THE TREATMENT OF SEWAGE. 247 



conditions of life for pathogenic bacteria and their bacterial competitors 

 are almost identical. All the biological methods (both land and 

 artificial treatment) have this end in view, viz. the consumption of 

 the food supply by non-pathogenic bacteria. 



We may distinguish three main methods of sewage disposal : 



1. Disposal without purification. 



2. Land treatment. 



3. Artificial treatment. 



§4. DISPOSAL WITHOUT PURIFICATION. 



In small villages the sewage is not usually collected together, each 

 house disposing of its own portion as best it can. The faecal matter 

 is usually covered with earth and periodically removed, whilst in the 

 better-class houses the same material is drained off, the pipes discharging 

 their contents into cess-pools. The liquid part gradually wells up to 

 the surface and as it passes through the earth its organic content 

 becomes gradually converted by the soil-bacteria, into substances 

 that cannot serve as bacterial food. The solid portion remains 

 in the soil, where it is out of harm's way and where it is speedily 

 disintegrated and chemically changed by the activity of anaerobic 

 bacteria. The other substances that in larger places usually find their 

 way to the sewage-drain are thrown broadcast on to any convenient 

 spot, such as a roadside or a neighbouring common. This method is 

 efficient enough for very small places, though it must detract somewhat 

 from the healthiness of village life. In most small towns situated on 

 or near the banks of a river or on the sea-shore, the sewage is con- 

 ducted into the river or sea as the case may be, with, in the majority 

 of cases, harmless results. We have explained above, the manner in 

 which purification is effected by rivers. The method is attended with 

 danger to the inhabitants of. the houses situated near the outlet of 

 the sewage into the river, and may injuriously affect their health. 

 The area of danger extends to at least four miles below the outlet 

 of the sewage. In the case of sewage conducted to the sea, there is 

 danger if bathing grounds are located in the immediate neighbourhood, 

 and there are cases known of epidemics of typhoid fever, caused by 

 the consumption of shell fish gathered from places near to the outlet 

 of sewage into the sea. 



