pie, it destroys the structure and qualitative composition of complex 

 organism-decomposing agents which results in reorganization of the entire 

 community or its restructuring occurs simultaneously. Extinction, a de- 

 crease in numbers, or an increase of pollutant-tolerant organisms are the 

 limiting conditions for uncontrolled emission of polluting substances into 

 water bodies. While the quality of water for drinking purposes gets worse, 

 requests to discharge more pollutants increase. 



The necessity has arisen to scientifically substantiate the maximum per- 

 missible emission (MPE) of pollutants into surface waters. It seems to me 

 that the scientific basis should be a balanced equality between the per- 

 missible level of pollution and limits on the amount of discharge. 



The role of toxic agents in all of the processes of waste assimilation 

 is tremendous because toxicants have a great effect on life processes of 

 pollutant-decomposing organisms. Even saprophytic bacteria, as is seen in 

 Figure 1, cannot maintain necessary biological activity in the presence of 

 toxic agents and they themselves cannot provide initially the processes of 

 self-purification. One should keep in mind that the nitrifying organisms 

 are more sensitive to many toxicants than are the saprophytic bacteria. 

 They lose, or decrease their biological activity with concentrations of cer- 

 tain toxicants being 10-- 1 (JO times less than those affecting saprophytic 

 bacteria (Table 1 ) . 



Substances in concentrations indicated are not completely harmless for 

 bacteria which mineralize organic substances. The BOD and the rate of NO2 

 and N0 3 formation are somewhat smaller than in the control, but decreases 

 are less than 25% of the control. In order to achieve a control level, the 

 processes of mineralization are increased to 3--5 days and in the presence 

 of certain substances, a longer period is required. 



Along with T.S. Balabanova we carried out tests on the breakdown of or- 

 ganic substances by microorganisms in a medium containing pyror-70. 



A method of separate determination of BOD, N0 2 and N0 3 in closed con- 

 tainers was used in the first series of tests. The nutrient solution was 

 prepared from river water, adding glucose and peptone; (NH^ S0 4 of NaN0 2 

 were added for the nitrifying agents. They were incubated at 25 C. 



In a second series of tests, open aquarium containers were used with 8 

 liters of solution (the same as in the first series). The quantity of or- 

 ganic substance was increased to a COM (chemical oxygen minimum) of 45 — 50 

 mg/liter of O2. The addition of pyror-/0 somewhat increased the COM (with 

 300 mg/liter pyror per 10 mg 02/liter). Air was blown continuously through 

 the aquarium. The temperature was 18-20 C. After a certain time samples 

 were taken for BOD 5, NO2 and NO3. The results obtained are shown in the 

 graphs in Figures 2 and 3. 



Both in the closed containers (Figure 2) and in the open aquaria (Fig- 

 ure 3) the processes of decomposition of an organic substance were sup- 

 pressed by the toxic agent--pyror-70 (2-bromo-2-nitro-l ,3-propanediol ); the 

 degree of suppression was greater the higher the concentration of pyror. 



