BACTEEIOLOGY OF WATEE AND SEWAGE 445 



stitute the ideal solution for the disposal of the sewage and 

 waste from the country home. 



The City Problem. — Rural communities are in some 

 respects quite as much interested in the disposal of sewage 

 from the cities as are the cities themselves. There is a gen- 

 eral tendency wherever possible for cities to empty their 

 unpurified sewage directly into streams or lakes or into the 

 ocean. This pollution of natural waters, of course, interests 

 those who live upon farms, through whose pastures the 

 streams pass, or which border upon lake, bays, etc. which 

 may be heavily polluted. It should be noted that in Amer- 

 ica at least, city sewage is practically universally water 

 carried. In general it is diluted when it is emptied into 

 streams. In some cases it is impounded in large reservoirs 

 and there treated with certain chemicals, such as alum, or 

 with acids which precipitate the solid materials as sludge. 

 This latter is eventually dried and may be used for ferti- 

 lizer. The supernatant liquid is allowed to pass without 

 further treatment into streams. In some cases where sewage 

 has been disinfected by the addition of chlorine or by the 

 addition of acid, the microorganisms which are present are 

 largely destroyed. Where the city is not so large as to 

 make it impracticable, septic tanks and filter beds of various 

 types have been employed. In recent years a modification 

 of this has appeared in the use of the so-called activated 

 sludge method.' It has been found by experience that if 

 sewage is drawn into a large tank and air bubbled through 

 it, it can be rapidly purified. The more insoluble materials 

 settle out in the form of a sludge and the clear super- 

 natant liquid is allowed to discharge into a stream. Con- 

 tact beds and sprinkling filters have also been used for pur- 

 poses of purification. 



