In other words, sewage includes mostly biodegradable pollutants such as human faecal matter, animal wastes, and certain dissolved organic compounds (e.g., carbohydrates, urea, etc.) and inorganic salts such as nitrates and phosphates of detergents and sodium, potassium, calcium and chloride ions.
Under natural processes most of the biodegradable pollutants of sewage are rapidly decomposed, but, when they accumulate in large quantities, they create problem, i.e., when their out into environment exceeds the decomposition or dispersal capacity of the latter.
Most cities of well developed countries like USA, Britain, etc., and some cities of developing countries like India have evolved certain engineering systems, such as, septic tanks, oxidation ponds, filter beds, waste water treatment plants and municipal sewage treatments plants for the removal of many harmful bacteria and other microbes, organic wastes and other pollutants from the sewage, before it is tipped into river or sec.
Sewage treatment is usually performed in following three stages: (1) Primary treatment, which removes large objects and suspended undisclosed solids of raw sewage and converts them into a biologically inactive and aesthetically inoffensive state, the slugde, a valuable fertilizer. (2) Secondary treatment, which supplies aeration and bacterial action to decompose organic compounds into harmless substances such as sulphate and water.
During later stage of secondary treatment, whole waste water is chlorinated (i.e., treated with chlorine) to reduce its content of bacteria. (3) Tertiary treatment, which removes nitrates and phosphates and releases pure water. These three stages of sewage treatment have become increasingly expensive and only in most advanced countries all the three treatments of sewage are done.
However, most Indian cities either lack any sewage or waste-water treatment plant or have inadequate sewage treatment facilities. Consequently, normally and especially during heavy downpour and floods, raw sewage or incompletely treated sewage is dumped into rivers which cause severe water pollution problems in following ways:
(i) Bacterial and viral contamination. Sewage wastes may contain pathogenic bacteria and viruses which are a threat to human health. Waterborne disease such as typhoid, bacillary dysentery, amoebic dysentery, botulism, poliomyelitis, and hepatitis all represent potential health hazards in sewage-contaminated waters.
Due to such kinds of sewage pollution waters of many ponds, lakes, rivers, sea-beaches in India and abroad have been prohibited for human use, whether for drinking, bathing, swimming or other sort of water recreation.
(ii) Eutrophication. According to Hutchinson (1969), the Eutrophication is a natural process which literally means “well nourished or enriched. It is a natural state in many lakes and ponds which have a rich supply of nutrients, and it also occurs as part of the aging process in lakes, as nutrients accumulate through natural succession.
Eutrophication becomes excessive, however, when abnormally high amounts of nutrients from sewage, fertilizer, animal wastes and detergents, enter streams and lakes, causing excessive growth or ‘bloom’ of micro-organisms and aquatic vegetation.
Most secondary sewage treatment plants, though, precipitate solids and inactivate most bacteria in domestic sewage, yet they do not remove the basic nutrients such as ammonia, nitrogen, nitrates, nitrites and phosphates. These nutrients stimulate algal growth and lead to plankton blooms.
Some plankton blooms, particularly those of blue-green algae produce obnoxious odours and tastes in waters. Others, such as the din flagellate blooms or “the red tide” of southern coastal regions, produce toxic metabolic products which can result in major fish kills.
Plankton blooms of green algae do not always produce undesirable odours or toxic products, but still create problems of oxygen supply in the water while these blooms exist under abundant sunlight, they contribute oxygen to the water through photosynthesis, but under conditions of prolonged cloudiness, they begin to decay and consume more oxygen than they produce, leading to oxygen depletion in the water.
Further, bacterial decomposition of organic wastes too requires oxygen and with heavy loads the oxygen contents of the water may diminish below the point where most fish cannot survive. As the conditions in the water become anaerobic due to increased oxygen depletion by bacterial decomposition or planktonic blooms, the breakdown products become reduced rather than oxidized molecules, many of which (e.g., hydrogen sulphide) produce offensive odours and tastes.
Because of this relationship between organic pollutants and dissolved oxygen, the most widely measure of water pollution is the biochemical oxygen demand or BOD. This is a test of the amount of oxygen needed for bacteria to break down the organic matter in a water sample over a period of five days typical BOD value for raw sewage run from 200 to 400 mg of oxygen per liter of water (therefore, 200-400 ppm). Water for drinking should have a BOD less than 1.
Excessive nutrient levels in aquatic system can also cause two other kinds of ecological problems. Primarily, they may lead to extensive growth of aquatic weeds such as Eurasian milfoil, water hyacinth, water chestnut, etc. Excessive growth of these weeds can impair fishing, bathing, fish spawning, shell fish production, and even navigation (Sculthorpe, 1967).
Secondarily nitrates can be converted in the human digestive tract by certain bacteria to nitrites, and the same transition may occur in opened cans of food even if they are subsequently refrigerated. Nitrites react with haemoglobin, forming methaemoglobin which will not take up oxygen.
Labored breathing and occasional suffocation result most severely in human infants. Nitrites may also react with creatinine (present in the vertebrate muscles) to form nitrosarcosine which can be carcinogenic (see Simmons, 1974; Southwick, 1976).
Most of the Indian rivers and fresh-water streams are seriously polluted by industrial wastes or effluents) which come along waste waters of different industries such as petro-chemical complexes ; fertilizer factories ; oil refineries ; pulp, paper, textile, sugar and steel mills, tanneries, distilleries, coal wateriest, synthetic material plants for drugs, fibres, rubber, plastics, etc.
The industrial wastes of these industries and mills include metals (copper, zinc, lead, mercury, etc.), detergents, petroleum, acids, alkalies, phenols, carbamates, alcohols, cyanide, arsenic, chlorine and many other inorganic and organic toxicants.
All of these chemicals of industrial wastes are toxic to animals and may cause death or sub lethal pathology of the liver, kidneys, reproductive systems, respiratory systems, or nervous systems in both invertebrate and vertebrate aquatic animals (Wilbur, 1969).
Chlorine which is added to water to control growth of algae and bacteria in the cooling system of power station may persist in streams to cause mortality of plankton and fish. Heavy fish mortality in river Sone near Dehri-on-sone in Bihar is reported to cause by free chlorine content of the chemical wastes discharged by factories near Mirzapur in U.P.
Mercury like other heavy metals such as lead and cadmium has cropped up as a toxic agent of serious nature. Mercury, a byproduct of the production of vinyl-chloride, is used in many chemical industries and it is also a by-product of some incinerators, power plants, laboratories and even hospitals, (Aaronson, 1971).
In Japan, illness and even death occurred in the 1950s among fishermen who ingested fish, crabs, and shell-fish contaminated with methyl mercury from Japanese coastal industries. This mercury poisoning produced a crippling and often fatal disease called Minamata disease. Initial symptoms of Minamata disease included numbness of the limbs, lips, and tongue, impairment of motor control, deafness, and blurring of vision.
Table 25.3. Some Indian rivers and their major sources of pollution:
Name of the riverSources of pollution
1. Kali at Meerut (U.P.)Sugar mills; distilleries; paint, soap, rayon, silk, yarn, tin and glycerine industries.
2. Jamuna near DelhiD D.T factory, sewage, Indraprastha Power Station, Delhi.
3. Ganga at KanpurGute, chemical, metal and surgical industries; tanneries, textile mills and great bulk of domestic sewage of highly organic nature.
4. Gomti near Luck now (U.P.)Paper and pulp mills; sewage.
5. Dajora in Bareilly (U.P.)Synthetic rubber factories.
6. Damodar between Bokaro and PanchetFertilizers, fly ash from steel mills, suspended coal particles from washe- ries, and thermal power station.
7. Hoogly near CalcuttaPower stations ; paper pulp, jute, textiles, chemical mills, paint, varnishes, metal, steel, hydrogenated vegetable oils, rayon, and soap, match, shellac, and polythene industries and sewage
8. Sone at Dalmianagar (Bihar)Cement, pulp and paper mills.
9. Bhadra (Karnataka)Pulp, paper and steel industries.
10. Cooum, Adyar and Buckinghum canal (Madras)Domestic sewage, automobile workshops
11. Cauvery (Tamil Nadu)Sewage, tanneries, distilleries, paper and rayon mills.
12. GodavariPaper mills.
13. Siwan (Bihar)Paper, sulphur, cement, sugar mills.
14. Kulu (between Bombay and
Chemical factories, rayon mills and tanneries.
15. Suwao (in Balrampur)Sugar industries.
Cellular degeneration occurred in the cerebellum, midbrain, and cerebral cortex and this led to spasticity, ligidity, stupor and coma. In Japan in 1953, due to Minamata disease 17 persons died and 23 were become permanently disabled (see Southwick, 1976).
The toxic and pathological effects of some heavy metal water pollutants have been tabulated in Table 25.4.
Table 25.4. Pathological effects of heavy metal water pollutants on man:
MetalPathological effects on man
1. MercuryAbdominal pain, headache, diarrhoea, hemolysis, chest pain.
2. LeadAnaemia, vomiting, loss of appetite, convulsions, damage of brain, liver and kidney.
3. ArsenicDisturbed peripheral circulation, mental disturbance, liver cirrhosis, hyperkeratosis, lung cancer, ulcers in gastro-intestinal tract, kidney damage.
4. CadmiumDiarrhoea, growth retradition, bone deformation, kidney damage, testicular atrophy, anaemia, injury of central nervous system and liver, hypertension.
5. CopperHypertension, uremia, coma, sporadic fever.
6. BariumExcessive salivation, vomiting, diarrhoea, paralysis, colic pain.
7. ZincVomiting, renal damage, cramps.
8. SeleniumDamage of liver, kidney and spleen, fever, nervousness, vomiting, low blood pressure, blindness, and even death.
9. Hexavalent chromiumNephritis, gastro-intestinal ulceration, diseases in central nervous system, cancer.
10. CobaltDiarrhoea, low blood pressure, lung irritation, bone deformities, paralysis.
Various industrial processes may utilize water for cooling, and resultant warmed water has often been discharged into streams or lakes. Coal-oil-fired generators and atomic energy plants cause into large amounts of waste heat which is carried away as hot water and cause thermal pollution or calefaction (warming). Thermal pollution produces distinct changes in aquatic biota.
A body of water at 30-35°C is essentially a biological desert and many game fish require temperatures for successful reproduction, although they will survive above that temperature. A temperature rise of 10°C will double the rate of many chemical reactions and so the decay of the organic matter, the rusting of iron and the solution rate of salts are also accelerated by calefaction.
Since the rate of exchange of salts in organism’s increases, any toxins are liable to exert greater effects and temperature fluctuations are likely to affect organisms. Thermal pollution is thus can exert a disruptive effect on aquatic ecosystems.
A result of intensive agriculture, earth moving for construction projects, poor conservation practices and downpour with resultant floods, is the increased production of silt in stream and lakes this load of particulate matter down primary productivity by decreasing the depth of light penetration. Silt may also interrupt or prevent the reproduction of fish, by smooth the ring eggs laid on the bottom.
Estuarine and oceanic pollution:
Often oceans are considered as so vast that they are virtually unlimited in their ability to accomodate the waste products of human civilization. But there are substantial evidence to indicate global pollution of coastal waters and open oceans due to dumping of domestic and industrial wastes, sewage, oil drilling in coastal waters, spilling of oil tankers, etc.
The oceans have in fact; become the final settling basin for millions of tons of waste products from human activities. For example, in the late 1960s West Germany was dumping 375 tons of sulphuric acid, 750 tons of iron sulphate, 20 tons of chlorinated hydrocarbons and 16,000 tons of gypsum wastes into the North Sea and North Atlantic every day. Table 25.5 shows a few of the industrial and agricultural pollutants reaching the ocean annually in the mid 1970s.
Table 25.5. Examples of industrial and agricultural pollutants discharged annually into the world’s oceans (after Southwick, 1976):
PollutantsEstimated annual discharge, 1970-75 (in metric tons)Source
1. Petroleum and industrial hydrocarbons3,405,000Offshore wells, oil tankers, industrial wastes.
2. Hydrocarbons (airborne)15,000,000Vehicles, industries, power plants.
3. Airborne lead350,000Vehicles.
4. Mercury100,000Industrial operations.
5. AMrin-Toxaphene (converted to dieldrin)25,000Agricultural and public health operations.
6. Benzene hexachloride50,000„
8. Polychlorinated biphepols (PCRs)25,000Plastic industries.
This tremendous burden of pollutants is evidently affecting the health and integrity of the world’s oceans. Due to oceanic pollution the marine biota has been seriously affected. Cousteau (1974) reported a 30 to 40 per cent decline in the overall productivity of pelagic organisms from shrimps to whales in the past 25 years.
He also observed a serious shrinkage of coral reefs in many tropical areas of world, and a displacement of these rich and varied communities with turbid and relatively barren waters. There has occurred a decline in populations of many fishes due to oceanic pollution. Oil spills have killed water birds, mammals, fish and vegetation (Hunt, 1965).