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 notmaterially 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.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 yeastfungi, 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.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.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.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.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 asspongy 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 mineralisationof 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 andvice 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 andvice 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 thedifficulties 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-organismswithin 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.(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 attainedand 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 willnot 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.(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.30·825·055·833·033·066·0Reduction, in per cent.6·6724·2415·45Raw Sewage.Dissolved matter, grains per gallon.Mineral.Organic and Volatile.Total.33·033·066·0Open Septic Tank Effluent.Dissolved matter, grains per gallon.Mineral.Organic and Volatile.Total.30·825·055·8Reduction, in per cent.6·6724·2415·45“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 knowwhether 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 tanks3001006 01,125,000Leeds tanks100607 7250,000Bearing this in mind it will not be without interest to examine the velocities employed during the Manchesterand 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:Manchester300′ 0″ × 12″1440= 2″·5 per minute.Leeds100′ 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 persecond 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.1Exeter1·00·521739441002Manchester0·445·584122371003”0·564·442333441004Leeds0·51·68⎫........5”1·00·84⎬average say28....6”2·00·42⎭........No.Name of Town.Rate of Flow.Length of Sojourn of Sewage in Tank.Velocity of Flow per minute.days.inches.1Exeter1·00·522Manchester0·445·583”0·564·444Leeds0·51·68⎫5”1·00·84⎬6”2·00·42⎭No.Suspended Matters in Sewage.Remaining in Tank.Destroyed and Liquefied in Tank.Leaving Tank in Effluents.Total.per cent.per cent.per cent.1173944100241223710032333441004........5average say28....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 orliquefied 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 mattersremaining in tank35””destroyed or liquefied in tank25””escaping in effluent40Total100These 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, CH473Carbon dioxide, CO26Hydrogen, H5Nitrogen, N (by difference)16Total100At 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.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.
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 notmaterially 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.
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 yeastfungi, 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.
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.
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.
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.
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 asspongy 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 mineralisationof 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 andvice 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 andvice 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 thedifficulties 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-organismswithin 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.
(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 attainedand 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 willnot 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.
(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.30·825·055·833·033·066·0Reduction, in per cent.6·6724·2415·45
Raw Sewage.Dissolved matter, grains per gallon.Mineral.Organic and Volatile.Total.33·033·066·0Open Septic Tank Effluent.Dissolved matter, grains per gallon.Mineral.Organic and Volatile.Total.30·825·055·8Reduction, in per cent.6·6724·2415·45
“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 knowwhether 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.
Bearing this in mind it will not be without interest to examine the velocities employed during the Manchesterand 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:
Manchester300′ 0″ × 12″1440= 2″·5 per minute.
Leeds100′ 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 persecond 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.1Exeter1·00·521739441002Manchester0·445·584122371003”0·564·442333441004Leeds0·51·68⎫........5”1·00·84⎬average say28....6”2·00·42⎭........
No.Name of Town.Rate of Flow.Length of Sojourn of Sewage in Tank.Velocity of Flow per minute.days.inches.1Exeter1·00·522Manchester0·445·583”0·564·444Leeds0·51·68⎫5”1·00·84⎬6”2·00·42⎭No.Suspended Matters in Sewage.Remaining in Tank.Destroyed and Liquefied in Tank.Leaving Tank in Effluents.Total.per cent.per cent.per cent.1173944100241223710032333441004........5average say28....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 orliquefied 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.
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:
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.
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.