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insured by an intelligent working of the bed. The extreme capacity values vary from 20 to 33 per
cent, of the whole space represented by the bed when full—including both coke and liquid. An
average of about 30 per cent, would represent the average permanent capacity of a working bed.
Three fillings of the beds per day were advocated by most of the witnesses, and the
quantity of sewage per day dealt with by an acre of bed, three feet in depth, was placed at
not more than 800,000 gallons. This is undoubtedly understating the capabilities of the bacterial
system. Deep beds have given quite as good purifying results as shallow beds.
The quantity of sludge which disappears by anaerobic action in the settling or "septic"
tank is variously estimated at from 20 to 60 per cent, of the whole amount deposited in the tank.
It is curious to note that Mr. Whittaker claims that no sludge disappears in his septic tank,
though he admits that large quantities of gas are found. (Questions 5742-65). In crossexamination
on this matter, he can hardly be said to have proved his statement.
It was generally admitted that a septic tank effluent is capable of being rendered nonputrescible
by one contact in an aerobic bed.
Professor Letts, in his evidence with reference to the purification of Belfast sewage, stated
that salt prevented the formation of nitrates. This perhaps accounts for the low proportion of
nitrates found in the effluents from the Crossness beds, as compared with that found in the
effluent from the Barking beds, since Crossness sewage contains an abnormal quantity of salt.
Dr. Adeney's evidence will be found to have a bearing on the present and probable future
condition of the river Thames.
Briefly, it may be said that although the experts differed in small matters of detail, they
agreed in the broad principles of sewage purification by bacterial means, and their evidence is
a strong confirmation of the results obtained by the experimental work carried out at the
Council's Outfalls.