E 

 a. 



c 



2 X ALGAE- 



X ALGAE 



From Bartsch, 1958 

 L_ 



INFLUEtJCE OF QUANTITY OF ALGAE ON B.O.D. 

 AUGUST 26, 1955 



FiQure 12 



million cells per ml. The algae added to one por- 

 tion of the dilution water-sewage mixture were 

 killed by short exposure to heat at 70°C; the others 

 were living. B.O.D. values with living and dead 

 algae were compared with values obtained when 

 the medium contained sewage alone (Figure 13) . 

 Although oxygen demand with dead cells was a lit- 

 tle greater, it is believed that the ultimate first- 

 stage B.O.D. (Lg) in either case would have been 

 about the same . Under identical laboratory condi- 

 tions, when dead Chlorella cells were added to 

 samples in relative quantities of 1 , 10, and 66 

 (18,330, 183,300, and 1,222,000 cells/ml. re- 

 spectively), the resulting B.O.D. values were 

 3.21, 3 .73 , and 7 .74 ppm. respectively . On the 

 basis of the Chlorella tests , it appeared that the 

 contribution to the five-day, 20°C B.O.D. ap- 

 proximated .2 ppm. per billion cells . 



Some workers believe that dense populations 

 of algae in polluted waters are always beneficial 

 and desirable. This view undoubtedly is supported 



by the common observation of high dissolved oxy- 

 gen concentrations in their presence and failure to 

 note the nocturnal depressions. Concurrent rapid 

 production of a more stable mass of organics in the 

 form of algae from readily putrescible wastes in ef- 

 fect postpones satisfaction of the oxygen demand to 

 some later time at a different place. When condi- 

 tions are favorable, such postponement may be de- 

 sirable; when unfavorable, as in examples cited, 

 algae make existing pollution conditions even 

 worse. The oxygen released to the water in algal 

 photosynthesis is momentarily beneficial in spite 

 of the frequent inefficiency in its use . To evaluate 

 equitably the algal source of oxygen, the processes 

 by which algae along with the other biota also con- 

 sume oxygen must be included in the tally. 



68 



