It will not be disputed that the deposition of the suspended solids in sewage is dependent on the rate of movement of the liquid, and that in a quickly moving liquid there will be less deposition than in a very slowly travelling liquid. -----------------+----------+---------+-----------+------------- Town. | Length. | Width. | Depth. | Contents. -----------------+----------+---------+-----------+------------- | | | | | feet. | feet. | ft. in. | gallons. Manchester tanks | 300 | 100 | 6 0 | 1,125,000 | | | | Leeds “ | 100 | 60 | 7 7 | 250,000 -----------------+----------+---------+-----------+------------- Bearing this in mind it will not be without interest to examine the velocities employed during the Manchester and Leeds experiments. The tanks employed in these have the dimensions given in the table on the preceding page. Now assuming that each tank is to be filled once in twenty-four hours we obtain the following velocities: Manchester (300′ 0″ × 12″)/1440 = 2″·5 per minute. Leeds (100′ 0″ × 12″)/1440 = 0″·84 per minute. Which means that in the Manchester experiments the velocity would have been three times as large as in Leeds; and it is clear that if the sewage of both towns was identical the results, so far as the retention of the suspended matters in the tanks are concerned, could not have been identical. As a matter of fact, considerably greater velocities have been used in the Manchester experiments, as will be shown later on. [Sidenote: Rate of flow through septic tanks should not be expressed by the length of sojourn in tank but by some linear measurement in a stated time.] It will be clear from this, that it is most misleading and erroneous to express the rate of flow by the length of the sojourn of the sewage in the tank, and that the velocity should in each case be expressed by some linear measurement in a stated time—probably inches per minute. The next point to consider is the velocity to be employed in septic tanks; and here it is not without interest to refer to the various experiments enumerated with their results in the next table. The difference in the results obtained, so far as the suspended matters are concerned, is probably due to the different character of the various sewages experimented with; but so low a velocity as 0·52 inch, as used in the Exeter experiments, does not appear to be necessary. In the Leeds experiments, it was found that the filling of the tank once in twenty-four hours gave the best results; and as the velocity then was 0·84 inch per second it will be somewhat near the mark to recommend generally a velocity of 1 inch per minute. On the assumption that the sewage shall remain twenty-four hours in the tank, this gives a length of tank of 120 feet, which is a very suitable one.