The Global Water Crisis: Our Inevitable Fate?

UV Tubes
The Solar Puddle
Solar Water Disinfection (SODIS)
The Science behind the Processes

The basis of all the UV purification methods is the high energy that UV light possess. This energy can not only be used to signigicantly raise the water temperature, but can also damage the DNA of the pathogenic microorganism and create free radicals that can oxidize cell membranes and cellular machinery. For more information about the chemical and biological processes that are at work please see: How Does UV Work?

UV Tubes

source

The UV tube consists of a mercury arch UV lamp that produces germicidal UV-C rays, incased in a chamber that allows for the controlled intake and outflow of water. The basic design concept is pictured above, but the actual system may vary in apperance dependingn on the material that is used to build the treatment chamber. Treatment chambers can be built out of local materials in order to minimize cost.

The UV lamp is normally a 15 Watt bulb that can either be powered by electricity or solar panels. The bulb emits light in the UV-C wavelength region, which is then concentrated on the untreated water that enters the chamber through the small inlet pipe. The UV-C light disinfects the water at a rate of 1L/minute, and the clean water is allowed to flow out of the treatment chamber through the outlet pipe.¹
source

Results from the Renewable and Appropriate Energy Laboratory (RAEL) at UC Berkeley (the developers of the UV tube) show that the UV tube is able to inactivate more than 90% of some disease causing bacteria, viruses, and protozoa like E. Coli, Gairdida, and Cryptosporidium.² The chart shows the percent inactivation of various pathogenic microorganism at different UV doses.

source:

Advantages
•Purifies water quickly and effectively
•Easy to operate
•Can be built with local materials

Disadvantages
•Only inactivates microbes, is not useful for removing chemicals, or clarifying the water
•Not Sustainable: In order to keep the UV tube working users have to keep buying replacement bulbs.
•Reduction in Efficiency: Just as light bulbs tend to go dimmer near before they burn out, near the end of its lifetime th eUV bulb will produce less UV-C rays, and will therefore give a lower UV dose to the water, reducing the number of organisms that the bulb can inactivate. The relatively high cost of the replacement bulbs may lead to people using the lamps even when they are at this limit.
•Expensive: Although $4 may not seem like a lot of money, for the people that need clean water the most it is actually a substantial part of their monthly income. The cost arises from several sources including the actual unit cost, the cost of electricity, and the cost of replacement bulbs.


source

For more information about the UV tube please see: UV Tube Project
Back to top

The Solar Puddle

source

The Solar Puddle is a solar water pasteurization method that utilizes the suns ultraviolet energy to kill pathogenic microbes. In 1864, French doctor Louis Pasteur found that one could kill disease causing microbes in liquids by heating the liquid to 55° C/131° F for several minutes. In the case of water, it has been found that heating water above 65° C/149° F for 6 minutes is needed to kill 99.999% of water-borne pathogens.

Figure (below): Solar Puddle design schematic. source

The solar puddle creates a greenhouse effect inside an hole in the ground, allowing one to heat water to the pasteurization temperature. As we can see in the design schematic below, the puddle consists of a water layer sandwiched between a solid insulation layer on the bottom and an air insulation layer on top. The air insulation is created out of two clear plastic sheets to allow for sunlight transmittion, while the topmost layer of the solid insulation is made out of a black plstic sheet, to allow for sunlight absorption. The plastic can be any material, ranging from expensive UV stablized plastics, to cheap polyethylene. The design of the system ensures that as the sun shines on the puddle solar energy is absorbed, heating the bottom layer, which then warms the water that is above it. Furthermore, the heat is slow to escape due to the air insulation above the water layer.³

Advantages
•Can purify large volumes of water
•Very effective in killing pathogenic organisms
•Non-labor intensive. Requires no additional energy.

Disadvantages
•Process is very slow
•Have to keep replacing plastic sheets
•Can only get rid of biological contaminants. Is not useful for clarifying the water, or removing pesticides or toxic metals.
•You need sun light for it to be effective
•Doesn’t work well for turbid (cloudy) water

For more information, please see: The Solar Puddle.
Back to top

Solar Water Disinfection (SODIS)

SODIS is another form of solar water pasteurization that is more cost effective than the solar puddle because it uses recycled plastic bottles as the reaction vessel instead of PET sheets that have to be purchased.

How do you use SODIS?

The SODIS process is diagrammed below:




source

Purification Steps⁴:
1) Clean the plastic bottles
2) Check tightness of cap
3) Fill bottles with rainwater
4) Place on corrugated tin
5) Let sit for six hours in sunlight. To increase energy absorption, the bottles can be placed on corrogated steel.
6) Water is now potable

There are many factors that affect whether SODIS is a suitable water purification technique. SODIS purification depends on sunlight, therefore regions that get a lot of sun, and little cloud cover, will be able to benefit from SODIS the most. The optimal usage region is between the 15°N/ S and 35°N/ S latitudes. Plastic bottles are the main materials needed for SODIS. Glass bottles can also be used, however they do not transmit UV-A rays as well as plastic bottles.⁴

To ensure maximum disinfection efficiency one must consider the type of plastic used and the shape of the vessel. Plastic bottles made of PET (polyethylene terephtalate) are better than plastic bottles made of PVC (polyvinyl chloride), because they don’t block out as much UV-A radiation. Secondly, the best vessels are shaped in such a way as to maximize the water surface area. Drink bottles are therefore not the best shape, which is why disinfection takes upto six hours. However, drink bottles are found every, and are therefore the cheapest and most practical solution.⁴

source
Advantages
•Low cost solution
•Very effective in killing pathogenic organisms
•Requres no additional energy

Disadvantages/Limitations
•Water has to be relatively clear for it to work.
•Weather has to be sunny.
•Doesn’t change the chemical quality of the water (purely kills biological pathogens)
•Not useful for large volumes of water

For more information, please see: SODIS

Back to top

Back to: On the Road to Cheap H2O