74 VETERINARY HYGIENE 



where it is disposed of in one of several ways. The sewage system 

 of a town may be a combined system or a separate system. The 

 former means that all the waste water and sewage, including excreta, 

 slop water, trade effluents and rain water, are conveyed in one 

 channel. With the separate system rain and surface water are 

 carried separate from the remainder or true sewage. The latter 

 is the preferable method. 



Sewage may be disposed of as follows : 



Discharge into the Sea. The outlet pipe must be carried well 

 out so that the outlet is always under water whatever the state of 

 the tide ; the outlet end is fitted with a valve flap to prevent the tide 

 from flowing up. 



Discharge into Rivers. Under the Rivers Pollution Prevention 

 Act of 1876 (which see) no solid or liquid sewage matter may be 

 allowed to flow into a stream unless the flow was practised before 

 1876, and then the persons responsible must take the best and most 

 practical means to make the sewage harmless. Nevertheless many 

 country houses discharge crude sewage direct into adjacent streams. 



Discharge into Cesspools. A common method of disposal of 

 house sewage in the country is to drain it into cesspools. A cess- 

 pool is a pit lined with brick, which may or may not be faced with 

 cement. If not made impervious with cement, the liquid drains 

 away into the surrounding soil and only the solids remain behind 

 for subsequent removal when the pit becomes full. In isolated 

 places where the subsoil is suitable the porous cesspool may be used 

 with advantage, but it must be so placed that there is no risk of 

 contaminating the water supply. The distance it should be placed 

 from a well or other water supply depends upon the nature of the 

 subsoil, and no arbitrary distance can be defined. 



The disposal of town sewage, where discharge into the sea is 

 not practicable, offers many difficulties and various methods are 

 adopted. 



Surface or Broad Irrigation or Sewage Farming. For the 

 sewage farming method of disposal about 1 acre per 100 of popula- 

 tion is required. The land must have a slight gradient and a suit- 

 able light subsoil. Clay is unsuitable. It is divided by narrow 

 shallow channels through which sewage is conveyed so that each 

 part of the land gets saturated in rotation. The sewage percolates 

 through the soil, and the affluent is carried away by subsoil drains, 

 which are laid about 6 feet deep, to discharge into a river or other 

 outfall. The land is used for the cultivation of grass, roots or 

 cabbages, and large crops are grown, though often of a rank nature. 

 If too much sewage is put on the land it becomes " sewage sick." 



