380 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1958 



of the sewage is likely to be neglected for the raising of crops. The 

 process is most applicable in arid countries. Although sewage farms 

 are used to only a limited extent in this country, they still serve to 

 treat the sewage of several large cities in Europe such as Paris and 

 Berlin. 



CHLORINATION 



Chlorine and chlorine compounds have been used since 1854 for the 

 treatment of sewage. However, it has been only during the past 50 

 years that chlorine has been employed extensively at sewage-treatment 

 plants. Except at very small plants chlorine is purchased in liquid 

 form in steel containers of different sizes, holding from 100 pounds to 

 30 tons. It is then fed as a gas in amounts that can be regulated manu- 

 ally or automatically by chlorinators (pi. 3, fig. 1) to any point of 

 application. 



The following are some of the uses of chlorine for treating sewage : 



1. Odor control. 



2. Control of trickling-fllter flies. 



3. Control of trickling-filter ponding. 



4. Reduction of biochemical oxygen demand. 



5. Disinfection of sewage. 



6. Control of aquatic life. 



ODOR CONTROL 



Chlorine has been widely used at sewage plants to control odors. 

 Many of the disagreeable odors are due to hydrogen sulfide. When 

 chlorine is added to sewage containing this gas the following reaction 

 occurs : 



Ch+H 2 8=2HCl+S 

 Chlorine can be applied to the sewage at one or more points in the 

 sewerage system, at the inlet to or at some point in the sewage-treat- 

 ment works. It is often preferable and cheaper to apply the chlorine 

 at one or more points in the sewerage system where the sewage is fresh 

 than to apply it at the sewage works where the sewage may be septic 

 and may contain hydrogen sulfide, 



CONTROL OF TRICKLING-FILTER FLIES 



Practically all trickling filters are the habitat of small flies, called 

 Psychoda alternata, which frequently create a nuisance in the vicinity 

 of sewage works. Of the many methods adopted to control them the 

 application of chlorine to the influent of trickling filters has proved 

 quite effective. A sufficient quantity is used for several hours at 

 weekly or biweekly intervals to reduce the adult fly population. 



CONTROL OF TRICKLING-FILTER PONDING 



One of the difficulties in operating trickling filters has been the clog- 

 ging of the filtering material with organic solids. The clogging is 



