384 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1956 



Several different designs of vacuum filters have been perfected and 

 are in general use. One widely adopted consists of a wooden drum that 

 is supported with its axis in a horizontal position in a lead-lined steel 

 tank. Attached to the outer surface of the drum are a number of 

 narrow wooden strips, parallel with and equidistant from each other. 

 These strips, which divide the periphery of the drum into a number of 

 shallow compartments, support a coarse-mesh screen, around which a 

 filter cloth made of wool, canton flannel, or some synthetic material is 

 wrapped. Vacuum pipes on the inside of the filter drum connect each 

 of the compartments with an automatic valve at one or both ends of 

 the filter. This valve connects with piping that supplies a vacuum to 

 the filter cloth. Sludge enters the filter tank through a sludge supply 

 pipe ; and as the filter drum rotates about its axis, a layer of wet sludge 

 about one-half inch thick adheres to the filter cloth. The differential 

 in air pressure between the surface of the sludge cake and the under 

 side of the cloth forces the water out of the sludge and through the 

 vacuum pipes away from the filter. By the time sludge cake reaches 

 the discharge side of the filter, its moisture has been greatly reduced. 



One essential step in preparing the sludge for filtration is to coagu- 

 late it with a suitable chemical such as alum, ferric sulfate, chlorinated 

 copperas, ferric chloride, or ferric chloride and lime. Of these, ferric 

 chloride is the most effective. Another step in the preparation of 

 sludge for filtration, especially if the material has been digested, is 

 to remove a considerable percentage of the bicarbonates, which are 

 formed as the sludge digests. Since they combine chemically with any 

 coagulant used, it is desirable to reduce their concentration so that less 

 coagulant be required. The concentration of bicarbonates is reduced 

 by mixing the sludge with a large volume of water or sewage effluent 

 and allowing the sludge to settle out from the liquid. The bicar- 

 bonates diffuse into the water and are removed. 



Sludge filtration is used in many cities in the United States. Three 

 of the most notable installations are in Chicago, Milwaukee, and Los 



Angeles. 



SLUDGE DISPOSAL 



The satisfactory disposal of sewage sludge is often a vexing problem 

 that confronts the operators of sewage-treatment plants. In most in- 

 stances sewage, after it has received partial or complete treatment, is 

 discharged into a watercourse, which quickly removes it from its 

 source; it frequently happens, however, that sewage sludge remains 

 to plague the operator. The following are the more common methods 

 of sludge disposal : 



1. Disposal in water. 



2. Disposal on land. 



3. Heat drying. 



4. Incineration. 



