10 SEWER DESIGN 



ing the approval for the construction of the extensions asked 

 for. 



Sometimes extensions to combined systems are approved 

 provided the dry-weather flow shall be treated in a disposal 

 plant and that there shall also be treated dilutions of from 

 3- to 5-fold, depending on the ordinary concentration of the 

 domestic sewage, the excess being discharged untreated into 

 watercourses where it is assumed, because of its greater dilu- 

 tion, it will not be dangerous nor cause offence. 



The desirability of the sewer acting as a ground-water 

 drain has been strongly urged. Waring says that the primary 

 object of sewerage is the removal of fouled waste water and of 

 subsoil water. Therefore he makes a line of drain-tile laid in 

 the sewer- trench an essential part of his system. The sanitary 

 advantages of a dry subsoil are sufficiently evident and are 

 not a question for discussion here. Whether the sewer-pipes 

 shall serve for the purpose or not is of some interest. The 

 disadvantages apparent are the uncertainty introduced in 

 determining the proper size of the pipe-lines, since the amount 

 of ground-water flow can only be determined when it is encoun- 

 tered, and then for that time only, the ground-water flow being 

 quite as variable as any other stream flow. Again, since the 

 height of ground-water seldom remains permanent, it is possible 

 that openings left to admit ground-water may at times allow 

 sewage to escape, thus polluting the soil and reducing the 

 carrying power of the sewage. It seems better therefore either 

 to provide a separate line of pipes for ground-water, discharg- 

 ing at near and convenient points, or else to arrange for the 

 ground-water to enter without the opportunity for the sewage 

 to escape. This may be done by providing special openings, 

 as in a pipe with siphon attachment patented by S. E. Babcock 

 of Little Falls, N. Y., and described in Engineering News, Vol. 

 XXVII, p. 66 1, or by a special pipe, laid where the amount 

 of ground-water makes it necessary and discharged into the 

 sewer at a convenient manhole. It is the general practice 

 to-day, however, to omit any such connection, and if drain- 



