Biological Sewage Disposal 245 



TRICKLING FILTERS 

 These filters are composed of beds about ten feet deep filled 

 with rough materials similar to those found in contact beds. After 

 preliminary anaerobic digestion, the sewage is sprayed on the 

 surface of the bed where aerobic decomposition continues the 

 breakdown process. It may require as long as three months to 

 build up a suitable zoogloeal mass within the bed, but once this 

 is established, sewage decomposition is very efficient. Too high a 

 concentration of industrial wastes, particularly those wastes from 

 chemical plants, may destroy large numbers of bacteria on the 

 beds, and the efficiency of the operation will suffer accordingly. 



SAND BEDS 

 A top layer of about eighteen inches of fine sand resting on 

 three feet of coarse sand and an additional three feet of coarse 

 gravel serves as an excellent bed upon which partially digested 

 sewage can be sprayed. After standing for a period lasting up to 

 several days, depending upon the temperature, another layer of 

 sewage can be applied to the sand bed where it undergoes aerobic 

 breakdown. When the organic matter builds up to an appreciable 

 thickness, it can be scraped off and the cycle repeated with a fresh 

 application of sewage. 



ACTIVATED SLUDGE 



A common method for the disposal of sewage is an aerobic 

 process in which air is forced through a large tank containing raw 

 sewage, resulting in the formation of activated sludge. The 

 bubbling air encourages the growth of aerobic bacteria and it also 

 keeps the solids in motion, which hastens decomposition. From 

 85 to 95% of the solids may be oxidized in such an aeration tank 

 in about five hours, if the oxygen content is maintained at one 

 part, or higher, per million parts of sewage. 



Since not all of the solids are decomposed in the aeration tanks, 

 it is customary to pump the treated sewage into settling tanks 

 where the solids are removed. Dried activated sludge contains up 



