For an evaluation of a possible permissible pollution level we must 

 make the balance between pollutants coming into a water body and the 

 capability to degrade them completely or render them harmless. One can ex- 

 press these relationships in a diagram in the form of a balanced equation: 



intake of pollutants (P) = decomposition (D) (self-purification) 



+ deposition in bottom sediments (S), or P = D + S. 



In order to prevent the bottom sediments from accumulating polluting 

 substances, we must permit only that quantity which can be decomposed 

 (P = D) in a unit of time. The capability of the water body to degrade 

 wastes plays a decisive role in this equilibrium. The higher the rate of 

 self-purification, the more pollutants that can be converted per unit of 

 time. Ideally, for maintaining water quality, it is necessary to achieve 

 an equality of P = D. In oligotrophic waters, such a balanced equality 

 essentially exists. In eutrophic waters, P > D and part of the organic sub- 

 stances are transferred to the bottom sediments. Cases where P < D are 

 not encountered in nature are difficult to find. The self-purification 

 capability of water can be increased several ways, primarily by increasing 

 the temperature and content of dissolved oxygen in the water (mixing, blow- 

 ing in air), and by selecting a complex of organisms. Of course, in- 

 creasing the assimilative capacity of a water body in a given time requires 

 an increase in the number of organisms which break down such toxicants 

 (phenols, hydrocarbons, etc.). An increase in the quantity of polluting 

 substances over the capability to break them down may result in an increase 

 in the number of saprophytic bacteria, fungi, protozoans and certain other 

 organisms, but assimilation is delayed and occurs after large community re- 

 organization. As a result, an accumulation of pollutants occurs in bottom 

 sediments and in the water mass which creates additional difficulties in 

 self-purification. Aquatic organisms play a decisive role in the equili- 

 brium of water. The more intensely they can convert the pollutants, the 

 cleaner the water will be and the larger the assimilative capacity of the 

 water body. Biological activity of aquatic organisms, in turn, depends on 

 living conditions. Toxicants introduced in any concentration will decrease 

 biological activity and with high concentrations completely suppress it. 

 Processes of growth, reproduction and effective conversion of the organism- 

 oxidizing agents will be affected by the toxic agents. The resulting 

 effect then will be determined by the nature of the toxicants and their 

 concentrations. Therefore, the permissible level of pollution (PLP) is 

 determined by the rate and degree of decomposition of the pollutant by the 

 aquatic organisms and they determine the maximum permissible emission (MPE) 

 of pollutants. A sequential connection of relationships reflecting a 

 balanced equilibrium is being established. Di agrammatical ly it can be ex- 

 pressed in the following way: 



complex of organisms ■*■ their biological activity "* PLP "* MPE. 



Man is primarily interested in maximum permissible emissions. Produc- 

 tion workers attempt to increase the size of MPE. This is economically 

 profitable and less troublesome. However, according to the feedback princi- 



