Water treatment
WATER POLLUTION is caused by the release of nitrates and phosphates (NO3-, PO43- and SO42) from untreated sewage, detergents and fertilizers into water bodies such as lakes, ponds and rivers. These result in the rapid growth of plants and algae which use up dissolved oxygen as they decompose. This results in oxygen depletion and death of aquatic organisms. This process is called eutrophication.
WATER TREATMENT
To obtain clean, potable water, many dissolved substances and microorganisms must be removed. This involves a series of steps.
1. AERATION
Water is stirred to allow oxygen to mix into it. Iron – ions are removed in this way as they precipitate out as iron hydroxide. This gives the water a brown colour.
2. SEDIMENTATION
Large particles are allowed to settle in sedimentation tanks. Tiny particles that cannot settle (clay) are removed by reacting with aluminium sulphate. The Al3+ ions react with the clay particles by coagulating the clay, that is, form clumps, which settle out of the water. The sludge at the bottom of the tanks is collected and dumped. Calcium hydroxide (lime) is usually added at this stage to control the acidity of the water. Lime is alkaline and raises the pH of the water.
3. FILTRATION
The water is passed through beds of sand and gravel to remove all solid particles. Adding carbon slurry helps to get rid of substances that can give the water an unpleasant odour or taste.
4. CHLORINATION
Chlorine is added to kill bacteria in the water. This helps to prevent diseases. Ozone or UV light can also be used. To remove excess chlorine, sulphur dioxide is added to get rid of the chlorine smell and taste.
ADDITIONS
Fluoride can be added to the water to prevent teeth from decaying. The pH of the water may also be adjusted to 7 before it reaches the tap, as chlorine is acidic.
ATTEMPT THE FOLLOWING QUESTION FOR PRACTICE QUESTION 1
(a) Construct a flow chart to show the processes involved in the purification of water, from its source (river or streams) to homes. Include equations where possible.
(b) Explain the importance of the chlorination process?
(c) Why is lime added in this purification process?
QUESTION 2
Two problems of hard water are scale and scum, as shown in the pictures below. (a) Explain the difference between scale and scum.
(b) Explain how hard water can be made soft using an ion-exchange column and washing soda.