1078 FRESH-WATER BIOLOGY 



over the sand filter, which was built without cover as it was used 

 only during warm weather. After rapid percolation through the 

 sand it was collected in well ventilated under-drains. After the 

 water had passed through the sand the beds were allowed to stand 

 exposed to the air so that they themselves became well aerated. 

 This method almost completely did away with the obnoxious odors 

 that had previously existed in the water supply of the city. In this 

 instance the part played by aeration was very important as experi- 

 ments had shown that the water could not have been filtered satis- 

 factorily by the ordinary processes of slow sand filtration. 



Mechanical filtration is also sometimes employed for the treat- 

 ment of algae laden water. Used in connection with aeration this 

 method may prove reasonably satisfactory, but special care must be 

 given to maintaining conditions of aeration throughout the process. 

 Sand filters are capable of satisfactorily removing the algae and 

 their accompanying tastes and odors if the growths are not too 

 heavy. 



The presence of algae in water tends to clog both sand filters and 

 mechanical filters to an unusual extent and increases the loss of 

 head, and, therefore, shortens the period of service and in general 

 increases the cost of filtration. Where water is stored before 

 filtration, or where it passes through settling basins copper sul- 

 phate is sometimes used as an auxiliary process antecedent to 

 filtration. 



Self-Purification of Streams. Various microscopic organisms 

 play an important part in the self -purification of streams. It has 

 long been known that rivers polluted by sewage and other waste 

 substances regain their purity to a considerable extent during their 

 subsequent flow. Various influences combine to bring about this 

 result, such as the natural death of pathogenic bacteria in an un- 

 favorable environment, the effect of sunlight, sedimentation of 

 suspended matter, oxidation of organic matter brought about with 

 the assistance of bacteria, and, what is of interest here, by the effect 

 of microscopic organisms. The cycle of changes by which nitroge- 

 nous matter is broken down by bacterial action and by which the 

 bacteria are destroyed by protozoa and other larger organisms, 

 the protozoa being devoured by rotifers and Crustacea, and these in 



