55 



original impurities, and a second decomposition is set up after 

 mixing with the stream, especially if so much lime or other agent 

 has been used as to render the effluent alkaline. In a river of 

 large volume (say, twenty times the volume of the effluent) a 

 flow of very many miles would be required to partially purify 

 itseK again ; and in a small stream purification is impossible. 

 It is an open question whether a second chemical treatment of 

 the effluent alone with the manganate of potash or soda, or other 

 oxidising agent, and a long flow in an oj)en channel might not 

 render it sufficiently pure to be admitted into the river at the 

 sacrifice of the manurial ingredients destroyed. This, however, 

 does not appear to have been tried. 



The treatment which most commends itself to sewage autho- 

 rities, and which is favoured by the Local Government Board, is 

 to filter the effluent through land, not over it. The soils most 

 suitable for the purpose, according to Denton, Tidy, Bateman, 

 &c., are sandy loam, or loamy clay, or stiflfish loam, and it is all 

 the better if the soil contains iron, as at Coventry. The quantity 

 of land required will depend upon whether it is to be used as a 

 filter alone, irrespective of the value of crops, or whether it is 

 intended to raise crops so as to recoup the cost of working. In 

 the former case, an acre for every 5000 of population would 

 suffice, but in the latter various estimates are formed from 100 

 to 1000 per acre of a fully water-closeted town. The land is 

 intersected with drains at a depth of five to six or seven feet, 

 according to the situation and soil, which enter a main drain 

 leading to the outfall. The surface is laid out in ridge and 

 furrow, and divided into several plots, so that the sewage water 

 can flow for a few hours on each plot alternately, by means of 

 sluices, which can be shut or opened as required. 



ItaHan Eye Grass, on account of its enormous appetite for 

 water, and rapid growth, is a specially suitable crop ; five crops 

 per year being easily raised. Mangolds, beetroot, turnips, 

 cabbages, rhubarb, &c., may also be cultivated. Animals fed 

 upon sewage-grown crops are as healthy and well fed and free 

 from disease as those fed on other i^roduce ; the milk and butter 

 and flesh of cows are in no way inferior. 



The action of the soil is twofold. First — It acts as a simple 

 filter, retaining particles in suspension. Second — It is a chemi- 

 cal laboratory. Franklin, Warrington, Muntz and Pasteur 

 have shewn that the ammonia and nitrates and organic matter 

 are oxidised by the agency of oxygen and living organisms in the 

 soil, which feed upon the sewage, destroying its noxious charac- 

 ter, and evolving nitric acid and carbonic acid, &c., for the 

 sustenance of vegetation. This process of nitrification or oxida- 

 tion is most interesting, and Mr. Dibden, a great authority on 

 sewage says, he has not the shghtest doubt that in future, where 



