370 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1956 



CHEMICAL PRECIPITATION 



At one time the chemical precipitation of sewage was extensively 

 adopted ; today, however, it is little used. The process consists of mix- 

 ing one or more chemicals with sewage to produce an insoluble or 

 slightly soluble floe, which enmeshes and precipitates particles in sus- 

 pension. The facilities required to treat sewage by this method consist 

 of equipment for dosing and mixing the chemicals with the sewage and 

 sedimentation tanks, in which the precipitated solids can be removed. 

 One or more of a number of chemicals are employed in the precipita- 

 tion process. Those most frequently used are alum, lime, ferrous sul- 

 fate and lime, ferric sulfate, and ferric chloride. Using chemicals to 

 precipitate the sewage solids increases the amount removed and conse- 

 quently produces a better effluent. Chemical precipitation will remove 

 70 to 80 percent of the biochemical oxygen demand and 80 to 90 per- 

 cent of the suspended solids and bacteria. Chemical precipitation 

 gives a degree of treatment intermediate between that obtained by 

 plain sedimentation and that by oxidation processes to be described 

 later. 



SEPTIC TANKS 



A septic tank (text fig. 3) is a sedimentation tank in which the 

 sludge remains for a considerable time and decomposes as the result 



Effluent pipe 



Y^ 



Influent pipe 



— |ff \-Z -0"—*}6 fW6"-l'r- 



Plan 



- 6'- 10" 



— | : f~/<f-~j£ H- 



CKl 



■■■ Surface of sewage ■•••' 





Grit chamber 



Cedar baffles- 



— J2-0" 



■ Sludge outlet 



Longitudinal Section 



Figure 3. — Typical septic tank for school or factory. (Bull. 16, Engineering Experiment 

 Station, University of Washington.) 



