It will be clear from the foregoing, that the changes going on in a septic tank are entirely different from those brought about in contact beds, and the question whether a septic tank is a necessity for the subsequent contact bed treatment, or whether it is a distinct disadvantage, can only be definitely settled when we know whether aerobic fermentation, i.e. decomposition, must in all cases be preceded by anaerobic fermentation, i.e. putrefaction, and to what extent, or whether such a succession of changes is not necessary.
[Sidenote: At Manchester contact beds accustomed to septic tank effluent did not at once purify raw sewage.]
It is interesting to note in connection with this point, that during the Manchester experiments it was established that contact beds, which have become accustomed to septic tank effluent, will not at once purify comparatively fresh sewage.
(_d_) _Velocity of Flow through Tank._—The velocity of flow through the septic tank is of great importance, as on it depends the size of the installation.
It seems to have become a habit to express this velocity by the length of the sojourn of the sewage in the septic tank—for instance, “the flow of sewage through the tank was such that it would fill it in twenty-four hours”; but as all tanks vary in size, and as in consequence the distance which has to be traversed by the sewage from the entrance to the exit in twenty-four hours is different in nearly every case, such a habit is, to say the very least, very misleading.
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.