Raking of beds not advantageous.
The raking of the surface does not materially affect the capacity of the bed, and it is better to scrape off the matters retained on the surface than to rake them into the body of the bed.
Renovation of filling material either partially or wholly.
It will be clear from these observations that, no matter how carefully the bed has been worked, sooner or later a time will come when the decrease of capacity becomes so pronounced as to render it impossible any longer to treat the daily flow of sewage with the available plant; and when this point has been reached a renovation, either partially or wholly, of the filling material becomes an inevitable necessity.
Minimum capacity of beds to be provided for.
To provide for this at the outset, and thus avoid the
difficulties of reduced capacity, it seems advisable to lay down, when designing the works, a minimum capacity, which will just allow the daily volume of sewage to be treated by the plant, and which when reached will necessitate the cleansing of the bed.
The idea, formerly frequently expressed, that the filling material when rationally worked need not be renewed or renovated, can no longer be maintained and is outside the reach of practical possibilities.
Underdrainage of intermittent contact beds.
At this place a word or two about the under drainage of intermittent contact beds may not be out of place. It is of the greatest importance that all drains should work well, and that the entrance of the sewage into them should not lead to disturbance in the filling material, especially should the tearing of portions of the filling material into the drain pipes be avoided.