386 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1956 



sludge or vacuum-filter cake. In other localities where the demand is 

 not great, the material is given away. 



HEAT DRYING SLUDGE 



When sludge is heat dried, its moisture content is reduced to 10 

 percent or less. In this condition the material is more easily handled, 

 it can be used as a base for fertilizer, and transportation costs are 

 reduced. 



Two methods of heat drying, which have been most generally 

 adopted in this country, involve the use of rotary heat driers and flash 

 driers. A rotary heat drier (pi. 4, fig. 2) consists of a steel drum with 

 its longitudinal axis, about which it slowly rotates, set in a horizontal 

 position. At the end of the drum where the wet sludge cake is added 

 a coal- gas- or oil-fired furnace is provided. As the drum rotates, the 

 wet sludge cake is lifted on the inside of the drum to such an eleva- 

 tion that the sludge falls through the hot gases back to the bottom 

 of the drum. By the time the sludge reaches the discharge end of 

 the drum, its moisture content has been reduced to 10 percent or less. 

 The outstanding rotary drier installation is in Milwaukee, Wis. 

 Rotary driers have also been used at Houston, Tex., Grand Rapids, 

 Mich., and Baltimore, Md. 



The flash drying system (text fig. 8) is the second method of heat 

 drying sludge that has been extensively adopted. Heat, which is 

 generated in an oil- gas- or coal-fired furnace, is supplied to a cage 

 mill or flash drier. Wet sludge, to which a known quantity of pre- 

 viously dried sludge is added, is introduced continuously into the drier 

 and is intimately mixed with the hot gases by means of a rotating 

 cage in the drier. The mixture of sludge and hot gases flow vertically 

 upward through a duct into a cyclone separator, in which the sludge 

 is separated from the gas stream. Flash driers have been installed in 

 Chicago, 111., Los Angeles, Calif., and Houston, Tex. 



SLUDGE INCINERATION 



One method of sludge disposal that is being more widely adopted 

 is burning it. The two types of incinerators most generally used are 

 multiple-hearth furnaces and furnaces used in conjunction with flash 

 driers. A multiple-hearth furnace (see pi. 5) is cylindrical in shape 

 with a number of horizontal hearths, spaced equidistantly apart. Pass- 

 ing vertically up through the center of the furnace is a motor-operated 

 shaft, attached to which are a series of rabble arms, suspended several 

 inches above each hearth. Wet sludge is introduced on the top hearth, 

 and as the rabble arms rotate they push the sludge from one hearth to 

 the next lower one. Several oil- or gas-fired burners, attached to the 

 side of the furnace, provide the necessary heat to initiate incineration. 

 The bottom hearth is furnished with an outlet for the discharge of the 



