A number of reasons have been given for the different stages of water treatment. The following are the more popular reasons. For surface water sources, the treatment technologies are largely based on coagulation and flocculation; and are basically to remove suspended material (turbidity) and color, and eliminate the pathogenic organisms that such water may contain.
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The objective of treating the groundwater supply is commonly to remove hardness and other minerals, as well as to eliminate the pathogenic organisms.
Water Treatment Processes
In general, the treatment of drinking water by municipal water systems involves a few processes:
Aeration: The water is mixed to liberate dissolved gases and to suspended particles in the water column.
Flocculation: The materials and particles present in drinking water (clay, organic material, metals, and microorganisms) are often quite small and so will not settle out from the water column without assistance.
To help the settling process along, “coagulating” compounds are added to the water, and suspended particles “stick” to these compounds and create large and heavy clumps of material.
Sedimentation: The water is left undisturbed to allow the heavy clumps of particles and coagulants to settle out. A typical sedimentation basin looks as represented in Figure 10.
Fig. 10: A Sedimentation Basin
Filtration: The water is run through a series of filters which trap and remove particles still remaining in the water column. Typically, beds of sand or charcoal are used to accomplish this task (Fig. 11)
Disinfection: The water, now largely free of particles and microorganisms, is treated to destroy any remaining disease- causing pathogens.
This is commonly done with chlorination (the same process used to eliminate pathogens in swimming pools), ozone, or ultraviolet radiation. The water is now safe to drink and is sent to pumping stations for distribution to homes and businesses.
Advanced Treatment
For water sources that are heavily contaminated, extensive chemical treatment and disinfection are expected. Examples of such advanced treatment are as follows:
1. UV Radiation
This type of water treatment uses a mercury arc lamp to kill pathogens in the water. Ultraviolet (UV) radiation kills most bacteria and some viruses, but is ineffective against cysts (such as Giardia) and worms.
Cloudy or turbid water can reduce the effectiveness of UV radiation. UV lamps should be replaced annually or as suggested by the manufacturer, as they become less effective with time.
2. Ozonization
Ozone occurs naturally in our atmosphere; in fact the ozone layer in our atmosphere protects us from ultraviolet radiation coming from the sun. In ozonization of water, electrically generated ozone kills bacteria and some other pathogens, and removes some pesticides.
In combination with an activated carbon or mechanical filter, ozonization oxidizes and precipitates out iron, sulfur, and manganese. Ozone does not produce any taste or odour in the water. Ozone generators are relatively expensive to install. Ozonization does not have any residual effect in the water, unlike chlorination.
3. Activated Carbon
Activated carbon filters absorb organic compounds and remove them from the water. These filters can remove volatile organic compounds, some pesticides, radon gas, hydrogen sulphide, mercury, and residual chlorine.
Activated carbon filters are often used in combination with other water treatments such as reverse osmosis, chlorination, and ozonization.
There are different types of activated carbon filters. Granular activated carbon (GAC) composed of loose granules of carbon, has some problems associated with their use. GAC filters accumulate the organic impurities they remove from the water, but these impurities can then become food for bacteria. Also, the filter can become saturated with organics, which are then released back into the water.
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Finally, channels can form between the granules in the filter, which reduces contact time between the water and carbon, resulting in less effective filtration. Solid block activated carbon filters (SBAC) are a solid compressed block of activated carbon.
In addition to removal of chemicals mentioned above, the carbon is so tightly compressed that it can filter out some cysts such as Giardia and Cryptosporidium. Because SBAC filters are so fine, they easily become plugged with particulate matter, and frequently need to be replaced.
They are also more expensive than granular activated carbon filters. Inadequately maintained carbon filters can become breeding grounds for bacteria, so the filters need to be kept clean and replaced as recommended by the manufacturer. If a carbon filter is unused for several days, run water through it for at least 30 seconds to flush any bacteria.