Water Pollution and Sewage: An Overview (With Diagram)

This article provides an overview on water pollution and sewage.

Water is absolutely essential for the existence of life (animal or plant). It provides a wonderful chemical medium in which all biological and biochemical processes occur. Water dissolves various nutrients, distributes them to cells and removes waste products.

About 60% of the human body is composed of water. One can survive without food for some weeks, but cannot survive without water for more than a few days. Besides drinking, man requires water for bathing, washing, livestock raising, industrial, and agricultural and various other purposes.

Water-a scarce natural resource:

Water, covering more than 70% of the earth’s surface, exists in all three states-liquid, solid and gas. Most of the water on earth is salt water, and is unsuitable for drinking and other needs of man. It is estimated that only around 0.007% of the earth’s water is available for direct human consumption. Thus, consumable water is a scarce natural resource.

Water is required for development and progress of people, and therefore a nation. In fact historically, the places of availability of water were the major determinants for the settlement of people. This is because besides regular use, water is essential to raise the crops. It is an accepted fact that a community with good water supply has good growth, progress and prosperity. For the reasons stated above, it is well recognized world over that there is an urgent need for water conservation and management. The Governmental policies also support this concept.

Dissolved Oxygen:

Oxygen is soluble in water, and this dissolved oxygen (DO) is essential to support fish and other aquatic life in water. The solubility of atmospheric oxygen in fresh water is around 7-15 mg/dl, depending on the temperature and pressure. Increase in temperature and salts in water decreases O₂ solubility.

Oxygen gets dissolved in water by one or more of the following ways:

i. Direct entry from the atmosphere.

ii. By aquatic plant (algae) photosynthesis.

iii. Introduction by artificial means (aerators).

Importance of dissolved oxygen:

A minimum level of dissolved oxygen at a concentration around 4 mg/dl is considered to be necessary to support healthy aquatic life. If good aerobic conditions are not maintained, anaerobic microorganisms take over and cause harmful effects. And this is what happens in polluted waters, particularly from the industrial and municipal wastes. Determination of dissolved oxygen is very important in water pollution. This is mostly carried out by measuring biochemical oxygen demand (BOD), utilizing microorganisms for the oxidation of organic matter.

Water Pollution:

Several kinds of natural, and man-made activities (domestic, industrial, agricultural etc.) contribute to water pollution. Water pollution is characterized by certain observable disturbances in the normal properties and functions of fresh water. These include offensive odours, decrease in aquatic life (e.g. fishes), bad taste and unchecked growth of aquatic weeds.

Ground water pollution:

The ground water is generally considered to be safe and is useful for drinking, agricultural and industrial purposes. In fact, ground water is less prone to pollution. However, in recent years, it is recognized that contamination with fluoride, arsenic and nitrate in the ground water poses a serious threat to human health.

Surface water pollution:

The rivers, lakes and reservoirs are highly susceptible for pollution due to natural and man- made activates (industrial, domestic, agricultural etc.). The routes for the entry of pollutants include sewage outfalls, industrial outfalls, and outfalls from nuclear stations. Surface water pollution is a major threat for the survival of life itself. It is therefore necessary that regular monitoring of various routes of water contamination is done and effective protective measures are taken to minimise the pollution.

Nature of Water Pollutants:

There are a large number of water pollutants which may be in dissolved, suspended or colloidal state.

The pollutants may be broadly categorized as follows:

i. Organic pollutants

ii. Inorganic pollutants

iii. Microbiological pollutants

iv. Radioactive pollutants.

The salient features of these pollutants and the sources of their contribution are briefly described.

Organic Pollutants:

Organic compounds as such are absolutely essential for the existence of life. However, their entry into water causes pollution resulting in bad odour and unpleasant taste. In addition, some of the organic compounds may be toxic or even carcinogenic to humans.

Sources of organic pollutants:

There are several natural sources of organic pollutants. These include the decay of leaves, plants, and dead animals, besides the release of organic materials from aquatic plants and microorganisms. Several synthetic organic pollutants (e.g. pesticides, herbicides, ethyl-benzene) and synthetic volatile organic chemicals (e.g. carbon tetrachloride, tetrachloroethylene) pollute water. Certain organic pollutants such as dioxin and polychlorinated biphenyls (PCBs) are very toxic, besides being carcinogenic to humans.

Inorganic Pollutants:

In the natural fresh water (source may be ground or surface), there occurs varying concentrations of inorganic chemicals. Most of them, in general, are within the acceptable limits.

Sources of inorganic pollutants:

The industrial and agricultural outflows pollute water with inorganic compounds e.g. metal complexes, inorganic salts, mineral acids, trace elements. Among these, water pollution due to toxic metals assumes significance e.g. lead, mercury, cadmium, nickel, arsenic and aluminium.

Microbiological Pollutants:

A wide range of microorganisms (viruses, bacteria, protozoa, algae, helminths) contribute to water pollution. A majority of these organisms are harmless to humans. However, there are certain organisms that cause diseases, referred to as water borne diseases. Such organisms causing some form of sickness in humans are referred to as pathogens or pathogenic organisms. A selected list of pathogenic organisms and the corresponding diseases is given in Table 56.1.

Sources of pathogens:

The pathogens are mainly present in the waste water and for most of them human feces are the prime source of pollution. The human waste can pollute the water from sewage outfalls or from a flow of waste from ground (mostly from failed septic system). The best way to control water contamination with pathogenic organisms is to prevent the human wastes (particularly feces) from entering potable water.

Radioactive Pollutants:

Water may get polluted with radioactive materials arising from nuclear plants; radioisotopes used in medical, industrial and research purposes, besides the processing of ores to produce radioisotopes. The radioactive pollutants are carcinogenic e.g. uranium, radium, thorium.

Waste Water or Sewage:

Sewage is the liquid waste or waste arising mainly from domestic (residential, institutional, commercial) and industrial sources.

Composition of Sewage:

The actual composition of sewage depends on the source from which it comes. In general, about 97-99% of sewage is composed of water while the rest (1-3%) is solids. The most important organic, inorganic compounds, and living organisms found in waste water are listed below.

Organic—carbohydrates, fats, proteins, amino acids and urea, besides the products of their degradation.

Inorganic—sand, mud, mineral ash, mineral salts, lead, arsenic, mercury and cyanides.

Living organisms — bacteria, viruses, algae, fungi and protozoa. Some details on pathogenic organisms and the corresponding diseases are given in Table 56.1

Types of Sewage:

The waste water (sewage) is broadly categorized as weak, medium and strong, based on the composition (Table 56.2). It must however, be noted that the composition of sewage is highly variable which mostly depends on the local conditions. The sewage mainly consists of solids, organic and inorganic compounds of nitrogen and phosphorus, chlorides, grease and volatile organic compounds. In Table 56.2, the data on biochemical oxygen demand (BOD), chemical oxygen demand (COD) and total coliform for the three types of sewage are also given.