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. Generallyspeaking, 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. Ifthe 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.thevelocity 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 thisstate, 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; and2. 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 4ft.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 veryspecial 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.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.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.…“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 thechemical 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.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 atall, the evil-smelling gases (NH3and 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.’”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. Atany 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 sucha 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 bedemonstrated 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 30to 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.
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. Generallyspeaking, 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. Ifthe 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.thevelocity 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 thisstate, 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 4ft.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 veryspecial 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.
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.
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.…
“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 thechemical 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.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 atall, the evil-smelling gases (NH3and 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.’”
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. Atany 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 sucha 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 bedemonstrated 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 30to 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.