UV (or Ultra-Violet) disinfection is the most common method of treating water to ensure that it meets rigorous EU bathing water standards and the UK standard for safe drinking water. It is frequently used when water is extracted from a borehole or stream, and sometimes also for rainwater. By treating the water before use, risks of the ill effects caused by bacteria and viruses are effectively eliminated. UV rays are harmful (hence why we need to protect our skin from strong sunshine) and in the right dose and intensity, they have the ability to kill microorganisms by destroying DNA.
How Does A UV Filter Work?
In water treatment systems, UV rays are generated by a UV lamp. The light that is emitted has a particular intensity such that it provides a certain dosage of radiation to the water passing over it. The water is passed slowly over a glass tube that holds the UV lamp, so that the light is shining directly on to the water whilst preventing the water from touching the lamp. It is important that the rate of water flow is not too fast because that would mean the UV lamp would not be capable of doing the job for which it is intended. In other words, the water would receive insufficient dosage to destroy any organisms present. For this reason, the UV unit must have sufficient flow capacity for the particular application. Where the water is being pumped, then the flow rate of the UV unit should match the pump’s flow rate.
UV units are almost always accompanied by one or more ultra-fine sediment filters. Their purpose is to remove all particles larger then 5 microns (0.05mm) prior to the water entering the UV unit. This ensures that there are no particles large enough for an organism to ‘hide’ behind and avoid coming in to contact with the light rays.
UV lamps are designed to a British Standard that ensures they have a minimum lifespan of 8760 hours whilst still operating at 98% efficiency. The lamp should therefore be replaced on an annual basis to ensure optimum performance.
Why Would We Need To Disinfect Rain Water?
The vast majority of rainwater harvesting systems that we install here in the UK are not intended to provide potable drinking water. If you think about the roof of a house where the water is harvested from, you will realise that birds and small animals probably contaminate the source. The water is perfectly good for tasks such as flushing toilets, irrigating a garden, and washing the car, but cannot be deemed reliably safe for personal use.
There are generally two scenarios that might justify installing a UV filter:
- Raising the water quality to bathing and drinking quality
- Treatment of water from a private supply
UV disinfection means the harvested rain water is safe for bathing and drinking although it should be noted that UV treatment only removes biological hazards and not chemical ones.
Remote rural dwellings are sometimes off grid as regards mains water supply. It is common for the water supply to be sourced from an underground spring or a well. In these cases, UV treatment is a sound precaution to protect a family’s drinking water.
Why Don’t You Make Rainwater Drinkable Too As A Matter Of Course?
The simple answer is that for most domestic properties it is not possible to collect enough water to meet total household requirements. And as the quality of mains water in this country is excellent, it makes sense to use the untreated rainwater for flushing toilets, laundry and irrigation etc. where cleanliness is more important than purity. Harvesting rainwater for flushing toilets, for example, replaces about 25% of a typical home’s water consumption and that is a very worthwhile contribution to the environment.
Installing a UV filter and a fine particle pre-filter would add considerably to the cost of the average domestic rainwater harvesting system, so unless this can be justified by significantly higher water saving. The reality is that unless you live in a house with a very large roof area, and / or live in a very wet part of the country, the cost generally outweighs the benefit. Commercial properties are different of course, and frequently have much larger roof areas from which to collect water. Even so, there is little point in treating water if it is simply used for flushing toilets.
Typical UV Filter Units
You can browse through the range of UV filters that we supply, as well as particulate filters for pre-UV treatment. We are always happy to discuss any special requirements that you may be considering and we specialise in designing customised solutions. Feel free to contact us today and learn how we can help to add an environmental selling point to properties through the addition of a rainwater harvesting system.
