filtration. So for example, Wisy® Vortex filters have a mesh rating of 280 microns (0.28mm) which will remove all but the finest particles without the mesh becoming blocked.
Pre-tank or in-tank filters will require cleaning, some more regularly than others, depending on the design. Filters with a horizontal mesh are likely to require cleaning more often than a vertical one. There are some more advanced models that have an in-built cleaning function using jets of water. There are even some filters manufactured from titanium that require no cleaning at all. One advantage of a pre-tank filter over an in-tank version is not having to open up the tank just to clean the filter.
For most purposes, such as flushing toilets, washing clothes and simple garden watering, a good quality pre- or in-tank filter will be all that is required. However, in some situations, further filtration or treatment may be needed, depending on the demands of the system. For example, certain industrial processes may require water to be filtered to a microscopic level, or some uses such as vehicle washes will require the water to be sterilised.
A much finer degree of filtration can easily be achieved through the use of particulate filters. These days such filters are generally in the form of a sealed housing containing a cartridge or bag which traps very fine particles to provide an even cleaner standard of water. Mostly these require the cartridge or bag to be changed periodically when they begin to clog up, though some of the coarser grades can be washed and
re-used. One important point is that such filters require pressure in order to function effectively; they work by the water being forced through. Hence such filters are generally only found on pumped water supplies.
More advanced versions have a stainless steel element with a ‘backwash’ function – to enable easy cleaning; this can either be done manually or automatically using a timer control. These are extremely effective and reduce maintenance, but are expensive items so are generally only used on larger systems. Another advanced method is the use of membrane technology. Here the water is forced under very high pressure through a layer of material with microscopic holes that trap the particles. Again such devices are generally only used on larger commercial projects.
More traditional filters are made up of layers of gravel and sand, – at different levels of coarseness. Water passes through by gravity, with different size particles removed as the water passes through each level.
While mechanical filters are effective at removing particles from water, they cannot remove bacteria. This is not an issue if the water is to be used just for WC flushing, washing machine use and garden watering. However, if the rainwater is also destined for personal use; drinking, showering, etc. or if there is a risk of water vapour being ingested (e.g. spray irrigation at a garden centre or nursery), then disinfection is an essential additional step.
There are several methods of doing this, such as chemical dosing, Reverse Osmosis (RO) or Ultra-Violet light (UV). RO systems tend to be expensive and waste a lot of water, and dosing systems rely on using hazardous chemical (e.g. chlorine). So the most common method is UV disinfection, being both safe and reliable if properly maintained.
Ultraviolet disinfection works by sterilising water as it passes over a glass tube where it is exposed to ultraviolet light of a particular wavelength and intensity. This process instantly sterilises the water by destroying any microorganisms present. The UV lamp will need replacing annually, and is a simple unskilled task.
If rainwater is to be used for drinking (this is rare, but if no other water supply is available there may be no choice) further treatment may be recommended in the form of carbon filtration. By passing the water through carbon, taste and odour are significantly improved and also discolouration to a degree. This is the same technology as used in the common household water filter jugs and under-sink units. Carbon is also effective at removing chlorine and other volatile organic substances (VOCs).
A Granular Activated Carbon module can be added to any rainwater system, or used as a stand-alone unit.
Our Guide To Rainwater Harvesting Systems
Find out more about rain harvesting systems from a buyer’s perspective in our free Guide To Rainwater Harvesting Systems. The e-book contains sections on filtration, storage tank options, pumps and power sources, among other topics. Click here to download your copy.