Intro, fig. 51. Pollution and recovery in a stream. As the oxygen dissolved in the water decreases 

 (curve at left), so do certain microorganisms and, in turn, the insects and fishes that depend upon 

 them for food (Eliassen, 1952). 



the oxidation of organic matter by bacteria is hastened 

 because sunlight can penetrate the clearer water and 

 produce oxygen through the agency of the more 

 numerous algae. Spirogyra and Euglena are often 

 abundant in the plankton, and Chironomid larvae 

 occur in enormous numbers on the bottom. Finally, 

 and often within a surprisingly short distance, the 

 stream returns to normal with clear water, clean 

 bottom, abundant oxygen, and with the wide variety 

 of organisms described elsewhere. 



A similar pattern is seen when organic matter is 

 dumped into lakes, but the effect of water movement 

 is less evident and in smaller bodies of water the 

 chief agents for recovery of oxygen are the algae. 



Trickling filters. — The ability of natural waters to 

 purify themselves is limited. For example, a stream 

 loses its ability to absorb organic pollution if its 

 microorganisms use up the dissolved oxygen faster 

 than it can be replenished (Eliassen, 1952). The rate 

 at which the micropopulation uses up oxygen depends 

 primarily on the amount of organic matter in the water. 

 This establishes what is known as the biochemical 

 oxygen demand (B.O.D.). Various methods have been 

 devised to reduce the B.O.D. to a level that a stream 

 can absorb. Perhaps the commonest method is the 

 trickling filter (intro. fig. 52). The various steps in 

 the trickling filter process are as follows: After 

 preliminary grease removal and chemical treatment, 

 if necessary, the sewage is run through a primary 

 settling tank. There, solids are taken out and pumped 

 to a sludge digestion tank where anaerobic bacteria 



act to reduce the material and methane gas is 

 produced. After treatment the dried sludge is used 

 as fertilizer. Meanwhile the fluid sewage is sprayed 

 onto filter beds where it trickles between rocks of 

 1- to 3-inch diameter to a depth of 3 or 4 feet. The 

 surfaces of the rocks soon become coated with 

 zobgloea, an organic film containing millions of 

 bacteria, algae, and other microscopic organisms. 

 Oxidation (and hence treatment) of the sewage takes 

 place as it passes over the zobgloea. For efficient 

 operation the filter must be rich in zoogloea but 

 still open enough so that the liquid can pass through 

 at a high rate. Psychodid larvae play a decisive role 

 in this process. They occur in enormous numbers in 

 filter beds where they feed on the zoogloea. Too 

 many larvae scour the rocks excessively and reduce 

 the effectiveness of the treatment. On the other hand, 

 too few larvae result in excessive growth of the 

 zoogloea which clogs the filter and produces "ponds" 

 of standing water that may halt the entire process. 

 Larvae have been maintained at optimal levels in 

 recent years by careful application of DDT in wettable 

 form at dilutions of 5 parts per million. After treat- 

 ment in the filter the effluent may be run through a 

 secondary settling tank and chlorinated to kill 

 pathogenic bacteria. Finally it is discharged into 

 a stream or other body of water which has been found 

 capable of carrying the reduced but nevertheless 

 considerable organic load. 



Oxidation ponds or sewage treatment lagoons. — 

 Another method for the treatment of domestic and 



