By what means may these effects be obviated?—In the Holborn and Finsbury divisions I suggested a plan of flushing the sewers, and of carrying off all the refuse by water. This plan has been adopted, and it is now in operation. The breaking up of the streets is avoided by the formation of side entrances; cast-iron flushing gates are fixed in the sewers; the ordinary flow of water in the sewers accumulates at these gates; the gates are opened, and the force of the water is sufficient to sweep off the deposit; and the system may be further extended.
What is the comparative difference in the expense of construction?—The cost of side entrances and flood-gates, as compared with the cost of man-holes, is from 6_d._ to 1_s._ less per foot lineal of the length of new sewers.
What other expense is attendant on this improved practice?—The main expense is the attendance of a man to shut and open the flood-gates.
The structural expense being lower, is the ultimate expense of cleansing lower also?—Yes; the expense of cleansing the sewers is about 50 per cent. less than the prevalent mode. Our expense of cleansing the sewers was about 1,200_l._ per annum; we save 600_l._ of that, and expect to save more; but to this must be added the saving to the public of the cleansing of the private drains, formerly choked by the accumulations in the sewers. This saving, on a moderate calculation, is found to be upwards of 300_l._ per annum. There is also the diminution of the escapes of gas from the old and continued accumulations.
During what intervals are deposits allowed to remain on the old mode?—The average is in one set of sewers about five years, and in another about ten years.
During which time the public are subjected to all the escapes of gas from the decomposing accumulation?—Exactly so. It could not, however go on so long but for heavy falls of rain or snow, which occasion partial clearances.
What is the effect of these accumulations upon the private drainage?—That the drains to the private houses are stopped: the first intimation of the foul state of the main sewer arises from complaints of individuals whose drains are affected; the accumulations in the private drains also occasions an expense to the individuals and much annoyance. By flushing the sewers this expense might be, and in Holborn and Finsbury division it is, avoided.