MICROBIOTA OF SEWAGE TREATMENT PLANTS IN STREAMS 6 



of streams. Mother Nature is a banker, the amount and strength of the 

 sewage discharged to the stream are the debt, or liability, and the dissolved 

 oxygen and other factors of self-purification are the assets. If the demand 

 for oxygen is excessive in relation to that available, the dissolved oxygen 

 is depleted and the stream goes into bankruptcy. At the opposite extreme, 

 the stream may be so large in comparison to the amount of waste discharged 

 into it that the sag in dissolved oxygen may be scarcely perceptible. 



Let us consider a stream which has been polluted to a moderate degree 

 so that it could be classed somewhere between the two foregoing extremes. 

 Space will not permit a discussion of the physical, chemical and biochemical 

 factors of self-purification; however, it is within the province of this paper 

 to demonstrate the role of the microbiota of a stream in the amortization of 

 the pollution debt. Mother Nature is an exacting banker. She expects the 

 pollution debt to be paid off without any needless delay, but she is also re- 

 sourceMl in changing the composition of the microbiotic association to cope 

 with the situation. The rigorous conditions of the environment caused by 

 dumping sewage into the stream are met at first by eliminating the sensitive 

 species from the microbiota. The increase in turbidity is a factor in limiting 

 the growth of phytoplankton, and the deposition of putrescible sludge makes 

 the bottom of the stream untenable for the growth of many insect larvae and 

 other sensitive benthic organisms. As a rule, the zooplankters dominate 

 the polluted stretches. These include the bacteria-feeding ciliate and flagel- 

 late protozoans as well as the saprophytic forms. Because of the greatly 

 augmented supply of food present in the stream below the point of sewage 

 discharge, the population of these organisms is usually large. 



As the sewage solids settle to the bottom to form sludge deposits, the 

 normal association of benthic organisms changes from a large number of 

 genera and species to a few hardy genera whose numbers usually increase 

 prodigiously. For example, it is not unusual to observe aquatic annelids 

 (sludge worms) in such numbers on a slightly submerged sludge bank that 

 they resemble the nap of a thick rug. The importance of these biological 

 workmen and of other scavengers of the benthos in the process of self-puri- 

 fication of streams has been demonstrated repeatedly. 



When recovery progresses to the point where there is some clearing 

 of the water, algae and chlorophyll-bearing protozoans may be seen to thrive 

 in the fertile water, sometimes to the extent of producing a water bloom. 

 The stream returns to its normal condition when the fertility decreases below 

 the point required for the maintenance of the algal bloom. At the same 

 time, hosts of bacteria, after consuming nutriment which is available to 

 them in the diluted sewage, starve and perish as they move down stream. 

 In this manner, a stream purifies itself. 



Sewage treatment plants are often called "condensed streams" where 

 self-purification is carried on by a succession of different organisms, each 

 utilizing the waste products of the preceding species. In such a system, 

 one should be able to trace out the various zones of self-purification, namely: 

 Recent pollution, active decomposition and recovery. While it is true that 

 the development of processes of biological treatment of sewage was aided by 



