200 Elements of Water Bacteriology. 



higher grade of purity than this should be attained. It^ 

 seems wisest at the present time to avoid fixing any gen- 

 eral standards of purity for sewage effluents. Each case 

 should be judged inteUigently on its own merits. In 

 general, however, where bacterial purification is indi- 

 cated at all, it seems fair to demand that the effluent 

 should be of such a quahty as not to increase materially 

 the bacterial content of the body of water into which it 

 is discharged. 



Before leaving the subject of sewage bacteriology, 

 brief reference must be made to the importance of bacte- 

 riological studies in relation to the processes of sewage 

 purification which bring about the removal of the organic 

 matter itself. Nothing is more necessary to the develop- 

 ment of the present art of sewage disposal than knowledge 

 of the micro-organisms concerned and of the conditions 

 which favor their activity; but such knowledge is woe- 

 fully deficient. Something is known of the nitrifying 

 organisms long ago discovered by Winogradsky. Such 

 recent work as that of Schultz-Schultzenstein (1903), 

 BouUanger and Massol (1903), and Calmette (1905), has 

 cleared up many points concerning these forms; but 

 much remains to be done. In regard to the reducing 

 action of bacteria in the septic tank and contact bed we 

 are almost wholly in the dark. Septic tanks work well 

 with some sewages and badly with others; and the pres- 

 ence or absence of the right bacteria is probably largely 

 responsible for the different results. In some cases, as at 



