Water that has been standing in a storage tank can become very dirty and unsafe for use.
You need to try and prevent the harmful growth of bacteria, mould and algae because these micro-organisms can cause disease in plants, unsafe drinking water and a difficult-to-clean water tank.
Using a UV light is a simple and inexpensive way to keep the water in your tank clean by preventing the growth of harmful (and revolting!) substances. To get technical, it will inactivate approximately 90% of various micro-organisms.
It is important to use a UV light in conjunction with a filter, to remove debris before water enters the tank. The filter will remove the bigger particles and the UV will inactivate the micro-organisms.
UV works in the same way that sunlight works to kill bacteria. (Leaving water in strong sunlight for a few hours also kills most of the bacteria).
9 Interesting Facts around UV:
- Ultraviolet (UV) means "beyond violet", violet being the highest "visible" colour of the spectrum
- We cannot see UV light, thus it is termed "invisible" light
- UV light energy is found at 200 – 400 nano metres, between visible light and x-rays
- It performs "invisible radiation" and penetrates DNA in cells
- DNA in bacteria cells and other micro-organisms then becomes inactive
- UV will not remove particles in water
- UV will not inactivate / kill ALL micro-organisms – some moulds, protozoa and bacteria are resistant, but it will kill MOST of them (over 90%)
- UV is responsible for the formation of Vitamin D in animals with skeletons
- It is best used together with a water filter, as suspended particles can shield some of the micro-organisms from UV rays
- It is inexpensive and low maintenance, compared with chemical water treatment
- A combination of UV (to kill most of the bacteria) and final chemical treatment is often used by professionals
Sunlight in space at the top of Earth’s atmosphere is composed of about 50% infrared light, 40% visible light, and 10% ultraviolet light.
The UV Spectrum
UV-A, UV-B, and UV-C are all part of the ultraviolet light spectrum:
- UV-A results in skin tanning and can be used to treat certain skin disorders
- UV-B has a high penetrating ability and results in sunburn. Prolonged exposure is responsible for some types of skin disease, skin ageing and cataracts
- UV-C has extremely low penetrating ability and is nearly completely absorbed by the outer, dead layer of skin where it does little harm.
UV-C is the type of light used in the UV Sanitizer.
Rainwater that has been cleaned by the UV sterilizer can usually be used for irrigation purposes.
It is a good idea to install a filter at the point of entry to the water tank, to prevent debris from entering the tank.
UV will help to sanitize the water, by inactivating ± 90% of various micro-organisms. In most cases, further water purification efforts are not required. Rainwater harvested from plastic water storage tanks, such as JoJo tanks, for example, can be used for irrigation.
If used for drinking, the water should be treated by:.
- Boiling – bring the water to a rolling boil and then allow to cool
- Adding household bleach – add one to two teaspoons bleach e.g. Jik or Milton (depending on how cloudy the water is) to a twenty litre bucket of water and leave it to work for two hours
- Chlorine – add 60 ml (1/4 cup) to one tank (5000 litres) water
Please note that it is also important to filter the water (up to 2 microns) before entering the tank.
If there is high turbidity in the water and it is very cloudy, it is a good idea to have an analysis of the water quality performed before starting.
Ozone can be used to help correct high turbidity.
Chlorine treatment may be advisable in severe cases.
Cleaning the Water Tank
Water tanks should still be cleaned on a periodic basis, usually yearly; or if the water looks cloudy. (This will be a much easier job if you have a UV sanitizer.)
For how to clean a home water storage / JoJo tank, read the article How to Clean your Plastic Water Storage Tank.
For commercial water tanks, see Cleaning and disinfecting a water holding tank