BACTERIOLOGICAL EXAMINATION OF WATER 451 



is. Filters may be divided, roughly, into those for low and high pressure. 

 The former are directly connected with the water main, while the 

 others simply have the slight pressure of the column of water stand- 

 ing in the filter. Many high-pressure filters contain animal cha renal, 

 silicate*! carbon, etc., either in a pressed condition or in one porous in 

 These filters remove much of the deleterious matter from the suspected 

 waters, but the majority cannot be depended upon to remove all bacteria. 

 Even those which are equipped for self-cleansing become in a little while 

 foul, and, if not cleaned, unfit for use. The best of the class are of porous 

 stone, such as the Berkefeld and Pasteur filters. These yield a water, if 

 too great pressure is not used, almost absolutely free from bacteria, and if 

 they are frequently cleansed they are reliable. A large Berkefeld filter 

 will allow sixty gallons of water to pass per hour. The Pasteur filter 

 is more compact and slower. From the best Pasteur filters sterile water 

 may be passed for two to three weeks; from the Berkefeld usually only 

 a few days. A simple typical low-pressure filter is that of Bailey Den- 

 ton. The upper compartment contains the filtering material, which 

 may be sand or charcoal, and is fed from a cistern or hydrant. After 

 a certain cjuantity of water has passed in, the supply is automatically 

 cut off until the whole amount has filtered. A filter easily made is the 

 following: Take a large-sized earthenware pot and plug the hole in 

 the bottom with a cork, through which pass a short glass tube. Upon 

 the bottom place an inch of small pieces of broken flower-pot; upon 

 this a couple of inches of well-washed small gravel, and upon this six 

 to twelve inches of well-washed, fine, sharp sand. Cover the sand with 

 a piece of filter paper and hold this down with a few small stones. 

 Mount the pot on a tripod, and it is ready for use. The paper prevents 

 the sand being disturbed when water is added, and as it also holds 

 most of the sediment, this can be readily removed. Every few months 

 the sand can be washed and replaced. Animal charcoal is not a good 

 substance for permanent filters, as bacteria grow well in it. Whenever 

 water is suspected, and there is any doubt as to the filters, it should^be 

 boiled for ten minutes; this will destroy all bacteria. This precaution 

 should always be taken in the presence of typhoid fever and cholera 

 epidemics. 



The Disposal of Sewage. 



The disposal of sewage is becoming a vital question with all towns 

 and cities which are not situated near salt-water outlets, since the 

 present tendency in legislation is to compel such towns to dispose of 

 their waste so that it shall not be a menace to drinking-water streams, 

 destructive to fisheries, or a nuisance to harbors. 



Methods of sewage purification depends upon the character of the 

 sewage and the kind of effluent desired. 



Two hundred thousand gallons of crude sewage may be filtered 

 upon an acre of land daily and an effluent obtained which will com- 

 pare favorably in every way known to the chemist and bacteriologists 

 with the best "mountain springs. This is, however, a slow process and 



