WaterWater constitutes a natural element indispensable for the development of life and human activities; it is difficult to imagine any type of activity in which is not used.

In our planet it covers 75% of its surface, but all the water is not in conditions apt for the human use. 97,5% of the water are salty, only 2,5% remainder is fresh water distributed in lakes, rivers, streams and dams; this minimum proportion is the one that we can use.

In order to satisfy different necessities the water is transformed into a resource. However not everyone has access to it. This happens by several reasons, from wich the unequal natural distribution of the water in the planet surface can be mentioned. This impossibility takes to situations of shortage, that does not have exclusively natural causes, but also social ones. This allows us to say that exists a strong relation between the possibility of supplying it and the development of society, because the greater the development is, greater is the capacity to obtain it and greater is the contamination.

The humanity requires water every time in greater amounts to make its activities. The greater water consumption is also due to the increase of agricultural practice of irrigation, to the great industrial development or the existence of consumption habits that, sometimes, imply their waste.

Multiple uses of water

Water consumption varies according to the type of activity for which it is used. The irrigation agriculture is the one that demands greater amount; followed by the industry and finally by consumption for domestic purposes.

In the case of agriculture, we must consider that by means of the artificial irrigation we manage to increase the food production. In the industrial process, the water is also essential: some industries use potable water to elaborate their products, whereas the majority uses it in their productive processes, like coolant or extender of effluents.

In the case of domestic consumption the use in personal hygiene we can name the washing of utensils, cooks, beverage, washing of cars, irrigation of gardens, etc.

At the present time, for example, agriculture represents more than 90% of the global continental fresh water consumption; the rest distributes between the industry and the domestic use. The problem of the distribution of the water with respect to the societies that consume it has generated a variety of technological answers. The old Romans constructed aqueducts and chain dumps.

The water pollution

The problem of the contamination of fresh waters is old known. One of the first historical testimonies constitutes the story about one of the ten plagues of Egypt (Exodus, 7, 14-25), in which the transformation in “blood” of the waters of the Nile river is described. This phenomenon was without a doubt due to the biological contamination produced by microorganisms. With the increase of the population and the sprouting of the industrial activity the pollution of rivers, lakes and underground waters increases constantly. The World-wide Organization of Health defines the pollution of fresh waters the following way: “It must be considered as polluted when its composition or its state is altered in such a way that no longer can be used in the same way that can be used in its natural state”.

According to the definition that the WHO gives for the contamination we must also consider the modifications of the physical, chemical and biological properties of the water, wich can make it lose their potability for daily consumption or its use for domestic activities, industrial, agricultural, etc., as well as the temperature changes caused by hot water emissions (thermal pollution).

In fact, there is a natural contamination originated by the rests of animals, vegetables, minerals and gaseous substances that dissolve gradually with water.

The organic materials, by means of natural biological processes of biodegradation in which aquatic decomposers take part (bacteria and fungi), are degraded to simpler substances. In these processes the amount of oxygen dissolved in the water is fundamental because the decomposers need it to live and to produce the biodegradation.

Water polluting substances.

There is a great number of polluting agents of water that can be classified in very different ways. A possibility generaly used is to group them in the following eight groups:

Pathogenic microorganisms.
They are the different types of bacteria, virus, protozoos and other organisms that transmit diseases like the cholera, typhus, diverse gastroenteritis, hepatitis, etc. In the developing countries the diseases produced by these pathogens are one of the most important reasons for premature death, mainly of children.

Normally these microbes arrive at the water in lees and other organic rest that produce the infected people. Because of this, a good index to measure the salubrity of waters is the number of present coliform bacteria in the water. The WHO (World-wide Organization of the Health) recommends that drinkable water must have 0 of colonies of coliform per 100 mililiter of water.

Organic remainders.
They are the set of organic remainders produced by the human beings, cattle, etc. They include lees and other materials that can be disturbed by aerobic bacteria, that is to say, in processes with oxygen consumption. When this type of remainders is in excess, the proliferation of bacteria exhausts oxygen, and no longer fishes and other beings can live in these waters which need oxygen. Good indexes to measure the contamination by organic remainders are the amount of dissolved oxygen, DO, in water, or the BDO (Biological Demand of Oxygen).

Inorganic chemical substances.
In this group are included toxic acids, salts and metals such as mercury and lead. If they are in high amounts they can cause serious damages to the living beings, diminish the agricultural yields and corrode the equipment that is used to work with the water.

Inorganic vegetal nutrients.
Nitrates and phosphates are soluble water substances that the plants need for their development, but if they are in excessive amount they can induce the disturbed growth of seaweed and other organisms causing the eutrophisation of waters. When these seaweed and other vegetables die, being decomposed by these microorganisms, oxygen is exhausted and the life becomes impossible of other alive beings. The result is a malodorous and unusable water.

Organic compounds.
Many organic molecules like petroleum, gasoline, plastics, plaguicides, dissolvents, detergents, etc…, finish in the water and remain, in some cases, long periods of time, because being products made by man, they have complex molecular structures difficult to degrade by microorganisms.

Suspended sediments and materials.
Many particles uprooted from the ground and dragged to waters, along with other materials that there are in suspension in waters, are, in terms of total mass, the greater source of contamination of the water. The turbidity that cause in the water makes difficult the life of some organisms, and the sediments which are accumulated destroy feeding sites or egg-laying of the fish, fills up lakes or marshes and obstructs channels, rivers and ports.

Radioactive substances.
Soluble radioactive isotopes can be present in the water and, sometimes, they can accumulate along the trophic chains, reaching concentrations considerably higher in some alive tissues that those that they had in the water.

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