One should give special attention to the following fact. The process 

 of decomposition of organic substances to complete mineralization occurs in 

 the presence of a toxic agent, but to accomplish this requires a great deal 

 of time. The shapes of the curves in Figures 2 and 3 indicates that sapro- 

 phytes and nitrifying agents were hardly suppressed in their activity under 

 the effect of pyror in the first test. The number of saprophytic organisms 

 is changed approximately along the same curve as the BOD, NO . and N0 3 . It 

 seems to me that this reflects the following phenomenon. Microorganisms 

 affected by a toxic agent die in a certain quantity. The more resistant 

 specimens remain and, after a certain length of time and a number of 

 generations, clones are produced which are resistant to pyror and which can 

 carry out biological oxidation and nitrification. But, for the formation 

 of a resistant clone, the greater the concentration of the toxic agent the 

 more time is required. 



One should note that a delay in the process of biological oxidation or 

 nitrification (first and second phases) by 10-20 days or more is in itself 

 an expression of pollution. If this occurs in a river in which the water 

 is flowing at an approximate rate of 3 km/hr, then the water is not purify- 

 ing (not breaking down organic substances) travels to a distance of 700- 

 1400 km from the emission source. This situation is intensified by the 

 fact that the processes of bacterial decomposition of organic substances 

 occurs in a strict sequence (BOD ^formation of N0 2 ^ formation of N0 3 ). 

 Therefore, according to the balanced equilibrium, the rate of pollutant 

 addition (P) must not be greater than the rate of decomposition (D). 



In conclusion, one can formulate the following basic positions on per- 

 missible levels of pollution: 



1. Different water uses permit different levels of water pollution. 

 The lowest levels a^e needed for drinking water supply and fisheries. 



2. Organic substances are broken down by different microorganisms in a 

 specific sequence. Toxic substances having an injurious effect on these 

 microorganisms suppress the processes of mineralization more strongly, the 

 higher the concentration. 



3. The maximum permissible emission (MPE) of pollutants into waters 

 must be limited by the permissible level of pollution (PLP) of a given 

 water at a given time. MPE, in turn, is limited by processes of self- 

 purification (D) in which many aquatic organisms, especially microorga- 

 nisms, participate. Their capability and the rate of decomposition of 

 pollutants (D) must be appropriate to the quality and quantity of pollu- 

 tants discharged. 



4. Among all of the chains mentioned the^e must be an equilibrium of 

 MPE = PLP = D. If MPE > D, then the water body will be polluted. 



5. One cannot make calculations of values for MPE and PLP without tak- 

 ing into account the peculiarities of the water body, the nature of the 

 pollutants and the season of the year. 



