366 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1956 



horizontal axis parallel with the surface of the sewage. Different 

 methods of cleaning line screens have been adopted, one of which is by 

 revolving brushes. 



Screenings, which are usually quite objectionable, are disposed of 

 by incineration, by burial, or by mixing with municipal garbage. One 

 method that has been practiced to a considerable extent for the past 

 25 years has been to mascerate the material in a grinding machine and 

 discharge it back into the sewage. This method of disposal is espe- 

 cially applicable where screenings from coarse screens are disposed of, 

 since the quantity of solids involved is small and the ground material 

 can be removed from the sewage by subsequent treatment processes. 



A device called a comminutor, which combines the properties of a 

 screen and a grinder (pi. 2, tig. 1), consists of a slotted rotating drum, 

 installed in a sewage channel. The sewage flows through the hori- 

 zontal slots in the drum. Attached to the outer surface of the drum 

 are a series of projections, on which the screenings are caught. As the 

 drum rotates, it passes through the teeth of a stationary comb and cuts 

 up the screenings into such a size that they flow through the slots with 

 the sewage. 



GRIT CHAMBERS 



After sewage has been screened, usually the next step in the treat- 

 ment process consists of removing the grit in one or more tanks, called 

 grit chambers. These chambers treat the sewage from combined 

 sewerage systems, which contains considerable quantities of sand and 

 similar materials in the discharge from highways and unpaved areas, 

 and in some instances are provided to treat the sewage from separate 

 systems. There is a variety of designs. The size of the tank (pi. 1, 

 fig. 2) governs the velocity of flow of the sewage passing through it 

 so that the grit settles on the bottom of the tank and the lighter organic 

 matter passes out with the effluent. The length is such that the sewage 

 is retained in the chamber about 1 minute, and the traverse cross sec- 

 tion is usually such that the velocity of the sewage is about 1 foot per 

 second. The quantity of grit removed varies from about 1 to 10 cubic 

 yards per million gallons of sewage treated. Grit is disposed of by 

 incineration and by making fills at sewage-treatment plants. 



TANKS 



SKIMMING TANKS 



As sewage contains considerable grease and oil, specially designed 

 tanks for the collection and removal of these materials have been used 

 to some extent. The tanks are generally small with detention periods 

 ranging from a few minutes to about 15 minutes. One type is rec- 

 tangular in plan with a V-shaped cross section. It is divided longi- 

 tudinally into three compartments by tw T o vertical walls, which extend 



