UV Water Disinfection

The Basics of UV Water Disinfection:

This outlines the basic method for UV disinfection but minor
adaptations might be necessary due to local conditions:

1. Water should be placed in clear plastic or glass bottles with secure tops.  Plastic bottles are best
     because they are widely used in developing countries and often discarded as litter.  Plastic bottles
     are also safer for children to handle and transport   The bottles should be cleaned thoroughly and
     any labels removed.  Ideal bottle volume is 1 to 2 liters, any larger is not as effective
     because there is too much water for the sunlight to penetrate sufficiently.

2.  Water that is cloudy or turbid should not be used.  Large, dark print should be legible when held
      behind the filled bottles. Turbid water can be poured in another vessel and allowed to settle. 
      The clear water can then be poured into bottles for disinfection.  If the settling method does not
      remove the turbidity, the water can be filtered through cloth or clean sand.

3.   Aerated water facilitates disinfection so an inch of air should be left at the top of the bottles, the
      top secured tightly, and then the bottle should be shaken vigorously for 15-30 seconds.  The air
      makes the viruses and bacterias more water soluble and therefore more easily disinfected by UV.

4.   The bottles should then be placed in full sun. Good locations include on a roof or a piece of
      corrugated metal. The duration of exposure is dependent on the amount of UV that is available.
      Many of these factors are listed below. General time frames are 6 hrs for up to 50% cloud cover and
      2 days for 100% cloud cover. If the water temperature reaches 50 degrees Celsius only 1 hr is required.

Bottle selection:

While any clear container will work the type selected will have an impact on the methods efficiency. Polyethylene
Terephtalate or PET plastic containers are preferred to PolyVinyl Chloride or PVC containers. This is because PVC
containers have more UV-stabilisators than PET containers. To determine if a container is PET or PVC can be
relatively simple. PVC containers often have a blue tint and have a pungent smoke smell if burnt. PET containers
smell sweet when burnt  (we don't recommend burning plastics and inhaling the smoke).

Selected bottles should have a water depth of 6 - 10 cm, at higher depths UV intensity is greatly reduced. Bottles
should be selected that have high surface area to depth ratios.

The bottles must also be replaced periodically . Scratches to the bottle surface will reduce the UV transmittance of the
container. Also UV light causes photoproducts to form on the outer surface of plastic containers. These photoproducts
present no harm themselves but will reduce the amount of UV light transmitted to the water.

If bottles are not available clear plastic bags with good seals can be used effectively. Pans or plastic puddles can also
be used as a replacement for bottles.

Should I Use UV Disinfection for My Water? ...
Factors Affecting UV Intensity

1. How is the weather? - Cloud cover reduces UV intensity approximately the same as sunlight intensity, so if the clouds are
    blocking about half the full intensity of the sun, you should leave your bottles out twice as long.  If you live in a persistently
    rainy climate UV disinfection of water probably isn't for you.

2. What season of the year are you in? - UV intensities are typically greatest during summer and weakest during winter. 
    In winter months, leave the bottles in the sun 1.5 times longer than in the summer.

3. Where do you live? - UV intensities are greatest near the equator, so if you live by the arctic circle you might want to
    consider a different method for purifying your water.

4. What time of day is it? - UV intensity is greatest around midday, so bottles should be put out in the late morning and
    harvested late afternoon. 

5. Elevation - higher elevations receive more UV radiation due to thinner atmosphere, so if you are living on a volcano or
    on a mountain near the equator you are ready to go!

6. Ozone thickness - varies based on time of year and global position

Limits to the Process:

While the method described above works very well to remove bacteria and viruses from drinking water the
effectiveness of removal of some types of cysts and worms is still being studied. The process also does not remove
chemical contaminants, pesticides, or heavy metals. If these contaminants are present other treatment options
should be explored or alternate water sources should be found. It can not be stressed enough that highly turbid or
cloudy water will not be treated properly by this method.

More information about this bottle UV treatment method can be found at the following sites:

Solar Water Disinfection

A Water Treatment Process used at Household Level

The Global Research Institute  www.grilink.org
          UV Disinfection in Action - northern Brazil


Back to our Solar Water Treatment Page
Back to FW5770 Projects Page

This page composed by:
Tim Martin
John Lyons
March, 2005