Rainwater is just water, right? Although rainwater is one of the cleanest sources of water available, each drop forms when water condenses around an atmospheric particle of dust, clay, chemical or salt. Roof tops, the primary surface area for rainwater collection, also collect wind-borne dirt, moss, leaves, bird droppings and other debris, which is washed into your guttering and drainage system.
If you are investing in a rainwater harvesting system, an effective pre-tank filter mechanism is a must. Pre-tank filtration minimises unwanted debris from entering the rainwater water storage tank, which is essential for water quality. If silt and leaf debris are left to accumulate in the tank, they will begin to decompose, leading to unpleasant odours and discoloured water. Whilst this may not be a major issue for watering your garden, it would be highly undesirable for household use or even for washing the car.
Different Types Of Filter
Several types of filter are available, and are generally mechanical, requiring only the gravitational flow of water to operate. They are usually either located within the vertical rainwater downpipes or fitted below ground into the stormwater drain run. Better quality below ground units are fully self-contained and carry vehicle traffic.
Some types can also be installed within the storage tank itself, though this option does require careful consideration.* The most important feature for any filter is its efficiency; ideally it should remove particles greater than 750 microns (0.75mm), and the finer the mesh the cleaner the water will be. A good filter should be to some degree self-cleaning in order to maintain a good flow of filtered water to the tank whenever it rains.
Mesh gutter leaf-guard filters provide basic filtration by preventing leaves and other large debris entering the downpipe. These can be difficult to keep clean due to height, and are generally not necessary if good downpipes or inline filters are used.
Downpipe filters can be incorporated into vertical downpipes for ease of removal and cleaning. Such units contain a plastic or stainless steel mesh to filter out leaves, insects and other debris. The filtered water is directed towards the tank, whilst all debris falls on through the downpipe in the normal manner. Because these filters fit in to the vertical pipe, they are typically used where the tank is above ground, but generally are only suited to pipes up to 110mm diameter.
In-line filters are typically used below ground but some models can also be wall-mounted for use above ground. These are designed to fit in to a horizontal drainage run and may incorporate a vertical or near-horizontal mesh surface. All such filters have a difference in level between inlet and outlet, and this may be very shallow (c.50mm) or significantly more with some types. Generally, the less shallow the profile, the finer the degree of filtration. Vertical mesh filters are largely self-cleaning whilst some better models of horizontal filters have an inbuilt backwashing feature.
*In-tank filters are available with some tanks and are typically pre-fitted in the tank when purchased. Whilst this provides a neat solution, such filters are generally of a lower quality and may not be as simple to maintain as a separate pre-tank filter. It is important to be able to access the filter for maintenance so if the tank is more than about ½ metre below ground, this can be difficult.
Larger roof areas and sites collecting high volumes of water may require two or more filter units, depending on the design of the drainage system and the total surface areas. Remember to inspect and clean your filters regularly, about every three months. Even self-cleaning backwash filters should receive a yearly inspection.
It’s impossible to prevent 100% of fine silt or dust from entering your rainwater storage tank, but through effective filtration, build up is minimised to approximately 1-2 mm per year. Quality tanks include additional precautions such as a fine strainer on the pump inlet and overflow syphons which skim surface debris. All underground tanks should also have protection against back-flow from the drainage system, to comply with the British Standard. By investing in high quality rainwater filtration, tank cleaning is markedly reduced to ten years or more. This involves cleaning the tank with a pressure hose, or by pumping out silty deposits when the tank is almost empty.
Without effective filtration, bacteria and silt build quickly and you may incur costly maintenance, due to clogged valves and faulty pumps. Pre-tank filtration is vital because it determines the overall quality of the water in your tank. Neglect to include good filtration and you risk increased component replacement, or in the worst scenario, create a health hazard.
In summary, the important things to look for when choosing a filter are:
- Ease of maintenance
Ongoing Peace Of Mind
At Rainharvesting Systems, our extensive range of WISY® Vortex and downpipe filters, and our self-backwashing HYDRAPRO® filters are suitable for use with above and below ground tanks. So, whether harvested rainwater is being used for home or commercial appliances, flushing the toilet, or to keep your land green and pleasant, you can gain the maximum cost saving benefits from your rainwater harvesting system.
For more information please download our new Guide to Harvesting Rainwater. This free buyer’s guide contains all the information you need to make an informed purchase decision. Click here to claim your copy.