Other Methods of Water Purification

In comparison to chlorination, other methods of water purification include boiling water, reverse osmosis, UV light, solar radiation and ozone. They are briefly summarized below.

1. Boiling water

Boiling is what is commonly used to disinfect water from microorganisms. Bacteria and protozoa are killed at the first bubble, and it takes about three minutes to kill the rest. It is the preferred method of water disinfection in rural Africa. However, it requires lots of fuel, mainly in the form of firewood or charcoal. As these natural resources are rapidly depleting in many countries, this water treatment method is not sustainable.

For many households, boiling with fuel is a prohibitive option for treatment of water because of cost or lack of availability of fuel, respectively.

2. Reverse Osmosis

Reverse osmosis (RO) is a water purification technology that uses a semipermeable membrane to remove larger particles from drinking water. Reverse osmosis can remove many types of molecules and ions from solutions, including bacteria, and is used in both industrial processes and the production of potable water.

RO is most commonly known for its use in drinking water purification from seawater, removing the salt and other effluent materials from the water molecules. Those industrial scale desalination plants can purify large quantities of water at a very low cost, but they cost hundreds of millions of US dollars. There are also small RO units available in the market but they can only purify small quantities of water per hour. Therefore, this technology is too expensive for water purification at the Base of the Pyramid.

3. Ultraviolet Light

This method does not provide a chemical disinfectant residual to protect the water from recontamination or microbial regrowth after treatment, for instance when water is poured into dirt canisters which is the common procedure of fetching and storing water in rural Africa. Additionally, many microorganisms are only inactivated but not killed. When the exposure to UV light stops, these microorganisms can become active again. Therefore, disinfecting water with UVGI does not meet a basic requirement for being applied in rural Africa.

On the other hand, exposure to UV light would immobilize Giardia and Cryptosporidium protozoa which cannot be killed with chlorination technologies - which is an unsolved problem worldwide.

>This technology cannot be applied at the POU for technical, operational and financial reasons.

4. Solar Radiation

Solar water disinfection is a type of portable water purification that uses solar energy to make biologically-contaminated water safe to drink. The SODIS method uses a combination of UV light and increased temperature (solar thermal) for disinfecting water using only sunlight and PET plastic bottles. At a water temperature of about 30 °C, a threshold solar irradiance of at least 500 W/m² (all spectral light) is required for about 5 hours for SODIS to be efficient. SODIS is a free and effective method for decentralized water treatment, but it needs a lot of time and must be handled properly. Although the number of microorganisms will be significantly reduced, some of them remain active in the water making it safer to drink but not really 100% safe. There are also health concerns about micro particles that will be dissolved away from the plastic when heavily exposed to sunlight.

In the coastal regions of West Africa where direct sunlight is rare in most times of the year, this method is not an option.

5. Ozone

Ozonation is used by many Western countries. Ozone is an inorganic molecule with the chemical formula O3 which is quickly decaying into common oxygen O2. Disinfecting water with ozone is energy-intensive. It involves ozone being bubbled through the water, breaking down all parasites, bacteria, and all other harmful organic substances.

However, this method leaves no residual ozone to control contamination of the water after the process has been completed. Therefore, disinfecting water with ozone does not meet the requirement for being applied in rural Africa.