388 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1956 



A still higher degree of treatment can be attained by adding chemi- 

 cals to the sewage for coagulation purposes. If more complete treat- 

 ment is necessary, trickling filters followed by humus tanks or the 

 activated-sludge process may be required. If the sewage flow is small 

 and sufficient land is available, intermittent sand filters may be used. 

 Chlorine can be used in conjunction with any of the foregoing methods 

 of treatment to reduce odors or to disinfect the effluent. 



COST OF SEWAGE TREATMENT 



The cost of sewage-treatment plants depends upon a number of 

 factors, some of which include the cost of the plant site, the presence 

 of foundation problems, the types of treatment units adopted, and the 

 volume and character of the sewage. Plants that provide partial 

 treatment and contain screening equipment, grit chambers, sedimen- 

 tation tanks, sludge digesters, and sludge-drying equipment will in 

 general cost from $10 to $30 per capita served, and plants that contain, 

 in addition to the foregoing facilities, provision for oxidizing the 

 sewage either by trickling filters or by the activated-sludge process will 

 cost from $15 to $50 per capita served. The yearly operating and 

 maintenance costs per capita will usually vary from approximately 

 25 cents to $2.00 for plants in the former group and from 50 cents to 

 $4.00 for plants in the latter group. These costs do not include fixed 

 charges. 



WHAT OF THE FUTURE? 



The growth of the urban population in this country is such that the 

 need to build and enlarge sewage- treatment facilities and sewerage 

 systems will continue perhaps indefinitely. Coupled with this need 

 is the increasing demand by the public for clean streams and water- 

 courses. It is estimated (McCallum, 1955) that $5,330,000,000 will 

 be required within the next 10 years to keep up with this demand. 



If the future advance in sewage-treatment continues as in the past, 

 a greater use of mechanical equipment seems likely. There are possi- 

 bilities that the efficiency of the activated-sludge process can be ma- 

 terially increased, that the time required to digest sewage sludge can 

 be reduced, that the efficiency of sludge filtration can be improved, 

 and that better ways of heat drying sludge will be developed. It is 

 most important that adequate funds and personnel be made available 

 so that research relating to sewage treatment can be continued on a 

 university, city, State, and national level. Much of the future ad- 

 vance in sewage treatment will depend on the emphasis placed on 

 research. 



SUMMARY 



Sewage-treatment processes have been perfected to such an extent 

 that most any degree of treatment can be obtained with a minimum 



