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VI.—FINAL CONCLUSIONS.
Before giving the final conclusions from the biological point of view as to the value of the
bacterial process in use at Barking and Crossness for treating London crude sewage, it is essential
to first make some remarks of a general and introductory character; otherwise a totally erroneous
impression might be formed, and it might even be considered that the conclusions arrived at after
two years' close study of the subject were of such a nature as to condemn the process. Thus, if no
note were taken of the chemical results and the practical side of the question and the
bacteriological results were considered by themselves, and not read as they ought to be side by
side with the chemical data, an impression might be gained which was not intended, and which
was quite remote from the truth.
Many of the points about to be alluded to have been dealt with in previous Reports, but it
is necessary to repeat them here for the reasons which have been stated,
I.—GENERAL CONCLUSIONS.
The Thames is a tidal river, and the "intakes" for waterworks purposes are situated high
up the river and are "cut off" by "locks" from the influence of the sewage discharge at the
Outfall Works.*
Although the bulk of London sewage is certainly very great the large size of the river
Thames must also be considered, as also the proximity of the "Outfalls" to the sea and the huge
body of water in the river at high tide.
The river water outside the influence of the discharges at the Outfall Works is already
polluted and contains practically all the bacteria found in raw sewage.
The body of Thames water which may be supposed to be directly affected by the discharge
of the effluents at Barking and Crossness is "brackish" and so could hardly be tolerated for
drinking purposes even by individuals who are indifferent to the physical characters and biological
composition of the water which they drink.
The bacterial coke-beds effect a reduction of at least 50, and it may be as much as 80 per
cent. in the amount of dissolved oxidisable and putrescible drganic matter in the raw sewage, as
compared with 17 per cent. effected by the chemical treatment.
The effluents from the coke-beds are apparently non-putrescible. §
The suspended matterst in the crude sewage are practically entirely removed as a result of
the biological treatment instead of only about 80 per cent. effected by the chemical treatment.
And recent experiments seem to show that this result may be obtained without loss of capacity in the
coke-beds if the sewage has undergone a previous process of rapid sedimentation.
In the biological treatment no chemicals are required, and no offensive sludge is produced.¶
Where an effluent is turned into a stream to be used for drinking purposes, the bacteriological
results are manifestly of more importance than the chemical; but when, as in the
present case, the river is not a potable one, and is already polluted, the converse probably holds
good; i.e., the chemical state of the effluent is of primary, and the biological of secondary
importance. In the first case the chief object is to guard against disease, and in the second case
the main consideration is to avoid fouling the river with putrescible matters to such an extent
as to constitute a grave public nuisance.
However desirable it may be to obtain an effluent chemically pure, and, at the same
time, bacteriologically above suspicion of danger, a process which leads to the solution of the
great mass of suspended matters in the raw sewage, which effects a striking reduction in the
amount of putrescible matters, which avoids the use of chemicals and the accumulation of offensive
sludge, which yields an apparently non-putrescible effluent, and which is at the same time
practicable, is one which presents such singular advantages over chemical treatment, and which
produces such satisfactory results in view of all the requirements of the case, that it would be
idle to deny its value or attempt to minimise its usefulness simply because it falls short of a
standard of absolute perfection.
Passing next to considerations which are purely biological in character, the following points
are worthy of note.
* It is assumed that no contamination of the underground water in the Thames valley takes place as a
result of the pollution in the lower reaches of the river affecting distant sources of water supply (e.g., deep wells).
If in the future reliable evidence to the contrary should be forthcoming, then the whole question might require
reconsideration.
§† It may be suggested that further evidence is required to show that an effluent apparently nonputrescible
remains so under all conditions, and also that an effluent free, or nearly so, from suspended matter
remains permanently in that condition. For it is conceivable that an effluent non-putrescible in itself might yet
give rise to a putrescent effect when discharged into a stream, and that substances originally in solution might
pass out of solution as the result, perhaps, of secondary degeneration. Nevertheless, from actual experiments
carried out by the chemists in charge at the Outfall Works, as well as by observation of the state of the river, this
would not seem to take place in the case of the discharge of the effluents at Barking and Crossness into the
Thames.
¶ As regards this paragraph and the preceding ones, the Second Report (Division 1, Section 1, No. 5, page 7)
may with advantage be consulted.