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"small quantity of lime, it may be possible to obtain
"good results by filtration over a fine bed only (such as coke
"breeze), and if the bed is five feet deep, an acre would probably
deal with 800,000 gallons a day."
Exeter septic tank system: "In this system about 700,000
"gallons per acre are dealt with, per diem."
It will be seen that the hours laid down by Mr. Dibdin for the
nefessary period of purification at each filling are seven. It is
therefore clear, allowing for complete rest once in every seven or
nine days, that the filter can only be used at most three times in 24
hours. It is, however, suggested that with a partially purified
effluent it might be possible to reduce the periods of rest and fill
the contact beds four times in the 24 hours.
The average flow at Kingston is about 2,000,000 gallons, so
that, for four fillings daily, the water capacity of the tanks must
be at least 500,000 gallons. The filtering material occupies twothirds
of a contact bed filter, so that the total holding capacity of
such tanks must be at least 1,500,000 gallons. Now, two acres
of filter tanks three feet deep (the levels hardly permit of greater
depth) have a total holding capacity of 1,631,080 gallons, so that
Messrs. Dibdin and Thudichum, in recommending 2 % acres, mide
allowance not only for a periodical rest of one day in seven or nine,
but also for the previous partial purification of the effluent.
Recent experiments have shown that if, instead of passing the
effluent over filtering materials, as in the contact beds, at intervals
of hours,it is sprinkled on at intervals of minutes, this minute subdivision
of the effluent permits it to trickle over the filtering material,
and carry with it into the interstices of the filter the air
necessary to feed up the bacteria with oxygen for the proper performance
of their work.
These filters are cheaper in construction, as retaining walls are
not needed, the only building work being the supports for the
sprinklers and the channels. A coarse open material, that will
allow free ingress of air, and of a hard nature that will not break