THE SEWAGE DISPOSAL PROBLEM 187 



and hence lead to their death when they have fulfilled their 

 function." 



There is no essential difference between the treatment of 

 sewage by land filtration or artificial filters. In each case the 

 purification, so far as it is not mechanical, is chiefly effected by 

 the agency of micro-organisms. 



Land Treatment 



It is a well-established fact that where the soil is suitable 

 and the area of land sufficient, the organic matters in sewage 

 can be thoroughly oxidised by land treatment. 



There are two methods of land treatment in use, viz. : 



(a) Intermittent downward filtration. 



(b) Broad irrigation. 



By the former method " the sewage is concentrated at short 

 intervals on an area of specially chosen porous ground as small 

 as will absorb and cleanse it, not excluding vegetation, but 

 making the product a secondary consideration." The purification 

 is partly due to the soil acting as a mechanical filter, separating 

 out and retaining the suspended matters in the sewage, but 

 much more to the destruction by organisms of the organic 

 matters in the sewage. These organisms are found in the 

 upper layers of all soils, and particularly of rich, loamy soils. 

 Sandy soils are not so efficient until they have been in use 

 for a time, while clay, peat, and the more retentive soils are 

 not really suitable media for this purpose. The selected land 

 is divided up into plots ; no plot receives sewage for more 

 than six hours in the da} 7 , therefore it has eighteen hours' rest 

 daily. It is necessary to underdrain the plots, say at a depth 

 of about four feet, and the sewage should be screened, filtered, 

 or precipitated before application to the land, and distributed 

 over each plot intermittently by branching carriers or some 

 other means. It is usual to allow one acre for each thousand 

 of population ; but if the sewage is clarified beforehand by 

 precipitation or other means, one acre may do for each five 

 thousand of population. Under favourable conditions the 

 effluent is found to be very free from organic matters and 

 organisms. 



(b) Broad Irrigation is " the distribution of sewage over a 

 large surface of ordinary agricultural land, having in view a 



