Inorganic Chemicals and Metals
Industrial and municipal waste waters and mine run offs cause various metals and metallic compounds to be released into water. These pollutants also enter the water bodies as acid rain and their sulphate and nitrate ions make water acidic. Most inorganic compounds are toxic, particularly, those of heavy metals such as mercury, chromium, lead, silver, cadmium etc., which come from paint and plastic production, metal and metal plating units etc. These substances get attached to the tissues of aquatic organisms, produce physiological poisoning and are therefore, capable of killing living organisms in water bodies.
Chemical waste, such as mercury, bioaccumulates in the food chains. It is taken up through the gills of fish and concentrated in the flesh. It can be dangerous to eat large quantities of big ocean fish, contaminated by mercury. In humans, mercury causes neurological symptoms, birth defects and death. In one international incident, a village of Minimata Bay, Japan suffered mercury poisoning and 52 people died eating fish contaminated by a chemical plant.The main constituents of mine drainage are sulphuric acid and iron compounds. These cause corrosion of metals and concrete, and are fatal to fish.
Alkalies discharged by industries such as textiles, tanneries, paper, etc., can also destroy aquatic life.Another problem for local fish is polychlorinated biphenyls (PCB's) from electrical transformers. This impairs reproduction in aquatic species and has been detected in the flesh of striped bass. In recent years, the levels of fluorides in the drinking water has increased considerably in different parts of India, causing 'fluorosis', an incurable bone disease. The toxic elements that are commonly present in municipal and industrial waste along with their adverse effect are given below.
Toxic Elements Commonly Present in Municipal and Industrial Waste Waters
| Element | Sources | Effects |
|---|---|---|
| Fluorine (F ion) | Natural geological sources, industrial wastes. | Causes bone damage, mottled teeth. |
| Lead | Plumbing, mining coal, gasoline. | Causes anemia, kidney malfunction and nervous disorder. |
| Arsenic | Pesticides, chemical wastes, mining by product. | Enzyme inhibiter, carcinogenic. |
| Mercury | Pesticides, coal, industrial wastes. | Highly toxic. |
| Beryllium | Nuclear power and space industries, coal. | Toxic carcinogenic. |
| Boron | Industrial wastes, coal, detergent formulations. | Toxic to some plants. |
| Cadmium | Industrial discharge, metal plating, Ni-Cd batteries, mining waste. | Causes high blood pressure, kidney malfunction, anemia and disorder of bone marrow. |
| Chromium | Metal plating industries. | Cr (VI) is carcinogenic. |
| Zinc | Metal plating industries, industrial wastes. | Toxic to plants. |
| Copper | Metal plating industries, mining. | Toxic to plants and algae. |
Suspended solids or sediments
In water, suspended solids of sand, silt and minerals result mainly from soil erosion. Solid particles that settle in reservoirs and dams cause sedimentation; this reduces their water storage capacity. Desilting of water reservoirs like dams, local tanks and lakes is a major expense for Governments.
Suspended particles in water bodies also block the sunlight required by bottom vegetation for photosynthesis. This reduces the availability of food to fish and the supply of dissolved oxygen.Radioactive substances
Contamination of radioactive substances such as wastes of uranium and thorium can be carried into water from nuclear power plants, during their mining and refining processes. Scientific and medical institutions, which utilize radioactive materials can also contaminate water bodies. These substances may cause radioactivity in living organisms and produce harmful effects.
Testing of Nuclear capability for war or peaceful purposes in oceans damages marine eco systems. The use of oceans for nuclear testing has damaged much of marine life in these water zones and nuclear explosions in oceans are now banned under an international treaty.Thermal discharges
Thermal Pollution results from power plant and factory discharges of water used for cooling purposes. These raise the temperature of a body of water. Nuclear power plants use vast quantities of water for coolants, returning the water somewhat heated. The increased temperatures have several adverse effects on water.
Even a few degrees increase in temperature causes the solubility of oxygen in the water to decrease. This occurs because dissolution of a gas in liquid is exothermic. The rates at which chemical reactions occur increase considerably due to high temperature and this results in faster depletion of dissolved oxygen because water wastes get assimilated faster. Aquatic life gets affected as the seemingly harmless thermal pollution lowers dissolved oxygen and increases respiration rates, killing fish in a positive feedback cycle (ever increasingly). The density and viscosity of water also decrease with increase in temperature. This results in faster settling of suspended solids. The rate of evaporation also increases appreciably with increase in temperature. This results in greater wastage of water in the form of its vapor and so on.Oil
Oil and oil wastes enter rivers and other water bodies from different sources such as oil refineries, storage tanks, petro-chemical plants, industrial effluents, oil tanker spillage and automobile waste oil.
Normal tanker operations and spillage from ocean going oil tankers accidents cause marine pollution and offshore contamination. Since oil is non-polar and insoluble in water, it floats and spreads rapidly into a thin layer and is very difficult to clean. The oil layer on the surface of water reduces the DO levels in water as oxygen transfer from atmosphere is prevented. At sea, oil layer is responsible for the death of birds. The oil penetrates the bird feathers thereby affecting their insulation and buoyancy. The birds experience difficulty in floating and flying and thus die.In 1989 an oil tanker belonging to one of the worlds largest oil refining companies, the Exxon Valdez Oil, spilt its oil on the North American coastline. More than 1500 km of Alaskan shoreline were contaminated with 11 million gallons of oil, coating tens of thousands of sea birds and killing at least 1000 sea otters. An entire bird population and their nesting sites were seriously effected.
Detergents
Detergents are surface-active agents, which cause variety of water pollution problems.
- They tend to inhibit oxidation of organic substances present in wastewaters because they form a sort of envelope around them.
- They are resistant to the action of biological agents and thus do not undergo biodegradation. Their elimination from municipal wastewaters by the usual treatments becomes problematic.
- They tend to produce stable foam in rivers. These foams extend over several hundred meters of the river water and are the most undesirable effect of surfactants.
Problems
5. Based on their nature, what is an alternative way to classify substances, which cause water pollution?
Solution
Depending on the nature of pollutant, water is polluted with three kinds of pollutants:
Physical pollutants
Sand, pulp, wood clips, leaves, rubber pieces, dead animals, etc.
Chemical pollutants
Metal salts, fertilizers, pesticides, oils, dyes, drugs, detergents, chlorinated materials, hydrocarbons etc.
Biological pollutants
Algae, bacteria, germs, worms and viruses etc.
6. How and why do organic wastes cause water pollution?Solution
Aquatic life in water depends upon the presence of dissolved oxygen. Unless a minimum amount of the dissolved gas is present in lakes and rivers, fish and other aquatic life will die. Many pollutants in the water are biodegradable and some are non-biodegradable. Complex molecules, which are broken down into simpler units are called biodegradable. Bacteria digest the organic waste such as untreated sewage, nitrogenous chemicals from fertilizers, phosphates from detergents, using dissolved oxygen and liberating CO2. The liberated CO2 helps the growth of algae and seaweeds, further adding to the pollution. As the process continues, the amount of oxygen in water decreases affecting aquatic life. Hence, such organic wastes must not be allowed to flow into the lakes and rivers from the industries. A control on such industrial effluents can check water pollution.
Control of water pollution
Some of the steps that are helpful for the control of water pollution can be applied before the polluted water enters a river, lake or pool. We can limit our use of phosphate-containing detergents, treat sewage wastes with chlorine to kill bacteria, and ban or reduce the dumping of toxic substances into our waterways.
- Use waste water treatment techniques such as:
Techniques
Physical processes
Comprising of screening, sedimentation, flotation and filtration.
Chemical processes
Some common processes are precipitation, coagulation and disinfections.
Biological processes
These are biological filtration and activated sludge process. In particular cases, the processes such as carbon adsorption, oxidation and reduction, ion exchange, reverse osmosis, electro dialysis, etc., are also used.
Use of septic tanks
Septic tanks should be used for each house especially in small towns and villages. This will reduce the flow of raw municipal sewage and human excreta into river, lake or pool.
Rivers, lakes, etc., should not be used for washing purposes and bathing. In this way, water sources do not get polluted with detergents and germs. In case a water tank pond or lake gets acidified, it can be treated with a base like lime (CaO) to raise the pH.Avoiding excessive use of pesticides
These are highly toxic substances causing contamination both in agricultural use and at the manufacturing level, if waste is not safely disposed off. Those pesticides that are not biodegradable should especially be avoided.
Use of low grade or polluted water
Efforts should be made to increase the use of above water by treating and recycling it. Some examples are: treatment of domestic sewage for industrial cooling; recycling of water in mining and similar industries where the water availability is less. Use of sewage water for producing bio-fuels used in matchsticks and construction industries. These efforts will save the fresh water from be polluted.
Problem
7. How are industrial effluents from paper mills treated?
Solution
Effluents from paper mills, textile mills and leather industries contain many toxic substances such as lignin, chlorine, ammonia, heavy metal salts, acids, alkalies, etc. The polluted water needs to be suitably treated before discharging into river or running water. First, the solid and suspended particles are removed by placing a number of screens of various mesh sizes. Then the water is allowed to settle in a tank where coagulants such as alum, lime or ferric chloride are added to allow the sediments to settle. The water is then subjected to the activated sludge process. Water is allowed to flow through towers filled with crushed rock. A layer of activated sludge (a mass of active microorganisms) is added. As the water trickles down, the microorganisms eat up the organic waste. Finally the water is filtered and chlorinated before letting it out into river.
