Prati’s Index (PI)

Prati’s Index -The need to determine the chemical, physical and biological characteristics of natural water resources is essential in view of their utilization particularly as a source of municipal water supply. The increased demand for water (WHO, 1966), as a consequence of population growth, agricultural and industrial development, has been accompanied almost everywhere by research oriented towards the definition of criteria and guides for surface water quality. In many countries the need for control of pollution has resulted in legislation and regulations (WHO, 1967) which have, however, been based on different criteria and different systems of classification as regards quality (LITWIN, 1965). Two main approaches seem m general to have been used. The first is based on the characteristics of effluents before their discharge into rivers; thus, by requiring a certain degree of treatment, both the enforcement regulations and the selection of type of treatment have been relatively simplified. The second approach, based on the quality to be maintained in rivers after dilution of effluents and thus taking into account the natural capacity of rivers for dispersion and self-purification, makes it more difficult to decide on the discharge of waste waters, particularly where several types of effluent are present in the same part of a river (MCKEE and WOLF, 1963).

In this last case, certain considerations by Kerri (1966, 1967) seem especially pertinent. On the basis of experience in the Ruhr river basin, he advocates discharge of effluents from treatment plants that are so designed as to fully utilize the dilution and self-purification capacity of rivers. The author suggests also an economic model facilitating the sharing of treatment costs, proportionately, among the various users according to their “polluting” capacity.

In the classification of river water quality, there are two main considerations, i.e. the use of the water and the concentrations of the pollutants discharged. The various classes are in any case determined by the concentrations, after dilution, of selected pollutants (or indices of pollution) according to a scale of proportion for the various levels of pollution. The application of this procedure, however, implies that the same polluting capacity is assumed for various pollutants (or selected indices) within a certain class, and no consideration is given to the cumulative polluting capacity of the various pollutants selected.

This note is a preliminary contribution towards the identification of an “implicit index of pollution” based on river water quality classification.

We have thus been looking for an index as a numerical expression of the degree of pollution, and which takes into account the various pollutants present at the same time (though measured separately). This index, increasing with the degree of pollution, could be used for the numerical evaluation of a purely qualitative characteristic expressed by the term “pollution”.

Our objective has therefore been to determine as many mathematical expressions as there are pollutants, to transform concentrations into levels of pollution expressed in new units which would then be the “units of measurement of pollution”.

These mathematical expressions are so constructed for each polluting factor, that their numerical value is no longer proportional to the concentration but to the “polluting effect” relative to other factors. In this way, although one pollutant may be present in smaller concentrations than another, it will have a larger number if its polluting effect is greater for example, on the basis of the standard values arbitrarily selected in TABLE l, in water containing l ppm of iron and l0 ppm of COD, the numerical value–representing polluting capacity–will be higher for iron than for COD (4-27 > l.).

It is obvious that the choice of the new unit of pollution is an arbitrary one since the evaluation of pollution is a relative proposition. However, it will have to represent coherently, as mentioned above, the relationship of the polluting capacity of the various contaminants selected. On this assumption, any pollutant or conventional index of pollution can be taken as a basis for reference and its scale (expressed as concentrations) used accordingly in a classification system, to arrive at the new numerical values as a measure of pollution.

It will be noted that no transformation through a mathematical expression will be needed for the polluting factor selected as referee. In fact, in the two classifications (the conventional and the new one), the values will remain the same, but for the difference that, while m the conventional classification these are expressed in concentrations (ppm, m 1-1 or other), in the new one they will be expressed as dimensionless numbers.

We then proceeded to transform the scales of concentrations of other pollutants (or parameters), according to the concept described above and taking into account their various polluting capacities, into new units related to the selected basic referee factor. The new proposed “unit of measure of pollution” can be assumed to be the arithmetic mean of the new values found for each pollutant or parameter.