LIEUT.-COL. ALFRED S. JONES, V.C. (continued 2)
Reading edition
Absorption of dissolved substances by soil.
More interesting still, and also more important, is the absorption of dissolved substances by soil. In this way is to be explained the decolorising effect and the retention of dissolved polluting substances such as are contained in sewage. In the same way soil has the power of destroying such poisons as strychnine, nicotine, coniine, etc., and the experiments of Falk and others go to show that ptomaines and toxines are likewise retained and rendered harmless by it. This absorbing power of soil is of the utmost importance in agriculture, and without it soil could not possess purifying powers for polluting liquids. It is quite true that in this process of purification other factors play an important part, but they could not come into play if this absorption did not exist.
The absorbing powers of soil are in some way dependent on the presence of micro-organisms and air, and in the absence of these they will soon come to a standstill.
IV. SELF-PURIFYING POWERS OF SOIL. NATURAL SELF-PURIFICATION OF SEWAGE.
Self-purifying powers of soil.
After these preliminary remarks it becomes necessary now to examine into the self-purifying powers of soil with special reference to sewage farms. Generally [Pg 53] speaking, the term “self-purifying powers of soil” comprises all those processes which go on on the surface and in the pores of the soil of sewage farms, and by which polluting liquids such as sewage become purified as these take place under natural conditions and in a natural medium, the process of land treatment of sewage is called—see previous observations—"the natural self-purification of sewage.”
Self-purifying powers vary with local conditions. Soil best suited for sewage farms.
It should be stated at the outset that the self-purifying powers of soil will depend largely on the soil itself and the local conditions under which they come into play, so that observations made in one locality will not be immediately applicable to others without making full allowance for the differences; this will be clear from the preliminary remarks as to the character and properties of soils made in the previous pages. As will be pointed out more in detail later on, a subsoil that combines great permeability for air with high retaining and absorbing powers, is best suited for sewage farms.
Let us now consider what becomes of water, sewage or any other polluting liquid containing organic substances after it has been poured out upon the surface of the ground, and for this purpose we will assume a subsoil of a suitable character and in fair condition for work with proper under-drainage.
Retention of liquid by pores of soil.
The liquid thus poured out upon the surface will sooner or later disappear in the soil, and will at first be retained in the pores of the zone of evaporation, which may be said to extend to the level of the under-drains. This retention is due to the retentive powers of soil.
Suspended matters retained on the surface, soil acts like a sieve. Coating of surface of the land. Removal of suspended matters generally an advantage.
Portion of the suspended matters will be retained on the surface and the rest will be strained out in a mechanical manner in the pores, the soil acting as a sieve more or less fine according to its character. If [Pg 54] the suspended matters are present in very large quantities it may happen that they will gradually form a coat on the surface of the land and choke the pores to the exclusion of air, and as this is a thing to be avoided in sewage farming it is in most cases advisable to remove them out of the liquid before it is poured upon the land.
The more finely divided the suspended matters are, the lighter the work of the land.
Even where such a removal has taken place there will still be left a certain portion of the suspended matters, and if these are in a finely divided state, such as is probably the result of their passage through fine strainers or pump valves, the work of the land will be considerably lightened.
Micro-organisms screened out in a mechanical way.
The micro-organisms contained in the liquid will be to a large extent screened out in a mechanical way with the suspended matters and deposited on the surface and in the upper layers of the soil.
Retention of matters in solution after removal out of the liquid is due to physical and chemical agencies.
The matters in solution will partly, after removal out of the liquid, be retained by the absorbing powers of the soil in the pores, a process that is due to physical and chemical agencies.
Absorbing powers gradually ripen.
It is well known that land which is being treated with sewage for the first time does not purify sewage so well as land that has been under systematic treatment for some time, and this is probably due to the absorbing powers, which gradually ripen until they have reached their maximum of efficiency. This process of gradual improvement seems to be due to the formation of a slimy coating round each particle of soil, which growth does not only assist mechanical filtration, but also possesses high powers of absorbing oxygen.
Depths to which polluting substances may penetrate into soil.
The depth to which polluting substances may penetrate into soil will probably differ in each case, but the following factors may be said to influence it, viz. the [Pg 55] velocity of the downward flow, the nature and degree of the polluting liquid, and the character of the soil. Where, therefore, the powers of the soil are over-taxed the polluting substances may reach the level of the underdrains and pass out through them, in which case the effluent will be but little better than the raw liquid. It must be the aim of careful management to avoid this.
Process of decomposition of organic matters stored in soil during periods of rest.
The polluting substances of an organic nature thus stored in the pores undergo here—and that probably chiefly during periods of rest—a process of decomposition or disintegration, which goes on until the whole of the organic matter has been converted into stable mineral forms.
Explanation of the term “self-purifying power of soil.”
This process of retention, absorption and decomposition of organic impurities is called “the self-purifying power of soil.”
After conversion substances are removed out of the soil by the plants, by the subsoil air and subsoil water.
The substances thus converted do not remain in the pores, but they are removed either by the plants, for which they act as food, or by the currents of subsoil air, or by the subsoil water, and as the removal of fertilising substances by the subsoil water indicates a waste it must be the aim of a careful management to utilise them as much as ever possible for the benefit of the plants.
Process of digestion. “Sewage sick.”
The whole of these intricate and very complicated changes may be likened to the process of digestion in animals, and when these digestive powers are overtaxed signs of sickness may be noticed as the inevitable result, which increase until, in sewage phraseology, the land becomes “sewage sick.” In this condition it remains until the flow of the polluting liquid is stopped, when after a period of rest—recreative period—the digestive powers gradually return and begin to do their work afresh.
Action of lime.
When the soil of a sewage farm has got into this [Pg 56] state, owing to having received heavy doses of sewage, the application of lime has proved very beneficial by accelerating the process of nitrification, and in this respect interesting experiments have been made on the Berlin sewage farms. The action of lime is said to be a twofold one.
1. It quickly attacks and splits up the organic matters and accelerates afterwards their decomposition and their utilisation by plants; and
2. It neutralises the excess of acid in the soil, and causes the latter to part with its carbonic acid.
Decomposition proceeds quickest at or near the surface.
The process of decomposition proceeds as a rule at a much quicker rate on the surface and in the upper layers of the soil, where, as already mentioned, the number of micro-organisms is greatest.
When carefully worked there is no time limit to the purifying powers of the soil.
It has been maintained that the soil of sewage farms will after a while silt up and cease to purify sewage, but the results obtained with carefully managed farms clearly disprove this, and under these conditions there appears to be no limit as to time to the purifying power of soil.
Depth of soil necessary for purification.
Concerning the depth of soil—evaporation zone—that is necessary for the successful retention, absorption and decomposition of sewage, no generally applicable rule can be laid down, as this will depend on a variety of factors, amongst which may be mentioned: the character and thickness of the top soil (humus), the nature and cultivation of the top soil; the character of the subsoil—its permeability for air and its retaining and absorbing powers; the surface slopes of the land and the level of the subsoil water.
Greater depths than 4 ft. will be rarely necessary.
On some farms a depth of 3 feet on an average has proved sufficient, and on others the drains have been laid at depths ranging between 3 and 6 feet, but very [Pg 57] special reasons ought to be shown for all depths over 4 feet.
Soil best suited for sewage farms.
Whilst practically no soil is entirely useless for sewage farming, with the exception perhaps of peat, owing to the quantity of moisture it contains, a soil that combines great permeability for air with high retaining and absorbing powers—such as a loamy sand with fairly large grains—is probably the best.
Clay soil not unsuitable for sewage farms, but it necessitates a greater area of land.
It has been maintained that clay, owing to its impervious character, is totally unsuitable for sewage farming, but the experience of such farms as South Norwood, Wimbledon, Warwick and Leicester disproves this. It is true, however, that as the purifying powers of the soil are restricted in a vertical sense to the upper layers, it may become necessary in places to extend the area of the farm beyond what would be necessary with a more pervious soil.
Changes observed in the heavy clay land at Leicester since sewage treatment was commenced.
It may not be without interest to draw attention here to some of the changes that have taken place on the Leicester sewage farm since the land has received regular dressings of sewage. When I was engaged in laying it out in 1888 my powers of locomotion over the land were greatly impeded during wet seasons by the inordinate amount of clay that adhered to the boots; but when engaged again for some considerable time on the land during the winter 1900 to 1901 this unpleasant peculiarity had completely disappeared even on land that had recently been sewaged. Through the action of the sewage the very dense clay had been disintegrated and become so pliable that, when trod upon, it crumbled to pieces. The colour of the soil had been changed from a yellowish-brown to a greyish-black, and altogether the land had been greatly improved by the application of the sewage.
Movement of liquid through the passage and capillary zones to the impervious layer.
[Pg 58] If more sewage is poured upon the land than the effluent drains can deal with—and here it may be well to bear in mind that on sewage farms in our climate on a broad average throughout the year about one-third of the total quantity is lost by evaporation—the excess will pass down between the drains from the evaporation to the passage zone, and if the flow of the sewage is not discontinued the downward movement in the passage zone may be continued until, after having traversed the capillary zone, the level of the subsoil water is reached.
Length of downward movement of water may be very great.
What length of time may elapse before this level is reached will entirely depend on local circumstances, but it will be clear from the preliminary remarks that the completion of this downward movement may in places and under certain conditions take a very long time.
Displacement of sewage held by the pores of the land by the fresh discharge of sewage upon the surface of the land.
In connection with this it is of importance to point out that not the fresh sewage which is poured on the surface of the land will at once pass into the lower layers, but a portion of the old sewage, which up to then was stored in the pores and is now displaced by the fresh discharge, so that the fresh raw sewage is retained and only purified sewage allowed to escape into deeper layers, which means that in its downward movement all sewage undergoes purification. Were this not the case the raw sewage might reach the effluent drains.
It appears time now to examine somewhat more closely the processes of decomposition and the products elaborated therein.
Factors that influence the process of decomposition.
Concerning the factors which have a favourable influence upon this process, some of them, such as permeability for air, high retentive and absorbing powers, have already been mentioned, and to these can be added moisture and warmth, the latter of which are always present in sewage.
[Pg 59]
Advantages of a systematic underdrainage.
One word here concerning the systematic under-drainage of the subsoil. Its chief function is, of course, the carrying away of the effluent water and by doing so to prevent the formation of a swamp, but after the land has done its work, and during so-called periods of rest, the under-drains act as ventilators of the subsoil and thus make it artificially more permeable for air, with the result that a drying-up action is set up and oxygen supplied for micro-organic life. For the purpose of improving the ventilation of the soil it may become advisable in places to connect the upper ends of the drains with a short upcast shaft. The mouths of the drains should always discharge above water so as to allow of a free circulation of air.
Micro-organisms that carry on the work of splitting up and converting organic compounds.
The work of splitting up and converting the organic compounds is primarily carried out by micro-organisms such as yeast fungi, mould fungi, algæ, protozoa and even by higher forms of life such as earthworms and insects.
To what extent in addition to these other agencies take part in this bio-chemical process is not yet fully elucidated.
Decomposition and putrefaction most complicated processes.
Fischer in his interesting book, ‘The Structure and Functions of Bacteria,’ observes (page 99): “The decomposition of dead animal bodies, of vegetable tissues, or of substances like stable manure, is far from being a simple putrefactive process. Side by side with the disintegration of nitrogenous bodies there are going on a number of fermentative changes by which non-nitrogenous compounds are being broken up, besides nitrification and other bio-chemical processes. For this reason it is always difficult and often impossible to determine the respective parts played by the different species of bacteria.…
[Pg 60] “The phenomena of putrefaction are so complicated that we do not know all of the compounds that arise during the process.… Very careful chemical investigations on pure cultures will be necessary before the chaos of phenomena presented by putrefactive bacteria can be arranged in something like order.
In the decomposition of proteids five or rather six stages may be distinguished.
"Proteids are split up by putrefaction into a large number of simpler compounds both nitrogenous and non-nitrogenous. The substances thus produced are precisely similar to those resulting from the artificial decomposition of proteids by fusion with caustic potash or boiling with hydrochloric acid or barium hydrate. Five groups may be distinguished:
Albumoses and peptones.
"1. Albumoses and peptones: soluble diffusible bodies closely resembling albumen. They are produced by the action on albumen of bacterial enzymes, similar to the enzymes (pepsin and pancreatin) which give rise to peptones in the digestive tract of man.
Aromatic compounds.
"2. Aromatic compounds; among others indol and skatol, which give the characteristic odour to human excrement; also some non-nitrogenous substances such as phenol, phenylacetic acid, and phenylpropionic acid.
Amido compounds.
"3. Amido compounds, all nitrogenous: leucin, tyrosin, aspartic acid, glycocol.
Fatty and aromatic acids.
"4. Fatty and aromatic acids, all non-nitrogenous and therefore having no part in the circulation of nitrogen; acetic, butyric, succinic and valerianic acids.
Inorganic end-products of putrefaction.
"5. Inorganic end-products of putrefaction: free nitrogen, ammonia, free hydrogen, methane, carbonic acid, methylmercaptan, sulphuretted hydrogen. It is probable also, but not certain, that phosphuretted hydrogen is formed and is oxidised at once by the free oxygen of the atmosphere.
Ptomaines.
"Most of these substances are formed also by the [Pg 61] chemical decomposition of proteids, but there is a sixth group which may be termed specific putrefactive products. These are the so-called ptomaines or putrefactive alkaloids.”
Some of these bodies are either not poisonous or only poisonous in large doses, whilst others, derived from putrid foods of various kinds (sausages, cheese), are highly toxic (ptomatropine, tyrotoxine).
Definition of some terms.
Concerning the work done by micro-organisms, it may not be out of place here to define the meaning of certain terms, and to direct attention at the same time to the modifications in the results brought about by the presence or absence of air during the various stages of the process.
The terms mineralisation, disintegration, oxidation, hydrolysis, bacteriolysis, nitrification, decomposition, eremacausis, putrefaction, fermentation, etc., have by many been used somewhat promiscuously, and this has led to a good deal of confusion, bewilderment and misconception. The cause of this has been undoubtedly our small amount of knowledge concerning this process and the changes brought about therein, but this would appear to be no reason why complication should be made worse. For the purposes of these remarks the undermentioned terms shall have the following meaning.
Mineralisation.
The term “mineralisation” is used for describing the whole process of the disintegration and conversion of organic into mineral matter, and no distinction shall be made between organic matter containing nitrogenous and organic matter containing carbonaceous substances.
Aerobic fermentation or decomposition.
When this process of mineralisation is carried on in the presence of sufficient quantities of air it is called “aerobic fermentation,” or “decomposition,” which is generally characterised by the absence of strong smells. [Pg 62] The process may then be called one of complete oxidation.
Anaerobic fermentation or putrefaction.
Where, however, the mineralisation proceeds in the absence of air the process is called “anaerobic fermentation,” or “putrefaction,” and it is then that very pronounced foul smells are emitted. The process may then be called one of incomplete oxidation.
Obligatory aerobes and anaerobes. Facultative anaerobes.
That class of micro-organisms which can only live in the presence of oxygen is called “obligatory aerobes,” and that which can only exist in the absence of this gas “obligatory anaerobes.” Between these two is the group of “facultative anaerobes,” which, while growing best with a plentiful supply of oxygen, are nevertheless able to exist with a very small amount, and even with none at all, although in this case their vitality is often much impaired.
Organic matters are first split up and then converted into mineral substances.
In the process of mineralisation two stages may be distinguished, viz. the first or disintegration stage, and the second or oxidation stage, i.e. the organic substances are first split up and afterwards converted into inorganic ones; and frequently these processes are taking place side by side and not after each other.
The splitting up of organic substances is frequently carried out in the presence of air.
It has been maintained—probably with a view to justifying the necessity of a septic tank—that the preliminary process of splitting up is best carried out in the absence of oxygen, but sufficient proof does not appear to have been advanced in support of this statement, and in some cases at any rate it is evidently carried out quite satisfactorily in the presence of air.
Concerning the presence or absence of oxygen, Fischer observes as follows:—
"The effects of the presence of oxygen are somewhat better understood. If air have free access, putrefaction (decomposition) may go on without any odour at [Pg 63] all, the evil-smelling gases (NH3 and SH2, for example) being oxidised at once to form nitrates and sulphates. Aerobic bacteria, too, such as the nitre and sulphur bacteria, bring about this mineralisation of organic nitrogen. Moreover, when air is circulating freely, there is no accumulation of intermediate products such as skatol or indol. It occurs on the surface of manure heaps, on the outer surfaces of carcases, and in well ventilated soil.
"In anaerobic decomposition (putrefaction proper), as in anaerobic fermentation, the organic molecules are at first only partly disintegrated, intermediate products such as leucine, tyrosine, skatol and indol being formed. In the absence of air these accumulate, and hence it is that putrefaction going on in the mud of ponds and ditches, or inside carcases, is accompanied by such evil odours.
"Although the details of the process vary considerably, according to the presence or absence of air, the ultimate products of decomposition and putrefaction are in both cases the same: namely, free nitrogen, free hydrogen, ammonia, methane, carbonic acid and sulphuretted hydrogen. These are also the end-results of the disintegration of the human body.
"After the organic nitrogen of decomposing substances has been converted into ammonia, and to a small extent into free nitrogen, the latter can at once be utilised by the root-nodule organisms and other bacteria in the soil, but the ammonia must undergo two further changes and combine with a base to form a nitric salt before it is available for plant life. These two changes are brought about by bacteria, which convert the ammonia first into nitrous and then into nitric acid; this process has been called ‘nitrification.’”
[Pg 64] It will be clear from the foregoing remarks that the process of mineralisation is a very complicated one, which under favourable conditions, for instance in the pores of an open soil, may come to an end fairly quickly, but which under very unfavourable conditions—such as the interior of large heaps of refuse—may last many years.
Chemical purification on sewage farms.
Concerning the chemical purification effected on sewage farms, i.e. the purification of the sewage as revealed by chemical analysis, it has been put on record over and over again, and is now fully and universally understood, that suitable land well managed is capable of changing even the foulest sewage to a perfectly clear water devoid of smell and danger, so that this point need not be laboured here. For instance, on the Berlin sewage farms the degree of purification attained has averaged for a period of 20 years 97 per cent., and on the farm at Gennevilliers—one of the Paris sewage farms—the effluent is so sparkling, bright and clear that the inhabitants drink it in preference to other available water.
Micro-organic purity of effluent from sewage farms.
But in reference to the purity of the effluent as to the products of micro-organic activity and pathogenic micro-organisms, it will be necessary to make a few observations with a view to remove misconceptions that have from time to time been put forward.
Ptomaines have not been found in effluents from well managed sewage farms.
The question whether the specific products of putrefaction, i.e. the putrefactive alkaloids “ptomaines and toxines,” are capable of doing further mischief by escaping with the effluent into the stream, may be answered as follows. These substances are fortunately very unstable, and the experiments conducted by Falk and others seem further to indicate that soil is capable of retaining them and of rendering them harmless. At [Pg 65] any rate there is no well authenticated case on record of these bodies having wrought mischief on sewage farms. (See here also the remarks made on pages 51 and 52 under the heading “The Absorbing Powers of Soil.")
Pathogenic germs on sewage farms.
It has further been maintained that the presence of pathogenic organisms on sewage farms might in two ways lead to mischief, viz. either by transmission through air or by transmission through water. The pathogenic organisms after spreading over the land might rise into the air through the movements of the atmosphere and then be carried about by it, or they might escape through the land and be conveyed with the effluent into the stream or river that takes the latter.
Pasteur’s fears as to mischief likely to be brought about by pathogenic micro-organisms on sewage farms not borne out by facts.
In connection with this point it may not be without interest to mention here that even the late M. Pasteur at one time of his career considered the wholesale spreading of disease germs on sewage farms might prove highly injurious to the public health of the neighbourhood. As he himself admitted, he based his fears on purely theoretical considerations and opposed, for this reason, the extension of the sewage farms in the neighbourhood of Paris. But when, later on, he was made acquainted with the results observed on the Berlin farms, he tacitly modified his views and ceased to oppose the extension of the Paris farms.
No well-authenticated case is on record where a sewage farm has acted as the focus of a local outbreak of typhoid fever.
Indeed, search as I might, I have not been able to discover one single instance where a sewage farm has acted as the focus of a local outbreak. On the contrary, during one or two small epidemics of typhoid fever in Berlin, no case of this complaint has been observed on the sewage farms of that city.
Experience on the Berlin farms.
Concerning the escape of pathogenic micro-organisms into streams and rivers, no case is on record where such [Pg 66] a thing has actually occurred: indeed, the very painstaking investigations on the Berlin farms have led to negative results.
Observations made at the Freiburg sewage farm.
Another sewage farm, that of Freiburg in Baden, has likewise been made the subject of careful and long-continued investigation by Dr. Korn, who, for the twelve months ending August 1897, made no less than 165 elaborate chemical and bacteriological examinations. Summing up his observations on the presence of bacteria in the effluents from subsoil drains, he remarks:
"Apart from the few exceptional cases of high numbers, generally speaking my experiments show that the number of germs in the subsoil drain effluents is relatively small, and even omitting these experiments, in which a dilution with subsoil water must have taken place, the number of micro-organisms is still so small that the effects of filtration through soil are clearly perceptible. In addition to this—and this is of considerable importance in forming a judgment—it must be borne in mind that the bacteria in sewage are principally derived from the intestines, whereas in the subsoil drain effluents the inhabitants of the intestines are either not present at all or only in very small numbers compared with the number of soil and water bacteria, which are always present. Out of 165 examinations I only succeeded in 18 cases in proving the presence of bacterium coli.”
Bacterium coli no longer a true criterion of sewage pollution.
It may be convenient to point out in this place that bacterium coli can no longer be looked upon as a typical inhabitant of the human intestines after the very elaborate investigations carried out by Dr. Weissenfels, who arrived at the following conclusions:
Dr. Weissenfels’ conclusions.
1. The so-called bacterium coli can be cultivated from almost every kind of water, and its presence can be [Pg 67] demonstrated in nearly every case, provided a sufficient volume of water is utilised.
2. It is not possible by the result of the experiments upon animals to decide whether the bacterium coli was cultivated from a pure or infected water, and the discovery of a virulent bacterium coli in any sample of water cannot, therefore, be regarded as a criterion that such water has been polluted with fæcal bacteria.
After these remarks, it would seem quite possible that the bacterium coli discovered in eighteen cases by Dr. Korn in the Freiburg effluents was not derived from sewage at all but from the ordinary subsoil water of the land.
The possibility of further mischief by pathogenic micro-organisms on sewage farms is exceedingly remote, if it exists at all.
Bearing these observations in mind, it is quite clear, therefore, that neither theoretical investigations, as available up to now, nor practical results, support the theory that pathogenic micro-organisms may do mischief on sewage farms, and one is forced to conclude that this possibility—if it exists at all—after systematic treatment on land is an exceedingly remote one.
Sewage farms reduce the quantity of final effluent.
Before concluding these remarks on the natural purification of sewage it is necessary to draw attention to another considerable advantage which it possesses over artificial sewage treatments, and that is the reduction in quantity of the effluent, which at times is very considerable, whereas in the artificial methods such a reduction is comparatively small.
Loss of liquid by evaporation and by plant life.
Spread over a large area of land, well cropped, evaporation is very active—especially during the summer months, when the flow of water in the brook that takes the effluent is as a rule at its lowest; and, in addition to this, the growing plants further abstract a considerable amount of the liquid that finds its way into the soil, so that the quantity of the effluent may not be more than from 30 [Pg 68] to 50 per cent. of the total quantity that was poured over the land. In the artificial treatment the evaporation is considerably smaller, and as plants are altogether absent the quantity of the effluent is probably about 90 per cent. and more of the total quantity of the raw sewage. This is a point of very considerable importance so far as the influence of the effluent upon the water in the stream that takes the same is concerned.
Although the subject of natural purification is by no means exhausted, it is now time to direct attention to artificial methods.
V. ARTIFICIAL SELF-PURIFICATION OF SEWAGE.
1. General Observations.
Enumeration of more important experiments.
A great many experiments have been made during the last ten years with artificial processes for the self-purification of sewage, and amongst the more important the following may be mentioned:
London experiments.
Sutton„
Exeter„
Manchester„
Leeds„
Sheffield„
Leicester„
York„
Hamburg„
Experiments have not been conducted on uniform lines.
A casual observer might, therefore, consider himself justified in thinking that all these experiments had added a great deal to our knowledge of the intricate changes taking place in these processes, but such a conclusion would not be justified in reality. For beyond settling questions of local importance by chemical analysis, the experiments, owing to a variety of causes, have not [Pg 69] materially enhanced the stores of our information, indeed not unfrequently the results obtained are apparently contradictory and bewildering.
An experiment must be looked upon as a question addressed to nature, and the answer will depend on the way the question has been put. If this way differs in every case it must be clear that the answer, too, will differ in every case, and it is this absence of uniformity which greatly reduces the general value of these experiments.
These remarks must not be misunderstood to convey the impression as if the experiments had not been conducted with care and skill! Far from it! Some of them have been made with the greatest skill and care and with the very evident desire to arrive at correct conclusions, and it is only when they are placed side by side with other experiments, with a view to deducing from them general conclusions concerning the processes at work, that great difficulties are experienced. The result of each experiment is governed by a large number of factors, which by slightly different manipulations may attain in this ever-fluctuating process different weights, so that the results may be contradictory, and it is only by arranging these factors on a common basis, as it were, and by addressing the questions to nature in the same systematic and uniform way, that good general results may be expected.
It is well known, for instance, that in some cases septic tanks have not given good results, whilst in others they have worked very well; again, continuous filtration has failed in some experiments, whilst in others, notably in the York experiments, it has given good results.
If, therefore, in future the mistake of the past is to be avoided, it will be necessary to settle on a common line of action in all experiments.
Attempt to evolve general theory.
[Pg 70] In spite of all the difficulties which beset such a task, an attempt will be made in the following observations to evolve some general theory concerning the processes at work in the artificial self-purification of sewage. Such a theory, it is quite clear, cannot be complete in the present state of our knowledge, and it is sincerely hoped that the many and serious gaps will be filled up by later investigations.
For convenience of reference the different forms of the process, such as are now employed, shall be dealt with separately, commencing with contact or oxidation beds.
2. Artificial Self-Purification of Sewage in Intermittent Contact Beds.
At the outset it may not be out of place to make a few remarks concerning the various names given to this form of application. The term “intermittent contact bed” is here used to distinguish this kind of bed from the “continuous contact bed,” frequently called “continuous filtration.”
Names of process misleading.
(a) Name of Process.—This process has frequently been called “biological process,” “bacteriological process,” “contact bed system” or “oxidation bed system,” but all these terms do not appear to define it sufficiently, as they do not cover the whole, but only phases or stages in the same; hence, they do not seem appropriate.
Biological process.
The name “biological process” is decidedly misleading, for besides biological agencies there are also at work physical (mechanical) and chemical ones.
Bacteriological process.
The term “bacteriological or bacterial process” is likewise erroneous, for besides bacteria a number of other micro-organisms participate in it—such as yeast [Pg 71] fungi, mould fungi, algæ, protozoa, and even higher forms of life, such as earthworms and insects.
Contact bed system. Oxidation bed system.
The expressions “contact bed system” or “oxidation bed system” are in so far inappropriate as they describe only portions of the process but not the whole. The term “contact bed” describes the first stage, and the term “oxidation bed” portion of the second stage only.
Term most suitable.
The term which seems most suitable of all is “artificial self-purification in contact beds,” as it includes every phase of this lengthy process applied in an artificial form; the term “natural self-purification” being applied to land treatment of sewage, as it is the only method in which the self-purifying powers are employed under natural conditions.
Working operations.
(b) Explanation of Process.—The cycle of operations commences with the filling of the bed, and during the same the sewage comes gradually in contact with the filling material. When the bed is full, the inflow is stopped and the sewage allowed to remain in contact with the material for some time. The bed is then emptied, and a period of rest is given it before the filling is commenced again.
Purification of sewage in full bed due to absorbing powers of filling material and only to a small extent due to activity of micro-organisms.
It has been held, that while the sewage is in the contact bed it undergoes a very rapid process of decomposition by bacteria, but it must be evident, that as the sewage—including filling—remains only for about two hours in the bed, the micro-organisms would have to work at an express rate. This fact alone is apt to make this theory very doubtful, but apart from it, it has been proved by experiments that the by far greater amount of purification—whilst the sewage is in the beds—is due to the absorbing powers of the filling material, which are derived from the surface attraction of its component particles.
[Pg 72]
Retention of suspended matters by bed. Absorbing powers of filling material.
The filling material retains in its upper layers the suspended matters, which it strains out of the sewage in a purely mechanical manner, much after the fashion of a screen, and when the bed is filled its absorbing powers come into play, which cause the removal of the dissolved matters out of the liquid and their retention on the surface of the particles. This latter process is probably a chemico-physical one assisted by the micro-organic life in the sewage.
Decomposition of organic substances by micro-organisms when bed is empty.
It is only after the bed has been emptied that the real activity of the vast number of micro-organisms commences, which is directed towards converting the organic substances into mineral ones. This process of splitting up, decomposing, disintegrating and mineralising organic waste products is an exceedingly complex one, which ever fluctuates according to the prevailing conditions, and which does not come to an end until finally stable mineral forms are reached. In the presence of a plentiful supply of oxygen, the process proceeds as a rule at a more rapid rate, and the intermediate forms produced are less complex than in the comparative or total absence of this gas; hence the progress of the process is largely determined by it. The amount of oxygen necessary for bacterial activity is partly abstracted, and with extraordinary energy, from the atmospheric air in the pores of the filling material, and a portion of the substances formed, such as carbonic acid and nitrogen—in gas form—escape into the atmosphere, whilst the remaining portions are washed out of the bed with other products, such as nitric acid, by the effluent.
Further remarks upon this process of mineralisation have been made in connection with the subject of natural self-purification of sewage, and these may be referred to here.
Effluent from bed practically raw sewage as far as its bacterial contents are concerned.
[Pg 73] The effect of the bed upon the bacterial flora of sewage is, as was to be expected, but very slight, and it is on record now that, as far as the micro-organic life is concerned, the effluent is to all intents and purposes raw sewage.
Silting up of bed.
Some of the substances contained in raw sewage remain in the bed, no matter how carefully the sewage has been previously strained, and these, in combination with the slimy surface coating of the component particles, the accumulation of mineralised substances in the pores, the consolidation of the bed, the disintegration of the filling material, and the liquid retained, lead gradually but surely to the silting or sludging up of the bed.
Theoretical original water capacity of bed.
(c) Water Capacity of Bed and Silting up.—The theoretical water capacity of the bed, previous to commencing operations, is the aggregate of the cubical space occupied by the pores or small passages between the particles forming the filling material, and the pores of the filling material itself; but in practice a certain amount of this space is occupied by air, which it is impossible to dislodge altogether in filling. The aggregate of the cubical space of the pores may be called the pore-volume.
It is difficult to lay down general rules as to what the original water capacity of a bed should be expressed in per cent. of the space occupied by the filling material, but speaking within fairly wide limits the following is somewhat near the truth.
Original water capacity with spherical particles of uniform size.
When the particles forming the filling material are fairly spherical and of equal size, the original water capacity of a bed amounts to about 38 per cent. of the space occupied by the filling material; but as in practice it is difficult to obtain spherical particles of uniform size, the original water capacity is found to range from 35 to 45 per cent. of this space.
Original water capacity with particles of different sizes.
[Pg 74] When, however, the particles are of materially different sizes, and when the smaller ones fill up the spaces between the larger ones, the original water capacity may sink down to as low as from 5 to 10 per cent. of the space occupied by the filling material.
Size of particles of filling material does, under certain conditions, not affect original water capacity of bed.
It has been further demonstrated that the water capacity of a bed is not affected by the size of the particles, provided the latter are spherical and of uniform size. In other words, the water capacity of two beds filled with material of different sizes is the same, provided the particles are spherical and of uniform size throughout each bed.
Silting up of bed during regular work. Rapid initial decrease of capacity. Consolidation of bed.
This original water capacity is, however, not maintained in regular work, as has been pointed out already. Basing the observations on regular work only, the original capacity decreases at first, after a new bed has been started or after an old reconstructed bed has been taken in hand, rapidly for some time and afterwards more slowly. Graphically expressed, this decrease is not represented by a straight line but approaches more nearly a parabolic curve. This initial rapid decrease is chiefly due to the consolidation of the bed.
Disintegration of filling material.
In connection with the movements in the bed tending towards its consolidation, it is also clear that the continual filling and emptying operations cause the smaller particles to be washed out of their original position and to be placed in the larger passages between the filling material, and if this process is assisted by the gradual disintegration of the particles composing the filling material, it is clear that the pores must become smaller and smaller in time, i.e. choked.
From these observations it follows that the filling material should be a hard substance, which will only to a limited extent be subject to this crumbling away process.
[Pg 75] But besides these there are, as has already been pointed out, other silting up agencies at work.
Water-retentive power of filling material.
Of the total quantity of sewage which has entered the bed a small portion will always remain in it owing to the water retaining power of the material. This power has sometimes been called “minimum water capacity,” but as this name is liable to be misunderstood, it is better to adopt here the term “water-retentive power” of material.
The quantity of the sewage retained by the bed varies with the material and pore-volume, and is due to adhesion and capillary attraction. The greater the pore-volume, and the greater the percentage of fine pores, the greater is the quantity thus retained. Clean gravel retains about 12 per cent. and fine sand about 84 per cent. of its water capacity—i.e. expressed per cubic yard of filling material, one cubic yard of clean gravel will retain about 10 gallons and one cubic yard of fine sand about 70 gallons of water.
Through draining a bed for several hours through evaporation and other atmospheric influence, a portion of the sewage retained is lost, but the quantity so lost will vary continually with the circumstances under which the bed is worked.
The water-retentive power of the filling material does not decrease with the working of the bed, but increases, which in a large measure is probably due to the slimy coat which forms round the surface of the component particles, and to which reference is made in the following paragraph.
Slimy surface coating of component particles.
A further silting-up agency is the slimy surface coating of the particles of the filling material. This accumulation is well known to all who have had to do with intermittent contact beds, and has been described as [Pg 76] spongy bacterial growth. The Manchester report for the year ending March 27, 1901, contains on page 62 the following passage: “This (spongy bacterial growth) is at once the cause of increased efficiency in the bed and loss of capacity. On examining the material of a contact bed in active condition, every piece is seen to be coated over with a slimy growth. If this is removed it soon dries to a stiff jelly, which can be cut with a knife. Under the microscope masses of bacteria and zoogloea will be found to be present.”
Accumulations of decomposed substances in the pores.
In addition to this slimy surface-coating of the particles, there are also found in the pores, especially in the upper layers of the filling material—and in fine beds more so than in a coarse bed—accumulations which are “akin to humus or garden soil.” They contain to a limited extent only putrescible substances, and appear to be the remains of organic matter decomposed by the activity of micro-organisms.
Periods of rest will not permanently restore portion of the original water capacity of the bed.
It was formerly maintained with considerable persistency that periods of rest would permanently restore to a systematically worked bed a portion of its lost water capacity, but such a contention has been proved to be wrong. It is quite true that immediately after periods of rest an increase of the water capacity is very noticeable, which is probably due to drying up processes within the bed during the rest, but such an increase is not permanent and is lost again more or less quickly; it is therefore only temporary and not permanent.
Where, however, a bed has not been systematically worked, i.e. where it has been worked at a greater rate than is suitable, and where in consequence of this a large quantity of undecomposed substances is stored in it, a period of rest may permanently restore a portion of the lost capacity; but this is due to the mineralisation [Pg 77] of these undecomposed organic substances during the rest.
It follows from these remarks, as has been stated above, that when the organic substances are regularly decomposed during systematic work a period of rest cannot materially affect the water capacity, and that where a considerable permanent restoration of the water capacity takes place the bed has not been properly worked.
Decrease of capacity is accompanied to some extent by increase of efficiency and vice versa.
It would, however, be incorrect to assume that the silting up of the bed affects its efficiency besides reducing the capacity. On the contrary! To some extent decrease of capacity is accompanied by increase of efficiency and vice versa!
Higher capacity of beds in summer than in winter.
At this point it ought to be stated that in the Manchester experiments (see page 61 of the report for the year ending 27th March, 1901) a higher average capacity is maintained during the summer than during the winter, which is no doubt due to the greater activity of the micro-organisms during the warm weather of the year.
Raking of beds not advantageous.
The raking of the surface does not materially affect the capacity of the bed, and it is better to scrape off the matters retained on the surface than to rake them into the body of the bed.
Renovation of filling material either partially or wholly.
It will be clear from these observations that, no matter how carefully the bed has been worked, sooner or later a time will come when the decrease of capacity becomes so pronounced as to render it impossible any longer to treat the daily flow of sewage with the available plant; and when this point has been reached a renovation, either partially or wholly, of the filling material becomes an inevitable necessity.
Minimum capacity of beds to be provided for.
To provide for this at the outset, and thus avoid the [Pg 78] difficulties of reduced capacity, it seems advisable to lay down, when designing the works, a minimum capacity, which will just allow the daily volume of sewage to be treated by the plant, and which when reached will necessitate the cleansing of the bed.
The idea, formerly frequently expressed, that the filling material when rationally worked need not be renewed or renovated, can no longer be maintained and is outside the reach of practical possibilities.
Underdrainage of intermittent contact beds.
At this place a word or two about the under drainage of intermittent contact beds may not be out of place. It is of the greatest importance that all drains should work well, and that the entrance of the sewage into them should not lead to disturbance in the filling material, especially should the tearing of portions of the filling material into the drain pipes be avoided.
By carefully arranging the position, number, size and fall of the master drains and branch drains, it is possible to reduce the resistance so as to allow of a fairly even flow of sewage through all the drains, and to prevent a great rush of water through the drains near the outlet end.
The presence of lime is of no consequence.
In passing it may not be out of place to point out that the view, formerly expressed, that an admixture of lime in some form would prove advantageous to the purification of sewage, is not supported by the experience gained.
Absorbing effect increases with the time of contact.
(d) Absorbing Powers of Filling Material.—The absorbing effect of any filling material seems to increase with the time of contact.
Absorbing powers increase until bed has become ripe.
It ought further to be pointed out that the absorbing powers of the filling material gradually increase until the bed has become ripe. This fact was formerly stated to be due to the development of the proper micro-organisms [Pg 79] within the bed, but it would seem to be chiefly due to the slimy surface coating of the particles of the filling material, or spongy bacterial growth, as it has frequently been called, which does not only assist mechanical filtration but also possesses high powers of absorbing oxygen.
Absorbing powers soon cease in the absence of micro-organisms and air.
But it cannot be open to doubt that the absorbing powers of the filling material are dependent in some way or other on the presence of micro-organisms, for Dunbar has shown that in the absence of micro-organisms and without periods of aeration these powers soon cease.
Oxygen is absorbed from air in the pores with great energy.
(e) Consumption of Oxygen by the Filling Material.—The oxygen necessary for the proper work of an intermittent contact bed is abstracted with great energy from the atmospheric air, with which the pores become filled during periods of rest. Through diffusion, and through the vacuum created by the processes of absorption, further quantities of oxygen are taken from the atmospheric air, even under difficult conditions, and, as pointed out in the Manchester report for the year ending 27th March, 1901, “there is, therefore, little need to force air into a bed.”
The oxygen taken up during aeration is not imparted to the sewage at the next filling and does not escape in the effluent.
The oxygen thus taken up is not imparted in gas form to the sewage during the next filling, and the effluents from intermittent contact beds are not saturated with oxygen. Dunbar states that the effluents of a satisfactorily worked bed frequently only contain one cubic centimetre of free oxygen per litre. Clowes reports a similar result in his third report on the London experiments.
The greatest quantity of oxygen is consumed during the oxidation of the products formed by micro-organisms.
There can be no doubt that by far the greatest quantity of oxygen is consumed during the process of oxidation of the products formed by micro-organisms from putrescible organic substances.
Consumption of oxygen and formation of carbonic acid not solely due to biological agencies.
[Pg 80] (f) Formation of Carbonic Acid.—Dunbar has shown by his experiments that the consumption of oxygen and the formation of carbonic acid is not solely due to biological agencies, but is to some extent the result of physico-chemical processes.
More free carbonic acid contained in the effluent than in the raw sewage. By far the greatest portion of carbonic acid escapes into the air.
He further reports that in his experiments the effluents contained on an average 100 milligram per litre more free carbonic acid than the raw sewage, and that the quantity contained in the effluents represents only a small portion of the total amount of carbonic acid formed during the whole process. The by far greatest portion of carbonic acid escapes into the air. Concerning the air in the pores of the filling material during periods of aeration, Dunbar states that it contains sometimes not less than from 6 to 10 per cent. carbonic acid.
Nitrogen escapes in gas form into the air.
(g) Nitrogen.—It is quite clear from all experiments that a considerable amount of the total nitrogen contained in raw sewage is abstracted by the filling material of intermittent contact beds, and it is interesting to ascertain what becomes of it! Does it accumulate in the bed? In that case, one has a right to assume that the satisfactory work of the bed would gradually cease! As this is, however, not the case, and as, further, the sludge formed in the bed, whether it be fairly fresh or very stale, only contains a very small amount of total nitrogen, we must surmise that the nitrogen after its retention by the bed escapes in gas form—like the carbonic acid—into the atmosphere.
The presence of nitric acid is not an unfailing guide for determining the satisfactory character of the effluent.
(h) The Formation of Nitric Acid.—Concerning the presence of nitric acid in the effluents from intermittent contact beds, Dunbar is of opinion that it offers certain means for forming an opinion of the processes taking place in the same, but that it is only in a subordinate sense an indication of the degree of purification attained [Pg 81] and must not be taken as an unfailing guide for determining the satisfactory character of the effluent.
Nitrifying bacteria always present in town’s sewage. Nitric acid is formed very rapidly, but only during periods of rest.
Nitrifying bacteria are always present in ordinary town’s sewage, but it would appear that other micro-organisms besides Winogradsky’s bacteria assist in the process of nitrification. Nitric acid is formed very rapidly, but only during periods of rest, and besides aeration other less powerful influences are at work.
Reduction of nitric acid when bed is filled from bottom with an upward flow.
It is further interesting to note that, according to Dunbar, the greater portion of nitric acid which has been formed during periods of aeration becomes completely reduced in a very short time, when the bed is filled with an upward flow from the bottom, and that only a small portion remains in the form of nitrous acid.
3. Artificial Self-Purification of Sewage in Septic Tanks.
Septic tanks only used in combination with contact beds.
Although it has never been claimed, and is further not open to doubt, that a septic tank alone and unaided by subsequent treatment in intermittent or continuous contact beds does not sufficiently purify the sewage, in these remarks the work of the septic tank only will be considered, as the treatment in contact beds will be dealt with separately.
Septic tank a suitable name.
(a) Name of Septic Tank.—A good many names have been suggested by different observers—such as “anaerobic fermentation tank,” “putrefying tank,” “liquefying tank,” “cess-pit,” etc.—but there appears to be no reason why the name “septic tank” should not be adhered to, as it describes sufficiently correctly the work done by the tank, which is chiefly of a septic nature.
(b) Covered or Open Septic Tank.—Before dealing with the processes taking place in a septic tank, it will [Pg 82] not be out of place to consider here, shortly, whether a closed septic tank confers advantages over an open septic tank sufficiently great to justify the considerably greater expenditure necessitated by its construction.
It is well known, that at Exeter in the first experimental installation of this process, the septic tank was covered in by an arched roof; but subsequent experiments made elsewhere do not seem to support the theory then advanced, that such a tank should be a closed one. This is chiefly due to the thick skin which, after a few months’ work, forms on the surface of closed or open tanks, and which according to locality and season may reach a thickness of from 1 to 2 feet; it is maintained then that this cheap natural cover does away with the expensive artificial cover.
In the report on the treatment of the Manchester sewage, by Messrs. Baldwin Latham, Percy F. Frankland and W. H. Perkin, it is stated on page 54, amongst the conclusions and recommendations, as follows:—"The anaerobic or septic process is found to take place as effectively in an open tank as in a closed one.” This conclusion does not appear to have been modified by the experiments made subsequent to the issue of this report.
In the Leeds experiment a similar result was obtained.
Closed septic tanks possess generally speaking no advantages over open ones.
Whilst it would, therefore, appear to be correct to say, generally, that closed septic tanks afford no material advantages over open ones, so far as the purification of the sewage is concerned, they may become necessary in special cases, when the smells emanating from open ones might create nuisances in crowded neighbourhoods.
The following remarks refer, therefore, equally to open as well as to closed septic tanks, and no distinction will be made between them.
The work in the septic tank is chiefly done by obligatory anaerobes.
[Pg 83] (c) Explanation of Process.—Although the processes taking place in septic tanks are at present but imperfectly understood, they may be said to be in the main due to anaerobic micro-organisms, i.e. due to such micro-organisms which carry on their life’s work in the absence of oxygen. They split up or peptonise the organic compounds in the absence of air, and the group of changes brought about by them has been termed “anaerobic fermentation” or “putrefaction.” During the same, it is claimed that a considerable amount of the sludge retained in the tank is liquefied or destroyed, and that the rest becomes so changed as to be denser than ordinary sludge, and to contain less moisture.
Dissolved matters entering and leaving the septic tank.
Concerning the amount and nature of the dissolved matters entering and leaving an open septic tank, the following is taken from the Manchester report for the year ending March 27, 1901:—
Manchester observations.
"A series of determinations have been made of the amount of dissolved matter entering and leaving the tank, by evaporating known volumes of the sewage and effluent after filtration through paper and weighing the solid residue.
"An average of six determinations (confirmed by similar observations in connection with the closed septic tank) gave the following results:—
Raw Sewage. Dissolved matter, grains per gallon. Open Septic Tank Effluent. Dissolved matter, grains per gallon.
Mineral. Organic and Volatile. Total. Mineral. Organic and Volatile. Total.
30·8 25·0 55·8
33·0 33·0 66·0
Reduction, in per cent.
6·67 24·24 15·45
Raw Sewage. Dissolved matter, grains per gallon.
Mineral. Organic and Volatile. Total.
33·0 33·0 66·0
Open Septic Tank Effluent. Dissolved matter, grains per gallon.
Mineral. Organic and Volatile. Total.
30·8 25·0 55·8
Reduction, in per cent.
6·67 24·24 15·45
[Pg 84] “A certain amount of loss of ammonia, as ammonium carbonate, will take place on evaporation in both cases, and this will probably be greater with septic tank effluent.
"An examination of the residue obtained by evaporating large quantities of open septic tank effluent (filtered through paper), shows that the mineral matters largely consist of iron oxide, from the decomposition of organic compounds of iron, and calcium sulphate. Among the volatile constituents have been detected ammonium carbonate, mercaptan-like compounds of very offensive smell, acetic and butyric acids. No evidence of the presence of amines could be found in the residue on evaporation, but by distilling large volumes of the liquid and carefully analysing the platinum salts obtained from the distillate, the presence of amines is indicated.
"Research in this direction is being continued; careful comparison especially will be made of the products obtained by evaporation and distillation of crude sewage and septic tank effluent respectively.
"The evidence, however, points to a breaking down of albuminoid and cellulose matter in the septic tank into simpler and to some extent volatile compounds. The reactions are probably hydrolytic in character, ammonia, amines, carbonic acid, water, and possibly alcohol, being produced.
"A further quantity of organic matter also disappears as methane, nitrogen and hydrogen.”
Must aerobic fermentation in all cases be preceded by anaerobic fermentation?
It will be clear from the foregoing, that the changes going on in a septic tank are entirely different from those brought about in contact beds, and the question whether a septic tank is a necessity for the subsequent contact bed treatment, or whether it is a distinct disadvantage, can only be definitely settled when we know [Pg 85] whether aerobic fermentation, i.e. decomposition, must in all cases be preceded by anaerobic fermentation, i.e. putrefaction, and to what extent, or whether such a succession of changes is not necessary.
At Manchester contact beds accustomed to septic tank effluent did not at once purify raw sewage.
It is interesting to note in connection with this point, that during the Manchester experiments it was established that contact beds, which have become accustomed to septic tank effluent, will not at once purify comparatively fresh sewage.
(d) Velocity of Flow through Tank.—The velocity of flow through the septic tank is of great importance, as on it depends the size of the installation.
It seems to have become a habit to express this velocity by the length of the sojourn of the sewage in the septic tank—for instance, “the flow of sewage through the tank was such that it would fill it in twenty-four hours"; but as all tanks vary in size, and as in consequence the distance which has to be traversed by the sewage from the entrance to the exit in twenty-four hours is different in nearly every case, such a habit is, to say the very least, very misleading.
It will not be disputed that the deposition of the suspended solids in sewage is dependent on the rate of movement of the liquid, and that in a quickly moving liquid there will be less deposition than in a very slowly travelling liquid.
Town. Length. Width. Depth. Contents.
feet. feet. ft. in. gallons.
Manchester tanks 300 100 6 0 1,125,000
Leeds tanks 100 60 7 7 250,000
Bearing this in mind it will not be without interest to examine the velocities employed during the Manchester [Pg 86] and Leeds experiments. The tanks employed in these have the dimensions given in the table on the preceding page.
Now assuming that each tank is to be filled once in twenty-four hours we obtain the following velocities:
Manchester 300′ 0″ × 12″ 1440 = 2″·5 per minute.
Leeds 100′ 0″ × 12″ 1440 = 0″·84 per minute.
Which means that in the Manchester experiments the velocity would have been three times as large as in Leeds; and it is clear that if the sewage of both towns was identical the results, so far as the retention of the suspended matters in the tanks are concerned, could not have been identical. As a matter of fact, considerably greater velocities have been used in the Manchester experiments, as will be shown later on.
Rate of flow through septic tanks should not be expressed by the length of sojourn in tank but by some linear measurement in a stated time.
It will be clear from this, that it is most misleading and erroneous to express the rate of flow by the length of the sojourn of the sewage in the tank, and that the velocity should in each case be expressed by some linear measurement in a stated time—probably inches per minute.
The next point to consider is the velocity to be employed in septic tanks; and here it is not without interest to refer to the various experiments enumerated with their results in the next table.
The difference in the results obtained, so far as the suspended matters are concerned, is probably due to the different character of the various sewages experimented with; but so low a velocity as 0·52 inch, as used in the Exeter experiments, does not appear to be necessary.
In the Leeds experiments, it was found that the filling of the tank once in twenty-four hours gave the best results; and as the velocity then was 0·84 inch per [Pg 87] second it will be somewhat near the mark to recommend generally a velocity of 1 inch per minute. On the assumption that the sewage shall remain twenty-four hours in the tank, this gives a length of tank of 120 feet, which is a very suitable one.
Septic Tank Experiments.
Rate of Flow and Deposition of Suspended Matters.
No. Name of Town. Rate of Flow. Suspended Matters in Sewage.
Length of Sojourn of Sewage in Tank. Velocity of Flow per minute. Remaining in Tank. Destroyed and Liquefied in Tank. Leaving Tank in Effluents. Total.
days. inches. per cent. per cent. per cent.
1 Exeter 1·0 0·52 17 39 44 100
2 Manchester 0·44 5·58 41 22 37 100
3 ” 0·56 4·44 23 33 44 100
4 Leeds 0·5 1·68 ⎫ .. .. .. ..
5 ” 1·0 0·84 ⎬ average say 28 .. ..
6 ” 2·0 0·42 ⎭ .. .. .. ..
No. Name of Town. Rate of Flow.
Length of Sojourn of Sewage in Tank. Velocity of Flow per 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 ⎭
No. Suspended Matters in Sewage.
Remaining in Tank. Destroyed and Liquefied in Tank. Leaving Tank in Effluents. Total.
per cent. per cent. per cent.
1 17 39 44 100
2 41 22 37 100
3 23 33 44 100
4 .. .. .. ..
5 average say 28 .. ..
6 .. .. .. ..
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 [Pg 88] 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, CH473
Carbon dioxide, CO26
Hydrogen, H5
Nitrogen, N (by difference)16
Total 100
[Pg 89] 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.
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.
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.
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.
[Pg 90] 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.
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.
[Pg 91] 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.
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 [Pg 92] 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.
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.
Tank velocity at Manchester.
At the Manchester tanks it is stated that a velocity of 3 feet 4 inches per minute is employed.
Velocity frequently adopted.
A rate of velocity now frequently adopted in this country for new works is 6 inches per minute.
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.
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.
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.
[Pg 93] 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.
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.”
[Pg 94] 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.
[Pg 95]
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 [Pg 96] 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 matters. 1. Removal of suspended matters.
2. Removal of from 75 to 95 per cent. of the dissolved organic matters. 2. Removal of from 50 to 75 per cent. of the dissolved organic matters.
3. Removal of pathogenic germs. 3. Nil. Effluent bacterially practically raw sewage.
4. Utilisation of large portion of manurial elements. 4. Nil. All manurial elements escape into the rivers.
5. Great reduction of quantity of liquid. 5. No appreciable reduction 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:
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