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12
As far as our present effluent is concerned, these bacteria must
be constantly supplied or replenished with oxygen from the air
in order that they may work to the fullest advantage. Sewage
effluent merely sprayed or run over cascades takes as long as four
days to effect the same amount of purification as can be obtained
in eight hours by bacterial action. It is thus clear then that the
purification of sewage effluent by filters is due to the action of
bacteria, and the way in which such purification is effected is
attained either by passing the effluent through contact beds as
recommended by Mr Dibdin, or by sprinkling it over open filters
as recommended by Stoddart, Candy, and others.
The methods of working contact beds is best explained by
quotations from Dibdin's book on sewage treatment and the reports
of experts to the Leeds and Manchester Corporations.
Dibdin's London experiments show: " That so large a quan"
tity as 1,000,000 gallons of sewage effluent can be treated per
" acre per day." In speaking of experiments at Sutton, he states :
" The proprietary filter gave the best results; but this was caused
" by the lower rate at which it was worked " ; and then he directs :
" Beds should be worked as follows—One hour to fill a bed, two
" hours to rest full, one hour to empty and three hours to rest
" empty. It would be useful to throw one tank out of action each
" day, thus giving them all a day's rest once in nine days "; and
with regard to the filtrate from coarse beds, he says: " It should
" be treated in precisely the same way." He certainly goes on to
state that these processes might be modified, but he has advised
you specially that 2.25 acres is required to deal with 2,000,000
gallons daily of Kingston effluent.
Report of the Manchester experiments : " Each acre, being
'• laid out in beds of 3.33 feet in depth, with four fillings
" per 24 hours, would be capable of dealing with half a million
" gallons of sewage in round numbers daily, allowing for one day
" per week rest."
Report of the Leeds experiments: " If the bulk of
" the solids in suspension is first precipitated with a