Potable water treatment
Potable water treatment

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Potable water treatment

5.7.2 Elimination of pathogens through solar disinfection

The lack of safe drinking water in many developing countries has prompted research into simple methods of disinfecting small quantities of water. One such investigation at the University of Beirut in the Lebanon revealed that 99.9% of total bacteria in a water sample could be destroyed by 300 minutes exposure to direct sunlight. In effect this means that if you left a sample of water in a translucent container, a lot of the bacteria in it would be killed.

Research to date has concentrated on transparent PET (polyethylene terephthalate) bottles, these being more robust than glass bottles and hence more practical for use in rural areas. It is important to first remove any particles in the water which may harbour or shield pathogens from the sunlight. Removal is effected by allowing any solids to settle out by sedimentation. It has been found that inactivation of pathogens is more effective if the water is fully oxygenated.

The following is a procedure which works well:

  1. Collect the raw water in a large jar and leave for about 12 hours, till the water appears clear. (Ideally, the turbidity should be reduced to below 30 NTU.) Pour the liquid above the residue (supernatant) through a piece of cotton cloth into a clean bucket.

  2. Obtain a clear plastic bottle and clean it and its lid with some safe (boiled) water. Paint half of it black. (An alternative is to have a black surface, e.g. a black bin bag or a piece of tyre, on which to lay the bottle.)

  3. Half fill the bottle with the clear water from (1) and put the lid on it. Shake the bottle vigorously for 30 seconds. This will ensure that oxygen from the headspace (the air space above the water) dissolves in the water.

  4. Fill the remaining half of the bottle with the clear water from (1).

  5. Lay the bottle on its side, and in such a position as to allow maximum sunlight to fall onto it. UV radiation from the sun reacts with the oxygen molecules in the water and, together with the heat from the sunlight, inactivates the pathogens. These pathogens in contaminated water sources are commonly viruses and bacteria, including Vibrio cholera.

  6. Leave the bottle in the sun for at least five hours. If the weather is cloudy, leave outside for two days.

  7. At the end of this period, the water should be safe for drinking.

The graph below (Figure 30) shows the decay rate of faecal coliforms with exposure to sunlight. The UV-A band (320–400 nm) of solar radiation is primarily responsible for the inactivation of the micro-organisms.

Figure 30
Figure 30 Elimination of faecal coliforms with UV-A radiation

UV-A radiation intensity on a sunny day in the tropics is generally 10–20 W m−2, while total solar radiation might be 500–800 W m−2.

Self-assessment question

Why is it necessary to have a black surface in step (2) above?

Answer

This will lead to an increase in the absorption of heat energy.

Self-assessment question

List the advantages and disadvantages of chlorine and ozone as disinfecting agents.

Answer

Chlorination

Advantages: cheaper than ozone; results in a residual effect to protect the water in the distribution system.

Disadvantages: does not kill all viruses; is not effective against spores and protozoa; can result in trihalomethanes being produced.

Ozonation

Advantages: acts in a short time and kills all bacteria, spores and viruses; reduces taste, colour and odour; does not produce trihalomethanes.

Disadvantages: The major one is that there is no residual germicidal effect, i.e. the water is not protected against subsequent contamination in the distribution system. Ozone can produce bromates if bromine is present, and these are toxic. Finally, ozone production requires complex equipment, and is expensive.

Self-assessment question

Which of the following would be applicable to MOGGOD systems?

A They are cheap and simple.

B They use gases produced off-site.

C The raw material is inexpensive.

D Since chlorine is produced, the same problems arise with disinfection by-products as can happen with conventional water treatment.

E None of the above.

Answer

C is correct.

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