PLANKTON 



47 



cally no aftertaste. Measures to control algal growth 244 - 248 in reservoirs 

 include periodic chemical treatment (with algicides such as copper sul- 

 fate), removal of accumulations from the shores before decomposition 

 occurs, and restricting the food supply of the algae by removal of tree 

 branches, leaves, and other floating debris. 



While removal or eradication of algae is desirable in reservoirs, quite 

 the opposite is true in sewage disposal units. Oxidation ponds have been 

 widely used for secondary sewage treatment. As summarized by Oswald 

 and his colleagues, 234 oxidative sewage treatment is based upon the fact 

 that organic matter decomposes rapidly in water. When in solution, it 

 is readily available to microorganisms and their enzymes. These convert 

 complex organic compounds into simple substances such as carbon dioxide 

 and ammonia. Because oxygen is the ultimate hydrogen acceptor in these 

 rapid conversions, it may become so depleted that aerobic oxidation is 

 halted. Thus the activated sludge and the trickling filter processes are so 

 designed that atmospheric oxygen is forced into the liquid phase at an 

 accelerated rate. Algal photosynthesis offers a completely different method 

 of supplying oxygen. Here the availability of oxygen is independent of 

 the physical laws normally governing aeration from atmospheric sources. 

 Bodies of water containing green algae may attain supersaturation with 

 dissolved oxygen up to three or four times the normal maximum of water 

 in equilibrium with air. The organic waste material is changed by the 

 cyclic activity of algae and aerobic bacteria into more algal cells. These 

 are then disposed into natural streams or allowed to settle and then be 

 digested anaerobically as bottom sludge. The particular merits of this 

 method lie in the minimal odor, nuisance, and operating cost. It is there- 

 fore considered ideal for small communities situated in mild climates. 

 Particularly in Texas and Southern California has there been reliance on 

 the use of sewage lagoons and oxidation ponds for adequate and economi- 

 cal treatment of sewage. Algal purification of drinking water is also some- 

 times employed; on the other hand, uninvited algae often clog standard 

 sand filtration plants. 



As previously indicated (section on Culture), Renn 233 has recom- 

 mended that oxidation ponds be used not merely for sewage disposal, but 

 also to convert nitrogenous wastes into food protein. This can be accom- 

 plished by encouraging the growth of algae (especially Cblorella, Scene- 

 desmus, and Chlamydomonas) which yield a high quality protein. 



A relatively new problem in sewage disposal concerns atomic wastes. 251 

 Coopey 252 demonstrated radioactivity in all forms of life in the Columbia 



