BACTERIAL PURIFICATION 281 



deep at the rate of 200 gals, per square yard, or i million gals, 

 per acre per day for septic effluent, with over go% purification 

 after settlement, the beds being occasionally washed out by 

 storm-water. 



American experience has been that, working with preliminary 

 septic treatment, contact filters are able to produce a non-putres- 

 cible effluent at rates several times as great as intermittent sand 

 filters, and sprinkling filters can produce corresponding results 

 even with much higher rates. The latter give a better oppor- 

 tunity for oxidation, and the films deposited on the filtering 

 material become detached from time to time and pass off with 

 the effluent. Contact filters are generally more expensive in 

 working than the sprinkler type with settling basins : the 

 effluent from the latter contains only from 3 to 5% of the 

 bacteria in the crude sewage, and in most cases requires no 

 further treatment before discharge. 



