It has been thought that the cost of precipitation would be covered, and even a profit gained, by the sale of the sludge. This hope, however, is not nearer realization now than it was in the time, now gone past, when chemical processes were relied on to turn sewage from a profitless into a profitable commodity. There is, consequently, less justification now than there was at that time for adopting a precipitation system for sewage disposal. It is entirely a question of carefully considering the engineering and financial points involved, regardless of the sanguine representations of interested or enthusiastic advocates of any particular system. As the estimated manurial value of the sludge which is precipitated from sewage by the addition of chemicals does not seem to be capable of realization, we think that probably the reason may be found in the fact that the chemicals arrest that process of decomposition which is essential to the conversion of the organic matters into nitrates for vegetation to utilize.
This explanation will be understood in the light of what we have already described in regard to "nitrification." If this view is correct, it would follow that the more completely and permanently the sludge is deodorized by the chemicals, the less capable is it of passing through the necessary stages of decomposition by which its manurial value can be realized. As mistakes are constantly being made in regard to the weights of sludge with varying degrees of moisture, the following table may be useful:
Tons. Per cent. Tons. Per cent. 100 of sludge with 90 of moisture = 50 with 80 100 " " " 33.3 " 70 100 " " " 25 " 60 100 " " " 20 " 50 100 " " " 16.6 " 40 100 " " " 14.3 " 30 100 " " " 12.5 " 20 100 " " " 11.76 " 15
III. SEWAGE DISPOSAL BY DISCHARGE INTO RIVER OR SEA.
We will next deal with the conditions which should be fulfilled where it is sought to utilize a river or the sea into which to cast the sewage of a town. If it can be ascertained beyond question that at the proposed point of discharge the currents at all times will carry the sewage right away, and will not at the same time produce mischief at a distance (which is often omitted from the consideration), then that arrangement may be accepted as a good one. This, however, seldom occurs.
