a) describe the importance of the water cycle b) discuss the methods of water purification (include desalination via reverse osmosis) c) discuss the importance of dissolved oxygen to aquatic life d) discuss the sources of water pollution (include nitrates, phosphates, heavy metals (lead and mercury), cyanides, trace metals, pesticides, herbicides, petroleum reside, suspended particles) e) assess the impact of the pollutants in part d) above on the aquatic environment
Module 3 Water & its importance
Water Cycle
Water cycle is essential for the continued supply of water on the earth’s surface. Our bodies are made of about 65% water and thus all metabolic activities require water to be present. We cannot easily make MORE water, so we must depend on the water cycle for our present supply of it.
Water Purification
In many cases, the water obtained from nature is not pure enough for our needs, whether it is drinking or bathing or even for industrial processes.
Hence a series of purification steps must be conducted in order for the water to be used for our various purposes.
Please note that desalination can also be considered a process of water purification via the use of reverse osmosis

Flocculation
Water from raw water reservoirs (natural sources) is collected in large tanks/basins for a sufficient time period to permit large particulate matter to settle down at the bottom. This material is removed and then the water is treated with flocculants such aluminium sulphate which form a floc that precipitates and carriers with it microorganisms on the surface.
Suspended organic matter settle onto the bottom of the tanks/basins. In this ways most substances that impart turbidity to water get coagulated.
Sedimentation
The water, after coagulation, is left in settling basin further for sufficient period to allow sedimentation of remaining materials. Sedimentation however considerably reduces microbial population of the water aside from removing most of the suspended particles.
Filtration
After sedimentation, water is subjected to sand filters to remove flocks of living organisms. The process of filtration is highly critical and important as it can remove protozoan cysts and also about 98-99% of bacteria from water. The water may also be filtered through activated charcoal to remove potentially toxic organic compounds and organic compounds that impart undesirable colour and/or taste to the water.
Disinfection
Disinfection is the final step is municipal water purification and it ensures that no pathogenic microorganisms are carried through water. For water supplies of small towns and localities sodium or calcium hypochlorite (NaOCl or CaOCl
2 respectively) may be used to disinfect water, but for larger cities, however, chloroamine (as opposed to chlorine) is now the method for disinfection.

Importance of Dissolved Oxygen
All aquatic life depend on the amount of dissolved oxygen (DO) present. This allows all aquatic life to conduct their metabolic processes essential to life. High levels of DO (above 90%) indicate “healthy” water bodies. DO can also be related indirectly to biological oxygen demand (BOD) , the higher the DO, the lower the
BOD. Once DO levels begin to drop, this usually indicates some form of pollution is present in the water body.
Sources of water pollution
Pollutant
Nitrates
Phosphates
Source
Manmade fertilisers
Effect on environment
Drinking water that gets contaminated with nitrates can prove fatal especially to infants that drink formula milk as it restricts the amount of oxygen that reaches the brain causing the ‘blue baby’ syndrome as well as eutrophication.
Eutrophication
Heavy metals (e.g. lead and mercury)
Cyanides
Trace metals
Pesticides and herbicides
Manmade fertilisers and detergents
Use of lead pipes causes dissolved lead ions to form in the water, improper disposal of mercury from industrial processes
Improper disposal after used in making fabrics mining waste and tailings, landfills, or hazardous waste dumps.
Run off from backyards, farms and golf courses
Lead affects the nervous system and can ultimately lead to death. Mercury stunts physical development of organisms
Acts as a poison
Hazardous effects on nervous systems and physical development
Accumulate up the food chain and cause impairment of physical development of aquatic life
Similar to pesticides Petroleum residues From underground storage tanks
Suspended particles Industrial processes e.g. cement manufacture
Respiratory problems, covers wide area in fine dust

Practice Questions
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