372 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1956 



objectionable characteristics. In the operation of Imhoff tanks the 

 top of the sludge in the digestion compartment is maintained well 

 below the elevation of the slots connecting the sedimentation and the 

 digestion compartments to prevent the reentry of sludge into the sedi- 

 mentation compartment, which would adversely affect the clarity of 

 the effluent. Decomposed sludge is withdrawn from the tank through 

 a pipe (text fig. 4), the lower end of which terminates just above 

 the bottom of the digestion compartment. One distinguishing charac- 

 teristic of Imhoff tanks is that gas-lifted solids in the digestion com- 

 partment, as they float upward, cannot reenter the sedimentation 

 compartment and contaminate the effluent. 



Imhoff tanks, which were first adopted in Germany, were a marked 

 improvement over septic tanks and were built at many sewage-treat- 

 ment plants in this country such as those serving Akron, Ohio ; Fitch- 

 burg, Mass. ; and Fort Worth, Tex. During the past 30 years, how- 

 ever, Imhoff tanks have been superseded by mechanically cleaned 

 sedimentation tanks and separate sludge-digestion tanks. 



SECONDARY TREATMENT FACILITIES 



In many instances the above-mentioned facilities do not furnish 

 the required degree of treatment. Such is the case if there is an 

 insufficient volume of diluting water into which the sewage effluent 

 discharges. The three principal types of supplementary treatment 

 usually called secondary treatment, are intermittent sand filters, trick- 

 ling filters, and the activated-sludge process. Although intermittent 

 sand filters were often employed 50 years ago, their present use is in 

 general restricted to those places where the sewage flow is quite small 

 and where a high degree of treatment is required. Either trickling 

 filters, or the activated-sludge process are widely adopted where sup- 

 plementary treatment is called for. The activated-sludge process is 

 more generally used than trickling filters where large sewage flows 

 must be treated. 



INTERMITTENT SAND FILTERS 



An intermittent sand filter consists of a bed of sand varying from 

 30 to 36 inches in depth. Each bed is surrounded by a low earth 

 embankment, and one or more pipelines with pen joints are provided 

 at the bottom of the beds to drain off the effluent. The raw sewage is ap- 

 plied, at stated intervals, to the beds to a depth of about 3 inches. The 

 sewage solids are strained out in the upper few inches of the bed, and 

 the effluent is usually quite clear with a low suspended solid content. 

 Between each application of sewage several hours are allowed to 

 elapse to permit the entry of air into the bed so that the entrapped 

 materials can be oxidized. From time to time it is necessary to re- 

 move and dispose of the solids caught on the surface of the beds. 



