06 SOME REMARKS OX SEWERAGE SYSTEMS. 



But on the other hand are the following disadvantages, i.e. : 



1st. It largely increases the quantity of sewage. 



2nd. It concentrates the refuse. 



3rd. And thus renders it more difficult of disposal. 



4th. If the sewer is leaky, it is liable to pollute the sub- 

 soil and any wells in its proximity. 



The ordinary water-carriage system is divided into two 

 distinct methods, called the Combined and Separate methods 

 of sewerage. In the former, the rainfall is admitted into 

 the sewers ; and in the latter, it is as far as possible ex- 

 cluded and carried off in separate conduits. The greater 

 portion of Bristol has adopted the former method ; but at 

 Westbury-on-Trjmi, where a sewage farm is in operation, it 

 is largely excluded. Where pumping has to be resorted to^ 

 the introduction of rain-water becomes a costly matter, It 

 is scarcely possible, however, to entirely exclude all rainfall 

 without introducing a duplicate system of drains for each 

 house ; and the rain-water falling upon the backyards and 

 roofs therefore often finds its way into sewers. This may 

 be just as well, for rain-water from such parts, especially 

 the first, after a drj^ period, is quite as polluted as ordinary 

 sewage, and only fit for sewers. In some places special 

 apparatus is designed for the purpose of admitting the light 

 rainfalls into the sewers, and allowing the heavier falls to 

 run into the ordinary channels. 



If the rain-water drainage can be separately provided for^ 

 and the sewer kept only for sewage, it is without doubt 

 much the best plan, es^Decially when the sewage has to be 

 dealt with. At the same time it must not be forgotten that 

 the drainage of the subsoil water is an advantage, and 

 should be provided for if necessary, although the trench 

 which is cut for the sewer often acts as a drain for this 

 purpose outside the sewer. 



