148 SEWAGE AND ITS PURIFICATION 



5. Precipitation and filtration through specially-prepared 

 filters, followed by irrigation — i acre for every 2,000 

 persons. ^ 



The Local Government Board have required, in the con- ^ 

 struction of special filtration areas, that provision shall be made 

 for— 



1. A rainfall and sewage calculated at three times, the dry- 

 weather flow. 



2. Above three times and up to six times to be treated on a 

 further special area of storm-water filters, and not until the 

 flow is above six times may it be discharged into a stream or 

 on to prepared land without passing through the filters or other 

 method of treatment. 



3. The capacity of the filters to be taken at one-third for the 

 fluid and two- thirds for the filtering material. 



4. A cycle of eight hours for filling, emptying, and rest for 

 aeration. 



In any system of sewage-farming the difficulties of controlling 

 the drainage area, so as to provide for the varying amounts and 

 qualities of the sewage, will always exist. If the land be sufli- 

 ciently porous and well drained to prevent its being water- 

 logged, and to allow the free passage of the effluent during wet 

 seasons, in dry weather it will permit it to run through too 

 rapidly, and the effluent will not be purified. A denser soil 

 adapted for ordinary weather will be entirely clogged by unusual 

 rains, and therefore unsuited for any broad irrigation scheme, 

 unless a very large area is available.^ 



Another obstacle is the distance to be traversed before reach- 

 ing a suitable site. Thus at Wigan the sewage had to be 

 conveyed to the farm for yl miles through a 27-inch cast-iron 

 main costing -f 50,000, with three siphons ; it took ten hours in 

 transit, and on arrival was more or less charged with 

 sulphuretted hydrogen and other foul gases.^ I have drawn 

 attention^ to a further disadvantage of land treatment — the 

 risk of polluting the subsoil water. The present Royal Com- 

 mission on Sewage* state that these effluents contain large 

 numbers of organisms, " many of which appear to be of in- 

 testinal derivation, and some of which are of a kind liable, 

 under certain circumstances, to give rise to disease";^ they 



^ See a paper by S. Krawkow, y. Landw., 1900, xlviii., 209, on "The Move- 

 ment of Aqueous Fluids in Soils." 



2 Institute of Sanitary Engineers, November 16, 1899. 



3 Engineering Conference, Institute of Civil Engineers, 1903. Section VI. 

 ^ Interim Report, 1901, p. 10. ^ Ibid., vol, iv., part i., section 9. 



