Water pollution is the contamination of water due to the addition of substances (i.e.,inorganic, organic, biological, radiological,etc.,) making it unfit for human use and growth of aquatic biota.
The pollutants belong to three categories, biological, chemical and physical. Biological water pollutants include various pathogens such as viruses, bacteria, protozoa, helminthes, algae. Chemical pollutants include organic wastes, organic biocides such as DDT, BHC and polychlorinated biphenyls. Inorganic chemicals include chemicals such as As, Pb,Cd, Ni, Hg, phosphates, nitrates, fluorides etc. Physical pollutants include hot water, oil spill etc.
Water Polluting Agents
Following are the main water polluting agents:
Household detergents are helpful for washing clothes and cleaning utensils. These are released into the drains which ultimately carry them to water reservoirs such as lakes, rivers, etc., thus polluting the water. The major constituents of these detergents include compounds of phosphate, nitrate, ammonium and alkyl benzene sulphonate (ABS).
Following are the effects of pollutants released in the water by detergents:
Alkyl Benzene sulphonate
It generally forms foam on water surface. ABS is non biodegradable and thus accumulates in water reservoirs. Higher concentration of ABS cause obstruction to aquatic organisms.
Inorganic phosphorus and nitrogen
Phosphate is most useful algal nutrient. It along with nitrogen stimulates algal growth and as a result the algae grows to cover almost the entire water surface. This excessive growth of algae is termed as water bloom. Algal blooms compete amongst the constituent algae for light and oxygen. At the same time algae releases toxins harmful for aquatic organisms. This causes the death of organisms resulting in the accumulation of organic matter. Water reservoirs rich in such organic matter are termed as eutrophic and the process is termed as eutrophication. The organic matter is decomposed by microorganisms leading to the scarcity of oxygen and more of carbon dioxide in water. This further causes the death of fish and aquatic animals. Lake Erie in USA is often given as an example of eutrophication.
Control measures of water pollution:
Following are some common methods used to control water pollution due to household detergents:
- Precipitation of phosphate by using lime, ferric chloride, calcium oxide, calcium hydroxide, alum or zirconium should be done.
- Replacement of non biodegradable alkyl benzene sulphonate by degradable linear alkyl sulphonates.
Sewage contains human and animal excreta, food residues, domestic wastes, soaps, detergents etc. It is always rich in bacteria and organic substances. The major part of the sewage consists of organic matter and biological pollutants. Besides this, organic phosphate releases inorganic phosphate which act as nutrient. Nitrogen occurring as pollutant is present as ammonical nitrogen. The other pollutants include suspended solids, heavy metals, cyanide, oil etc.
Water reservoir always maintain a balance between available oxygen and decomposition of organic matter. This capacity is lost due to addition of excessive sewage and other wastes. The recycling and self regulation of the system is disturbed. The rate of reoxygenation becomes slower than that of deoxygenation. At this time, the degree of pollution is measured by biochemical oxygen demand (BOD). It is the amount of oxygen which the microorganisms require for biological oxidation. Values of BOD are higher in polluted water. Lesser values of BOD indicate complete comparatively clean water. Polluted water is likely to cause water borne diseases such as typhoid, dysentery, gastro enteritis, hepatitis, etc.
There are two types of treatments of sewage; primary and secondary. The primary treatments include physical processes such as sedimentation, flotation, etc. Secondary treatment consists of use of microorganism in oxidation ponds or activated sludge process. Oxidation pond is a shallow water reservoir where human fecal matter is collected. It is rich in organic substances and stimulate growth of algae and bacteria.
Bacteria decompose the organic matter and release CO2 whichis used by algae in photosynthesis. Oxygen released during photosynthesis reduces water pollution. In this way in oxidation, ponds algae and bacteria show symboisis.
The processes occurring in oxidation ponds kill infectious bacteria leaving non pathogenic bacteria and harmless organic matter.
Reverse osmosis is a relatively new process which may help in recycling clean drinking water. In this process water moves from its region of lower concentration to its region of higher concentration i.e., against the concentration gradient. By this process ammonia, nitrates, phosphates and some other soluble substances could be separated from polluted water.
Several industries release their wastes into the drains which carry these effluents to water reservoirs such as lakes, rivers, etc.
The waste consists of the following:
1. Inert suspensions such as dust, coal, ash etc.
2. Poisons such as acids, alkalies, phenol, cyanides, mercury, copper, zinc, etc.
3. Inorganic reducing substances such as ferrous salts such as sulphides, sulphites etc.
5. Organic residues such as effluents from farms, tanneries from nuclear reactors, paper mills, sugar factories, etc.
Following are some of the effects of the metals occurring in polluted water:
- Mercury: Chlorine and caustic soda manufacturing industries release mercury. It reaches human beings through the food chain by aquatic organisms such as fish. It affects the nervous system causing madness leading to death. Japan’s Minamata bay is a famous example of disaster of eating mercury polluted fish causing large number of deaths.
- Lead: The outlet pipe carrying effluents release lead. It accumulates in animal tissues and damages them. Continuous accumulation of lead in men over a long period of time affects the gastrointestinal tract, neuro muscles and central nervous system.
- Copper and zinc: These metals collect in the members of mollusca and affect their growth, metabolism and reproduction.
- Cadmium and chromium: These metals accumulate in the body of marine organisms and cause their death.
- Fluoride pollution: Fluorine occurs in water, soil and air. It causes dental fluorosis. Use of fluoride containing water causes stiffening of bones, mottling of teeth, outward bending of legs from knees (i.e Knock-knee syndrome).
Industrial effluents also contains several other toxic chemicals. It is therefore important to treat industrial wastes to remove polluting ingredients through various processes specified by pollution control board. It is necessary to modify the manufacturing process to prevent formation of harmful wastes.