374 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1956 



When a trickling filter is first put in service, purification of the 

 sewage is not great. However, within about a month or more the 

 filter media becomes coated with slime, containing a multitude of 

 lower forms of life such as bacteria, fungus, fly larvae, spiders, and 

 many types of worms. As the sewage trickles over the surface of this 

 slime, the carbonaceous and nitrogenous materials are oxidized by 

 bacterial action with the production of carbon dioxide, water, and 

 nitrates, and with a reduction of 60 to 85 percent in the oxidizable 

 matter. As the bacteria and other forms of life in trickling filters are 

 affected by temperature, a better effluent is produced in summer than 

 in winter. 



The quantity of sewage that can be treated on trickling filters varies 

 from 1 to 2 million gallons per acre per day, up to 30 million gallons 

 per acre per day or more. Prior to about 20 years ago from about 1.5 

 to 3 million gallons per acre per day of settled sewage was applied to 

 trickling filters, or about 12,000 to 24,000 persons were served per acre 

 by a filter G feet deep. Experimental work and many full-scale instal- 

 lations have since indicated that considerably greater applications of 

 sewage can be made, and that by pumping back a portion of the 

 effluent to the filter influent much larger quantities of sewage can be 

 treated with no decrease in efficiency. 



Trickling filters are used in many parts of the world for treating 

 both small and large sewage flows. During the past 20 or 30 years in 

 the United States, where the sewage from large cities requires sec- 

 ondary treatment, the trend has been to provide activated-sludge units 

 instead of trickling filters. The three largest trickling-filter plants in 

 the world are those serving Baltimore, Md. ; Bradford, England; and 

 Birmingham, England. 



The effluent from trickling filters usually contains from 50 to 100 

 parts per million of suspended solids. In order to remove these solids 

 the effluent is almost always treated in sedimentation tanks, which are 

 frequently called humus tanks. They generally have a detention 

 period of about 2 or 3 hours and as a rule are provided with mechanical 

 equipment for removing the sludge. 



ACTIVATED-SLUDGE TREATMENT 



The activated-sludge process, which was invented in Manchester, 

 England, consists of bringing sewage into intimate contact with air 

 and biologically active sludge. As a rule the sewage is first clarified 

 in preliminary settling tanks and then permitted to flow continuously 

 through aeration tanks with a detention period of several hours. 

 Activated sludge is added to the sewage entering the aeration tanks 

 in amounts varying from about 10 to 30 percent by volume, and air 

 is added to the mixture. From the aeration tanks the sewage and 



