The biological method uses biological filtration. One method is that 

 proposed by S.N. Skadovskiy--a method in which a cascade arrangement of 

 selected aquatic organisms in the water purifies it. In the upper portion 

 of the cascade large sections of the bottom are populated with filter 

 feeders such as freshwater mollusks (Unionids and Anodonts), which are cap- 

 able of filtering up to 2000 liters of water per day at a distribution 

 density of up to 70 individuals per square meter. These are followed by 

 water plants, which reduce the dissolved nitrogen in the water to a minimum. 

 Biological communities growing on surfaces as overgrowths will further re- 

 duce the nuisance organisms. Such communities reduce the number of phyto- 

 plankton cells by 60-70%, saprophytic bacteria by 70-80% and intestinal 

 bacilli by 30-50%. 



The mechanical method is based on various filter systems through which 

 the water is filtered and suspended particles are removed. During heavy 

 bloom, however, the filters quickly plug and they must be cleaned, sometimes 

 every 20-30 minutes. The efficiency in such filters fluctuates from 20-60%. 



The physical method of combatting bloom is based on the destruction of 

 algal cells using ultrasonics or an electric current. The shortcoming of 

 this method is the need for additional equipment to remove the slimy mass 

 that is obtained. 



The chemical method is most used for preventing blooms. To date, hun- 

 dreds of chemical compounds have been tested as algicides to suppress the 

 development of algae. Copper sulphate and chlorine are utilized most fre- 

 quently. Their effectiveness, however, depends on the acidity of the en- 

 vironment. The copper ion in copper sulphate is toxic for algae, therefore, 

 the effect of this ion depends on the concentration of hydrogen ions, i.e., 

 the lower the pH, the more toxic the ion becomes. Since pH is greater than 

 7 in the majority of water bodies, the effectiveness of this agent is con- 

 siderably reduced. Active complexing of copper ions with ligands in indus- 

 trial water supplies also often leads to a reduction in the toxicity of 

 copper sulphate. 



The chlorine in hypochlorous acid is toxic. Because of its strong oxi- 

 dizing efect, chlorine penetrates into plant cells and damages vital 

 centers. Chlorine concentration also depends on the pH of the solution and 

 it increases only with a reduced pH. 



Besides pH, the selection of algicides depends on the capacity of the 

 algae to adapt to the effect of the preparations. If only 2-3 species of 

 the 50-60 algae species encountered in a water body adapt, then difficulties 

 can occur for the industrial utilization of the water since these species 

 can cause a bloom and occur in huge numbers. The selection of different 

 chemical substances which are toxic to these specific organisms can be the 

 best solution. A collection of 2-3 algicides provides the possibility of 

 completely suppressing blooms in the water. However, in selecting algi- 

 cides, one must consider the features of the water body being treated. The 

 substance should possess toxicity with regard to the greatest number of 

 algal species, have the capacity to penetrate easily into vitally important 



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