To sum up the general conclusions to which my experience points, and which I trust may prove useful to district councillors, they are as follows:—
1. In works of sewerage, limit and regulate, as far as possible, the volume of sewage by excluding subsoil water and clean surface water.
2. Where the outfall sewage enters the disposal works provide a pair of open catch-pits (or grit-chambers), each twice as wide as, and 2 feet deeper than the sewer, with sluices allowing the sewage to pass through one pit at a time in its free course, while the other pit is being dried and the deposited detritus dug out. The depth below sewer invert may be more than 2 feet, and length of catch-pit is immaterial, but I confine its width to twice that of the sewer in order to conserve sufficient velocity in the current to carry forward organic matter, paper, etc., and leave only clean sand and gravel in these catch-pits.
Continuing the course by open channel (of same width as outfall sewer), it should expand to five or six times its width, forming the screening chamber, and thence discharge into the
3. Depositing tanks. These are best formed in concrete with smooth surface, with a semicircular level weir from which the liquid overflows into a semicircular collecting open carrier leading to the aerobic process on land or contact bed.
The semicircles above referred to are struck from centre of the inlet to depositing tank with a radius of 50 feet or more.
The weir level should be at least 1 inch below that of invert of inlet, and the depth of tank immediately under this point should be governed by consideration of the facility of drawing off the sludge by valve at that depth to the sludge drying beds by gravitation if possible, or pump if necessary, and
from this sludge emptying valve the smooth concrete bottom of tank slopes up to the semicircular weir above described.
The bottom and sides of such a tank should be made with the best Portland cement and finest granite chippings wrought to a smooth surface, so that the sludge may be easily swept clean away with a squeegee to its outlet valve, as it is very necessary to have the tank thoroughly washed after each emptying if my view of the clean mode of sewage disposal is to be carried out.
But with the dirty mode, on the contrary, some of the sludge only should be drawn off and the septic anaerobic action preserved continuously in the tank itself, whereas I prefer that action to have its early and less offensive course in the tank and its completion in a drying bed mixed if possible with farm-yard manure.