[Sidenote: Continuous contact beds still in an experimental stage.]
It is necessary to make at this point a few short observations on the
artificial self-purification in continuous contact beds.
This method of artificial purification has frequently been called
“continuous filtration,” but it will be much better to reserve the
term “filtration” for the percolation of water through fine material,
such as sand, and to call the continuous flow of sewage through
coarser material continuous contact bed treatment, as the processes
going on during the same are more analogous to those going on in an
intermittent contact bed than to those taking place in a waterworks
filter.
Formerly it was attempted to use the same kind of contact bed for
continuous treatment as is used for intermittent treatment, but, as
was to be expected, the results obtained were so unsatisfactory that
the experiments had to be discontinued. Now somewhat different forms
are utilised, which are mostly protected by patent rights, and the
mode of distribution has also been altered by the introduction of
patent distributors or sprinklers, which cause the sewage to fall in
very thin streams upon the filling material.
In the Manchester experiments, the proprietary continuous contact bed
does not appear to have given satisfactory results. Better effluents
were obtained at Leeds, and at York the results obtained are said to
have been very good.
On the whole, however, it is but right to say that the experience
gained so far is not sufficient to entitle us to form definite
opinions, and for this reason it will be better to await further
results.
VI. MANAGEMENT OF PLANTS FOR THE ARTIFICIAL SELF-PURIFICATION OF
SEWAGE.
[Sidenote: Plants for the artificial self-purification of sewage
require very careful handling.]
It was formerly frequently concluded that neither septic tank nor
contact beds required careful superintendence, but that they could be
worked by automatic machinery and left to themselves. It was therefore
maintained that the working expenses of plants of this nature would be
next to nil. This was, however, not Mr. Dibdin’s view, who, after
years of careful study, came to the conclusion that they were delicate
pieces of mechanism which required careful watching.
Since, Mr. Dibdin’s conclusions have been amply confirmed by all
careful experimenters.
For instance, Mr. Fowler, the chemist in charge of the Manchester
experiments, observed before the Royal Commission on Sewage Disposal
as follows: “It is a delicate operation (the management of septic tank
and contact beds), which requires careful watching! There is no doubt
whatever about that!” (Question 5651.)
Again, the conditions of successful working of contact beds, laid down
by the same gentleman on page 64 of the Manchester report for the year
ending March 27, 1901, are ample proof of this, and they show very
clearly how extremely careful the supervision of such a plant ought to
be, and that in the hands of inexperienced men it will soon come to
grief.
Professor Percy Frankland stated in his evidence before the Royal
Commission, that in his opinion land required less skilled supervision
than contact beds. (See Questions 9937, 10071-74.)
A similar view was expressed by Mr. H. M. Wilson, the chief inspector
of the West Riding of Yorkshire Rivers Board. (Question 6380.)
VII. SOME OBSERVATIONS ON THE DEPOSITION OF SUSPENDED MATTERS IN
TANKS.
[Sidenote: Definition of the term “deposition.”]
The term “deposition” shall here be held to mean the precipitation of
the suspended matters without chemicals or other artificial means,
i.e. the unaided subsidence of these matters at such a rate of flow
that septic action is not set up within the tanks.
The question that is of interest here, is: Which is the most
favourable rate of flow of the sewage through the tank, so far as
the deposition of the suspended matters is concerned? To some extent
the answer to this question will depend on the special characteristics
of the particular sewage under consideration, but for general purposes
the following observations will not be without interest.
Although of very great importance, this question does not appear to
have received very general consideration, as the available number of
careful experiments is but small.
[Sidenote: Tank velocity at Barking.]
It appears that the calculated velocity in the channels of the
precipitation tanks at Barking is about 4 feet per minute, and that
with this velocity about 77 per cent. of the suspended matters were
deposited in the year 1894.
[Sidenote: Tank velocity at Manchester.]
At the Manchester tanks it is stated that a velocity of 3 feet 4
inches per minute is employed.
[Sidenote: Velocity frequently adopted.]
A rate of velocity now frequently adopted in this country for new
works is 6 inches per minute.
[Sidenote: Frankfort experiments.]
In the settling tanks at Frankfort on the Main there are deposited
about 84 per cent. of the suspended matters, with velocities ranging
from 9½ inches to 16½ inches per minute.
[Sidenote: Cassel experiments.]
With a velocity of 7 inches per minute, it is stated that at the
Cassel sewage works 97 per cent. of the suspended matters are retained
in the tanks.
[Sidenote: Hanover experiments.]
At Hanover a set of interesting observations has lately been made, on
tanks 246 feet long, with a view to ascertaining the most advantageous
rate of flow.
With a velocity of 9·44 inches per minute, 62·7 per cent. of the
suspended organic matters were precipitated, with a velocity of 14·17
inches per minute 61·7 per cent. were deposited, and with a velocity
of 35·43 inches per minute 57·3 per cent.; from which figures it will
be clear that there is not much difference in the result on the
suspended matters between these velocities.
Against these results must be placed the results obtained with septic
tanks, where, as has frequently been stated, a velocity of 1 inch per
minute and a sojourn of twenty-four hours in the tank may be expected
to lead to a deposition of about 60 per cent. of the suspended
matters.
[Sidenote: Reduction of cost.]
Where, therefore, a previous septic treatment of the sewage by
anaerobes is not necessary, it is clear that the substitution of
ordinary settling tanks for septic tanks will be accompanied by a very
considerable reduction of cost.