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31
The Superintendent (Mr Sylvester) has during the year
removed a large quantity of this useless material from the
Filters and it is the aim of the Council to entirely replace it
so soon as possible by the hard core and burned cinder ash
which is procured from the dust collected in the district.
With the Filter Beds hitherto so inefficient, it is a matter
of some surprise that the effluents should have been so satisfactory,
but it must be remembered that in addition to a very
thorough chemical treatment of the sewage, the effluent from
the settling tanks is not entirely dependent upon niter beds for
its further purification, for it is subsequently exposed to considerable
surface flow over grass fields before it enters the
brook. It is a fact, well known to those who have had special
experience in the methods of purifying sewage effluents that
such surface flow, even when over a stiff clay soil, brings about
an improvement in the effluent which is remarkable, in that it
far exceeds that which would be conjectured on chemical
and physical grounds.
During the year I have performed a very large number of
analyses, with the object of ascertaining if
(a.) We were using more lime than was necessary.
(b.) or using more alum than was necessary.
(c.) or whether these two substances were combined in
their best proportions.
I considered that there was probably some waste of
chemicals, except perhaps in those cases where the sewage
came up very black and thick, and I thought that perhaps
some saving could be effected in chemicals without in any
way affecting the satisfactory character of the effluent; and if
so the amount and causticity of the sludge would also be
thereby reduced—another very positive advantage.
The recommendations of the late Dr. Tidy were that the
reagents employed should be alum and lime, and that they
should be added to the amount of 7 and 10 grains respectively
to the gallon of sewage.