349 
Sewage Disposal and Rivers Pollution. 
tillage implements ; but it is evident that much of the filtering 
power of the land must be lost on every occasion before matters 
arrive at this pass. Previous clarification of the sewage by addition 
of a few grains of lime per gallon before running it into settling- 
tanks removes injurious iron .compounds if present, and produces a 
rapid settlement of slimy suspended matter, giving in both cases 
an effluent eminently adapted for application to land. On the other 
hand, it introduces the new question of sludge disposal, which, how- 
ever, is not the bugbear it used to be, the difficulty being now 
overcome in several ways, and with greatest ease where there is land 
attached to the sewage works. In London, where there is no land, 
and the quantity of sludge produced is so enormous, they as yet 
see no alternative for treating the bulk to carrying out to sea in 
a fleet of steam barges. At Wimbledon, Wolverhampton, and very 
many other places, the sludge is filter-pressed into semi-dry cakes, 
which are sold at a nominal figure or given away to farmers who 
will cart them away as manure. At Birmingham it is pumped up 
into wooden conduits and conveyed to a porous section of the 
sewage farm, where the liquid portion speedily drains away, leaving 
the sludge in a condition to be dug into the ground. At each of 
these three towns, and many others, the clarified effluent from the 
liming tanks is employed to irrigate an adequate area of land, with 
satisfactory results as regards purification, and this has always 
seemed to the writer the correct way to employ effluents from pre- 
cipitation processes. It must be remembered that little of the 
manurial value of the sewage is removed in the sludge, and that the 
application of the effluent to the growth of crops, considered in 
itself and apart from the rest of the treatment, is a paying process. 
Of course the claim has been, and is still, made by the anti-irri- 
gation enthusiasts that chemical precipitation is of itself a sufficient 
purification, avoiding the necessity for land altogether, and pro- 
ducing an effluent fit for discharge into any stream. In practice 
this is seldom realised. The use of lime alone produces an unsatis- 
factory effluent, and to obtain a decent one it is in most cases 
necessary to supplement the lime with a little sulphate of iron, 
sulphate of alumina, or other chemical or patent nostrum. By 
such means a fair effluent may be obtained with trial quantities of 
sewage experimented on in the laboratory, but it is difficult to pro- 
duce a good effluent day after day at the sewage works when the 
changing quality of the sewage is not watched over by a skilled 
chemist ready to proportion the precipitants to the actual charge to 
be dealt with. This at any rate may be said, that whatever the 
quality of effluent produced by chemical precipitation, it is infinitely 
inferior in point of purity to the effluent from simple lime precipi- 
tation after the latter has been filtered through soil or similar 
purifying material ; whilst there can be no doubt that by a 
combination of precipitation with soil filtration a perfectly satis- 
factory effluent can always be obtained. There are, indeed, two 
cases where chemical precipitation by itself may still be the best 
procedure. As a stopgap, or a means of partial purification faute 
VOL. V. T. 8. — 18 A A 
