248 OUTLINES OF BACTERIOLOGY. 



§ 5. LAND TREATMENT OF SEWAGE. 



This is done by one of two methods, known respectively as Inter- 

 mittent Filtration i and Broad Irrigation.^ 



By the first method the sewage is placed on the land and allowed 

 to filter through the soil, the efiluent being drained off by soil 

 pipes (Fig 131). Before reaching the soil pipes, the sewage has 

 therefore to pass through a certain depth of soil. The passage is 

 marked by far-reaching changes in the sewage, which, as will 

 be explained below, becomes radically altered in constitution. The 



Fig, 131. — To illustrate land-treatment by iiitei-mittcnt filtration. Sewage after 

 treatment in septic tank (a) is allowed to filter througll soil (b) and is then 

 collected by soil-pipes {dd. .). (After Lafar.) 



filtration is made intermittent, because a break is necessary to 

 secure adequate aeration of the soil. By the second method the 

 sewage is run over a large area of land, the idea being to 

 spread the sewage over the surface of the soil as a thin layer, so 

 thin in fact that the work of purification takes place mainly on the 

 surface. The sewage is not drained off and the vegetation is not 

 affected, in fact the chief endeavour is to secure a maximum growth 

 of vegetation consistent with purification. As in the other method, 

 the laying of the sewage must be intermittent in order to secure 

 proper aeration of the soil. 



One or other of these two methods is usually employed in places 

 where plenty of land is available, about one acre to every 100 of 

 the population being required. Every kind of land can be utilised 

 for this purpose, with the exception of peat and stiff clay lands. 

 These are so wanting in porosity, that a very large tract of land would 

 be required for the purpose, so large in fact that land-treatment in clay 



'The two names inlermittent Jiltralion and broad irrigation have not been 

 happily chosen and have lately been replaced by the self-explanatory terms 

 dovmward /ftrraiion.and surface irrigation respectively. In the text we shall 

 use the older terms because of their familiarity. 



