1870.] of Seivage Irrigation. 23 



may be to the farmer, it cannot be relied upon as a source of much 

 income to the town whence it is obtained. The theoretical value 

 of sewage has been calculated by some of our most eminent 

 chemists and others, and the results arrived at vary from Id. to 

 upwards of 2d. per ton. Practically, however, it has been found 

 that this is too high, and that its real value — that is to say, the 

 price which a farmer could afford to pay for it — does not exceed 

 from %d. to Id. per ton, and in estimating the probable returns from 

 the sale of sewage it will always be safer to adopt the lower figure. 



The cost of the application of sewage for irrigating land appears 

 to be dependent on a number of local conditions, and, consequently, 

 to vary considerably. It would seem, from the data collected by 

 the Commission appointed by the British Association, that in many 

 instances the outlay requisite for this purpose would exceed what a 

 farmer could be expected to incur, and that in such cases, at least, 

 it would be proper to regard this outlay as coming under two 

 distinct heads, viz. that which a town may reasonably be expected 

 to bear for the mere object of getting rid of its refuse, and that 

 which a landowner or farmer may be able to incur for the improve- 

 ment of his land. It is probable that when viewed in this light 

 the application of liquid sewage to land would become a source of 

 revenue to towns only under special favourable circumstances, but 

 that, in opposition to the opinions which have been somewhat 

 hastily formed in certain cases, it will more frequently entail some 

 amount of expenditure on the towns themselves. At the same 

 time the benefit to land and the improvement in the condition of 

 rivers to be realized by this mode of dealing with liquid sewage 

 can scarcely be matter of doubt or uncertainty any longer. 



For carrying out a system of irrigation it is necessary, of 

 course, that the sewage should, in the first place, be brought by 

 channels or drains to the neighbourhood of the fields to be irrigated, 

 where the more solid parts are separated from the liquid by allow- 

 ing it to settle for a time, or, as is more generally the case, by a 

 coarse system of filtration. For the distribution of the liquid, four 

 different methods have been applied, viz. : 1. That known as the 

 hose and jet system. 2. Sub-irrigation, or the distribution of 

 the fluid below the surface of the ground. 3. By means of surface 

 channels. And, 4. By total submersion. In deciding, however, 

 which is the best system for distributing sewage, two things should 

 be kept in view : the first is that all arrangements for its distribu- 

 tion should be as simple and inexpensive as possible; and the 

 second, that owing to the constant quantity of sewage to be dealt 

 with, the arrangements must be capable of being worked at all 

 times and seasons. With these preliminary remarks we proceed 

 now to describe briefly the principles upon which the different 

 methods of distribution, above referred to, are carried out. 



