770 rnocEEDiNGs of section ii. 



not only to jireveiit the Hoatino- about of solid oljnoxious 

 niattpr, and its de))osit upon the foreshoi-o, or its settling at 

 the bottom of the watei* in places where it may reappear 

 later on in the form of mud-banks, left I)are at every tide, 

 but the effluent sewag'e should also l)e deprived of its putre- 

 factive capabihties, so that there shall be no diseng-agement 

 of noxious gas from it in case a large body of it should be 

 left stagnant for a time — say, at low water. Tliis purification 

 cannot be effected by the lime ju'ocesses of precipitation, as, 

 though they fulfil the requirement of rendering the effluent 

 clear enough, they do not render it imputrescible. What is 

 required is a ])rocess that will thoroughly oxidize tlie oi-ganic 

 matters of the effluent, and this the, lime processes do not 

 do. As far as the really jiracticable is concerned, I believe 

 we are confined, for the proper effecting of this oxidation, to 

 either, first, filtration by earth in the way of percolation or of 

 surface irrigation, or, secondl}^ to some iron process. 



First, as to filtration by earth. The adoption of this 

 system requires the obtaining of a sewage farm Avith a 

 suitable soil, and of an area of about one acre to each 100 of 

 the population. Jn this provision of land, as well as in 

 all other matters connected with sewage disposal on this 

 system, enough land must be obtained, not only for the 

 immediate ])resent, but also for the requirements of a more or 

 less definite future. As this land should be at a distance from 

 the habitations of the people, an in-igation farm entails the 

 necessity of a great length of outfall sewer; and as it is 

 cheaper to construct one server of sufficient carrying capacity 

 than several sewers which shall collectively have that 

 capacity, this outfall sewer must be a comparatively large 

 one ; and all the more so in anticipation of the future. 

 Moreover, as a seaside city is, in some of its parts at least, a 

 low-lying city, it is inevitable that some of its sewage, if not 

 all, will have to be raised by steam power from a lower to a 

 higher level. To economise the cost of this power it is 

 usual to limit this lift to the smallest height that will secure 

 the flow of the sewage by gravitation on to the farm — a 

 limitation that tells upon the size of the outfall sewer as it 

 tells upon its gradient, for it natiirally takes a larger sewer 

 to convey a given quantity of sewage in a given time at the 

 slow rate of a flat gradient than at tlie quick rate of a 

 steeper gradient. 



The effect of all these influences upon the economical 

 aspect of the system of sewerage is not confined to the size 



