Treating surface water for human consumption

I want to buy pure water (sachet water). Sachet water is a common packaged water in Nigeria. I wonder how many of these packaged water is pure and safe drinking. You will take a packaged water and the distinct taste/odour tells you that the water is not pure.

The sustainable development goal number 6 looks to ensure the presence of potable water and sanitation for all. However this goal have not been met. Access to clean water is something that has eluded billions of people worldwide. It is estimated that 3 out of 10 people lacks safely managed water services.

Water that is not properly managed treated can contain pathogens and food chemicals as a result of certain human or animal activities. Numerous bacterial viral pathogens can survive in contaminated water thus posing health problems to anyone that consume the water. Escherichia coli O157:H7, Vibrio cholerae, Salmonella enterica, Campylobacter jejuni, are examples of pathogens found in contaminated water. They cause a wide range of diseases ranging from diarrhoea, cholera, vomiting, to nausea and so on.

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Water treatment is thus needed to control these diseases, therefore, limiting their transmission via human or animal. The main objective of water treatment is to provide water that is safe for human consumption, this means the water should be chemically and microbiological safe for human consumption. We treat water in order to remove the contaminants that may cause health problems and to remove the contaminants that may affect its aesthetic values which include taste, colour, and odour. Treating water involves many processes. The choice of the treatment process to be utilized depends on the source, nature, volume, and quality of the water.

A nearly clean water requires basic process to make the water wholesome for consumption. However, a very dirty water will require complex processes. It is just like trying to treat wastewater for consumption, it is not going to be an easy task.

In many communities, surface water serves as the main water source. Surface water is often contaminated by microorganisms due to man and animal activities. Defecating either by animals near surface water can transmit pathogens (if present) from the faeces into the water body either by erosion birds etc. Hence there is a need to treat water surface water before consumption by the public.

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Surface water treatment processes

Surface water is treated in a process that consists at least four to five steps.

First, the water is stored in a reservoir that is usually exposed to sunlight. This not only serves as storage but it also aids self-purification through the action of sunlight and sedimentation. The sunlight naturally reduces the pathogens in the water and at the same time allowing larger particles in the water to settle thus reducing the turbidity. In some surface water, such as rivers, storage and pre-sedimentation may not be required and an alternative such as pre-chlorination is used. It involves adding chlorine to the surface water before additional steps are followed. This reduces the number of faecal bacteria, pathogens, algae, and animals found in the water. The chlorine remaining in the water ensures that there are little or no microbial activities in the next treatment steps.

The partially treated water is then transferred to a basin and chemicals such as Ferric sulfate, Aluminium sulfate are added. Most of this suspended organic or inorganic material in the water will settle if given enough time. However, some particles are too small to settle on time, a particle 0.001 mm in diameter require about 55 hours to settle, thus the need for coagulants. Coagulants need to be added and the water is mixed slowly to increase the rate of the particles collide in a process known as flocculation.

Colloids do not settle in time due to the electric charges they carry. The coagulants neutralise the charges on the colloids thus facilitating the agglomeration of the suspended particles in the flocculation stage. A typical example of these is when Alum is added into water. The chemical reaction that occurs produce positive positively charged aluminium ions (cations). The positively charged aluminium ions will then bind to the negatively charged particles (anions). This reaction cause a reduction in the anionic colloids resulting in the formation of flocs. This flocs settle down in the basin thus reducing the concentration of suspended particles to be removed by filtration. This process is known as sedimentation.

The water is then passed through rapid sand filters to further purify it. Sand size ranging from 0.5-1.0mm in diameter traps the flocs and remove a large amount of the bacteria.

Slow sand filter can also be used to treat the water further to ensure it meets the standard. Sand with size ranging from 0.3-0.6mm with biofilm covering the surfaces is employed. When the water passes through the bed of sand, fine particles, microbes, viruses, and organic matters are filtered out.

After filtration, the water is disinfected. This usually involves chlorination, although ozone, ultraviolet irradiation can also be used. It is the last step in treating surface water. When chlorine is used, the chlorine dose must be present in an adequate quantity to leave residual/free chlorine. The free chlorine helps to prevent the growth of microorganisms that may have entered the water through other means such as distribution system. It provides another line of defense against these organisms. You may have noticed a bleach-like smell in your drinking water. You should know by now that there is no cause for alarm. It is the free chlorine that is there. It is particularly useful when the water is distributed over a long distance.

Chlorination efficacy depends on the degree of purity achieved in the previous treatment steps. A major concern in chlorination is the development of disinfection-by-products such as trihalomethanes, thus chlorine dosage should be carefully selected not only because of the by-products that are carcinogenic but because of satisfying the chemical demand of the water. Adequate time is necessary before the water is distributed to the consumer.

Following this steps will produce portable water suitable for consumption

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References

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Prescott's Microbiology: Ninth Edition. Joanne Willey, Linda Sherwood, Chris Woolverton Essential microbiology: Stuart Hogg