F. W. MOHLMAN 361 



DISINFECTION 



The discharge of raw or treated sewage into streams, lakes, or the sea near sources 

 of water supply, bathing beaches, or oyster beds usually requires more complete re- 

 moval of Bad. coli and pathogenic organisms than can be effected by any of the treat- 

 ment processes now in use. It has been found that chlorination is the most satisfac- 

 tory method for obtaining bacterial removal. Present practice seems to favor Im- 

 hoff-tank treatment followed by chlorination, wherever sufficient dilution is available 

 to satisfy the oxygen requirements of the settled effluent. 



The efficiency of chlorine as a bactericide for sewage and sewage effluents has 

 been known for many years. The pioneer work was done in 1907, under Phelps's^ 

 direction at the Massachusetts Institute of Technology. Calcium hypochlorite was 

 used on raw-sewage and trickling-filter effluent at Boston, septic-tank effluent at 

 Red Bank (N. J.), and trickling-filter effluent at Baltimore (Md.). The amounts of 

 chlorine required were determined, ranging from 5 to 10 parts per million for fresh 

 Boston sewage, from 10 to 15 for septic sewage, from 5 or less for trxkling-filter 

 effluents. The reductions of Bact. coli for Boston raw sewage with 10 p. p.m. chlorine 

 were from 35,000 to 24 per cubic centimeter, or 99.9 per cent, and for septic sewage 

 at Red Bank, with 11. 5 p.p.m. chlorine from 205,000 to 75 per cubic centimeter, or 

 99.9 per cent. However, when the chlorine was reduced to 7.5 p.p.m., one test showed 

 a reduction of from 220,000 to 16,000, or only 92.7 per cent. The reductions for trick- 

 ling-fflter effluents were, at Boston with 3.4 p.p.m. chlorine, from 47,000 to 380 per 

 cubic centimeter, or 99.2 per cent, and at Baltimore, with 2.2 parts per million chlo- 

 rine from 2,000 to 200, or 90 per cent. In the latter tests residual chlorine of 0.6 was 

 obtained after two hours' storage. The Jackson-bile medium was used for Bact. 

 coli counts at Baltimore. 



Many investigations have been made since those of Phelps on the use of chlorine 

 as a bactericide, but liquid chlorine is now used instead of calcium hypochlorite be- 

 cause of the ease of application. The amounts required in operating plants have 

 usually been greater than those determined by Phelps. The amount of chlorine re- 

 quired is now adjusted by determinations of residual chlorine. The chlorine demand 

 varies with the organic matter, sulphides, nitrites, or other reducing compounds 

 present in the sewage. Tiedeman^ has recently studied chlorination of an Imhoff efflu- 

 ent. When maintaining residual chlorine of from o.i to 0.5 p.p.m., he obtained more 

 than 99 per cent reduction in Bact. coli. His experiments indicated that the chlorine 

 penetrated the insoluble solids and effected almost as great bacterial reduction as was 

 obtained in the liquid effluent. 



Chlorine is not efficient, however, for sterilization of raw sewage, as the coarser 

 suspended solids are resistant to penetration. When residual chlorine is maintained 

 and Bact. coli is absent from i c.c. or less, the evidence is quite convincing that patho- 

 genic organisms are destroyed. This assurance does not apply to industrial wastes, 

 for example, where enormous amounts of residual chlorine are necessary to eliminate 

 B. anthracis from tannery wastes. 



' Phelps, E. B.: The Disinfection of Sewage and Sewage Filler Effluents, Water Supply Paper 22p, 

 U.S. Geol. Siirv., pp. 1-73. 1909. 



='Tiedeman, W. v. D.: "Efficiency of Chlorinating Sewage Tank Effluents," Eng. News-Rec, 

 98, 944-4S. June 9, 1927. 



