220 Introduction to the Study of Science 



encourages their rapid growth. Algae, masses of thread-like 

 plants commonly called " pond-scum," flourish in still water, 

 if there is a sufficient food supply. Alga3, in themselves un- 

 desirable enough to demand instant removal, point to a still 

 more serious condition on the watershed or in the storage reser- 

 voir. The food of these plants consists of nitrogen compounds 

 or nitrates. Nitrates come from three sources : (1) from drain- 

 age from richly fertilized fields; (2) from decaying organic 

 matter, such as dead animals or plants, on the watershed or 

 in the reservoir ; and (3) from sewage or human waste products. 

 None of these has any place on the watershed or in the reser- 

 voir. The presence of alg83 may, therefore, be taken as a warn- 

 ing that a thorough examination should be made in order to 

 find and eliminate all sources of pollution from the collecting 

 and storage areas. 



89. The sand filter. The schematic section of a sand 

 filter (Fig. 65) shows more clearly than a description the 



essential features of its con- 

 struction. There are several 

 kinds in use, and all of them 

 are reasonably satisfactory. 

 Those of the type illustrated 

 remove, after still storage, 

 approximately 99 per cent of 

 the remaining bacteria and 

 other microorganisms. As 



don of a sand filter. Qf 



including all interest on original investment, operation, repairs, 

 supplies, and depreciation, is comparatively small, estimated at 

 about $1.00 per 100,000 gallons of water, it does not seem diffi- 

 cult to justify its use. Even where water is supposed to 'be pure, 

 as from a mountain stream, filtration minimizes the possibility 

 of accidental contamination. 



Such a filter as that illustrated will furnish water at a rate 

 of about two gallons an hour for each square foot. This is 



