LAND TEEATMENT OF SEWAGE. 249 



and peat districts is impracticable. This is emphasised by the Royal 

 Commission on Sewage Disposal (1898), in their Interim Report. 



" We doubt if any land is entirely useless, but in the case of stiff 



clay and peat lands, the power to purify the sewage seems to 



depend on the depth of the top soil . . . We are however forced 



to conclude that peat and stiff clay lands are generally unsuitable 



for the purification of sewage, that their use for this purpose is 



always attended with difficulty, and that where the depth of top 



soil is very small, say six inches or less, the area of such lands 



which would be required for efficient purification would in certain 



cases be so great as to render land treatment impracticable." 



As to the changes that take place in the sewage, we have already 



explained that these are essentially the same as those which take place 



when organic manure is placed on the soil. The rich organic matter is 



eagerly seized upon by organisms that are present in the soil, and by 



those that are already present in the sewage. 



The ultimate result will be the production of a number of relatively 

 simple substances which will be quite unfit for pathogenic bacteria to 

 thrive upon. Owing to the hardiness of the soil-bacteria, and owing 

 to the conditions under which the decomposition is taking place, the 

 pathogenic bacteria do not get a chance to multiply inordinately. 

 That partial multiplication takes place is unavoidable, and effluents after 

 land treatment cannot be regarded as absolutely harmless. The 

 justification of land treatment lies in the fact that after treatment 

 the number of pathogenic or potentially pathogenic bacteria is very 

 small, and in addition, the nutriment of these bacteria has disappeared, 

 so that subsequent multiplication will not be possible, at any rate 

 multiplication for which the sewage is responsible. 



In the broad irrigation method aerobic bacteria are most active, and 

 of these the nitrite- and nitrate-bacteria play perhaps the most important 

 role, so that in place of the organic matter, a number of harmless 

 and useful inorganic products are obtained. In the intermittent 

 filtration method anaerobic bacteria will take more prominent parts 

 in the decomposition, because the work is mainly carried on under 

 the ground. These organisms appear to produce unstable substances 

 that are readily attacked, especially by oxygen. We do not know 

 much of the work carried out by the anaerobic bacteria, but it seems 

 probable that one or two molecules of oxygen or hydrogen, nitrogen 

 or carbon, are snatched from a large and complex molecular group, the 

 group being then left in a very unstable condition and in such an 

 altered form that it readily undergoes oxidation when subsequently 



