Chapter VI: Part II: By H. Alfred Roechling (4)

-----+------------+-----------------------+
| | Rate of Flow. |
| +-----------------------+
| | | |
No. | Name of | Length of | Velocity |
| Town. | Sojourn | of Flow |
| | of Sewage | per |
| | in Tank. | minute. |
| | | |
-----+------------+-----------+-----------+
| | | |
| | days. | inches. |
1 | Exeter | 1·0 | 0·52 |
| | | |
2 | Manchester | 0·44 | 5·58 |
| | | |
3 | “ | 0·56 | 4·44 |
| | | |
4 | Leeds | 0·5 | 1·68 } |
| | | |
5 | “ | 1·0 | 0·84 } |
| | | |
6 | “ | 2·0 | 0·42 } |
-----+------------+-----------+-----------+

-----+-----------------------------------------------
| Suspended Matters in Sewage.
|-----------+-----------+------------+----------
| | | |
No. | Remaining | Destroyed | Leaving |
| in | and | Tank | Total.
| Tank. | Liquefied | in |
| | in Tank. | Effluents. |
| | | |
-----+-----------+-----------+------------+----------
| | | |
| per cent. | per cent. | per cent. |
1 | 17 | 39 | 44 | 100
| | | |
2 | 41 | 22 | 37 | 100
| | | |
3 | 23 | 33 | 44 | 100
| | | |
4 | .. | .. | .. | ..
| | | |
5 | average | 28 | .. | ..
| say | | |
6 | | .. | .. | ..
-----+-----------+-----------+------------+----------

[Sidenote: Septic tanks reduce the sludge difficulty to some extent, but do not altogether remove it.]

(_e_) _Destruction and Liquefaction of Sludge in Septic Tanks._—It was formerly maintained that the employment of a septic tank did away with all sludge difficulties, and one sees even now advertisements to that effect, that there is “no sludge” with a septic tank; but experience everywhere does not bear out this contention. On the contrary, there must be sludge with a septic tank, and the only question one has to consider is, to what extent does a septic tank reduce the quantity of sludge?

The table above contains the results obtained in the various experiments, and from these it would appear as if on an average, with a velocity of 1 inch per minute, 25 per cent. of the total sludge would be destroyed or liquefied in a septic tank. Generally speaking, therefore, the following figures will be somewhat near the mark, where the plant is worked systematically and carefully supervised.

Per cent.

Suspended matters remaining in tank 35
“ “ destroyed or liquefied in tank 25
“ “ escaping in effluent 40
---
Total 100

These figures mean that 35 per cent. of the total suspended matters will have to be dealt with as sludge, 25 per cent. will be destroyed or liquefied in the septic tank, and the remaining 40 per cent. will be deposited on and in the contact beds.

It has already been pointed out that it is claimed that the septic tank sludge is denser and contains less moisture than ordinary sludge, and that about half of it is mineral matter.

As previously stated, at Manchester a reduction of about 16 per cent. in the dissolved matter has been observed in the open septic tank.

(_f_) _Formation of Gas in Septic Tank._—It was at one time suggested that the gases formed in septic tanks during anaerobic fermentation might be utilised for lighting or heating purposes, but anyone well acquainted with the subject will admit that such a use is outside the range of practical possibilities.

At Manchester, 100 gallons of sewage evolved in twenty-four hours about a cubic foot of gas, which on an average contained:

Per cent.

Marsh gas, CH{4} 73
Carbon dioxide, CO{2} 6
Hydrogen, H 5
Nitrogen, N (by difference) 16
---
Total 100

At this rate 1 million gallons of sewage will evolve 10,000 cubic feet of gas, or 0·2 tons of gas, in twenty-four hours.

[Sidenote: Septic tank effluent more suitable for nitrification.]

(_g_) _Mixing Action of Septic Tank._—There is one advantage possessed by a septic tank which cannot be disputed, and that is the mixing action going on within it. The fresh sewage on its arrival becomes mixed with stale sewage, and, owing to the rising of lumps of sludge from the bottom, and other causes, the contents of the tank become of a more uniform composition, which must entail a corresponding advantage for the subsequent contact bed treatment.

[Sidenote: The septic tank effluent is so far as bacterial purity is concerned practically raw sewage.]

(_h_) _Micro-organisms in Effluent from Septic Tank._—Although the available number of experiments on the micro-organisms contained in the effluent from a septic tank is not large, yet they support the conclusion which one would form by analogous reasoning, that so far as the bacterial flora is concerned the effluent is practically raw sewage.

4. CONTINUOUS CONTACT BEDS.

[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.

VIII. CONCLUDING REMARKS.

Since the foregoing observations were penned, the Chairman of the Royal Commission on Sewage Disposal has delivered a very interesting inaugural address, in August last, at the Congress in Exeter of the Institute of Public Health, to which attention ought to be drawn at this point.

According to _The Times_ he is reported to have stated as follows: “He regretted that he could not give some idea of the probable date at which the Commission would issue its final report and recommendations. They would soon, he hoped, be able to publish the results of a prolonged investigation into the treatment of sewage on land; and their experts were now making elaborate parallel examinations of some of the processes of filtration by artificial means. But he feared that they would ultimately be obliged to bring their proceedings to an arbitrary close; for, however much they could learn, he was quite certain they could never come to a point at which they could say there was nothing more to be learned. The subject was inexhaustible.”

These very guarded observations are almost in direct contrast with the very positive assurance of some enthusiastic supporters of artificial treatments, who a year or two ago did not hesitate in proclaiming throughout this country that the panacea for all sewage difficulties had been discovered, and that the investigations of the Royal Commission were a mere matter of form and a foregone conclusion.

To all those who did not share these very sanguine expressions of faith, and who were painfully aware of the great gaps in our knowledge of the processes taking place in sewage purification, these words of Lord Iddesleigh will prove an assurance that the commissioners are not swayed by popular likes and dislikes, however fascinating they may be, but that they are earnestly endeavouring, in an impartial manner, to throw such light upon this abstruse question as will enable them to arrive at correct conclusions.

For a like purpose the foregoing remarks have been written; and if the facts recorded in the previous pages, and the opinions expressed therein, should prove of assistance to anyone in forming correct views, the labour spent on them will be amply repaid.

POSTSCRIPT.

Since the foregoing remarks were written, I have been somewhat struck with the views expressed at one or two meetings by some of those who ought to have the full facts of the case at their fingers’ ends. There seems to be a considerable vagueness as to the sanitary results to be obtained by either the natural or one of the artificial methods of sewage treatment, and with a view to making this point quite clear I have prepared the following comparative statement (see next page), which, I trust, will show at a glance what one may expect from either system.

This statement has not been prepared from experimental installations, where, as a rule, better results are obtained than in actual every-day work; but it refers to fairly large works dealing from day to day with the whole town’s sewage. It has further been assumed, that the plant both for the natural as well as for the artificial treatment is suitable, and managed carefully and on intelligent lines.

As in all sewage treatments the sanitary results have first to be considered, I have only dealt with them in the statement, but even if I had extended it to economic considerations the result would have been practically the same.

From a careful examination of the facts recorded in the statement it follows that with natural treatment we get five distinct advantageous results, against which we have to place only two on the side of the artificial treatments; but this means, that if we wish to bring up the results of these treatments to those obtained by the natural treatment, we have to supplement them by three further treatments for the extraction of pathogenic germs, and of the manurial elements, and for the reduction of the liquid.

It is these facts which ought to be carefully considered by all those who wish to study the comparative advantages of these two systems, or who have to decide on a definite method to be employed in a particular case, and no step ought to be taken before every one of these five points has been very carefully weighed. Generally speaking, that system will be preferred which confers the greatest number of advantages.

STATEMENT.

_Results to be obtained from_

(A) _Natural Treatment._ (B) _Artificial Treatment._

1. Removal of suspended 1. Removal of suspended
matters. matters.

2. Removal of from 75 to 95 2. Removal of from 50 to 75
per cent. of the dissolved per cent. of the dissolved
organic matters. organic matters.

3. Removal of pathogenic 3. Nil. Effluent bacterially
germs. practically raw sewage.

4. Utilisation of large portion 4. Nil. All manurial elements
of manurial elements. escape into the rivers.

5. Great reduction of quantity 5. No appreciable reduction
of liquid. of quantity of liquid.

LONDON: PRINTED BY WILLIAM CLOWES AND SONS, LIMITED,
GREAT WINDMILL STREET, W., AND DUKE STREET, STAMFORD STREET, S.E.

Transcriber’s Note:

Words and phrases in italics are surrounded by underscores, _like this_. Chemicals are displayed with the numbers surrounded by braces, e.g. CO{2}.

Footnotes were numbered sequentially and moved to the end of the section in which the related anchors appear.

Wide tables were split for easier viewing on small screens.

An unprinted letter “S” was added to sidenote, “Springs.” Obsolete and alternate spellings were not changed.