Chapter IV: Part II: By H. Alfred Roechling (2)

Later on it is on record, that Cagniard de la Tour in France, about the year 1825, and Schwann in Germany, about the year 1836, expressed the view, that organised substances—micro-organisms—played some role in fermentative and putrefactive changes. Almost diametrically opposed to this were the views authoritatively laid down by the then star in the chemical horizon, Justus von Liebig, who, about the year 1845, maintained that these changes were brought about by the dead inert matter itself—by molecular movements in the same—and not by organised substances, the presence of which in fermenting or putrefying substances was purely accidental. So great was Liebig’s authority then, that many almost blindly adopted his views, and the strife that commenced around these opposing views was fought with the greatest bitterness. But the stronghold of old ideas, which were gradually but surely being supplanted by new ones, could not hold out for ever against combined attacks, however stoutly it was defended by its designer, and its final downfall came about the year 1860, when a young Frenchman, Pasteur, established beyond doubt by his ever classical researches, that fermentation and putrefaction were, in the first instance, due to living organisms and not to dead matter. Pasteur further demonstrated that living organisms were also the cause of some and probably of all zymotic diseases.

So far, so good! But unfortunately the methods of biological research employed by M. Pasteur were very cumbersome and left otherwise much to be desired, so that his discoveries could not be fully utilised and extended, until in 1882 Robert Koch of Berlin published his new methods of investigation. This was the signal of raising the floodgates of biological (bacteriological) research throughout the world with this result, that the flood waters pent up until then inundated practically other branches of scientific investigation and drowned their individual life for some time to come.

During this interval, 1860 to 1882, investigators who wished to study the organised impurities in sewage had to proceed by indirect methods. They had no means of ascertaining by direct biological experiment the number and character of the micro-organisms contained in sewage: all they could do, was to determine chemically the dangerous nature of the sewage by the amount and origin of organic matter it contained, which would probably act as food to the germs; and the greater this amount was, so it was inferred, the greater would be the number of germs it harboured and the more dangerous its character.

This was the condition of things at the time the second Rivers Pollution Commission carried out its investigations, which in many respects, and rightly too, are still considered standard investigations. It cannot be surprising, therefore, that, being without proper means of biological examination, and having to rely chiefly on chemical methods only, the Commissioners came to the conclusion that the changes brought about in sewage purification were due to mechanical and chemical agencies!

It is frequently a matter of the utmost difficulty to ascribe, after the lapse of half-a-century, a new theory to one special author, as several investigators may have been trending the same way quite independently of each other, but may not have been equally successful in the matter of their publications becoming generally known. Theories, as a rule, do not drop out of the clouds like meteorites, they force themselves gradually upon men’s minds and are elaborated by them until ripe.

Bearing this in mind, and subject to further research, it would appear as if Alexander Müller had been the first to apply Pasteur’s general theories as to decomposition, fermentation and putrefaction to the problem of the self-purification of sewage. He made his experiments in 1869 and published them in 1873. Since that date a very large number of investigators have been at work on similar lines, and whilst it would lead too far to deal with them minutely, it ought to be stated that the results of their labour confirmed the view of living organisms playing a very important part in the decomposition of sewage. Among the many names prominent in this respect are those of Schloesing, Müntz, Hatton, Warrington, Sorby, Winogradsky, Percy Frankland, Dupré, Emich and Dibdin. That set of researches, however, which has done more than any other to consolidate the theory of bio-chemical changes taking place in the self-purification of sewage are the investigations of the Massachusetts State Board of Health, which were commenced in November 1887, and are still being continued.

Since 1895 a large number of additional experiments have been made, which will be dealt with more in detail later on, but speaking generally they have not materially increased our knowledge of the processes taking place in sewage purification.

Summarising the remarks on the theoretical aspect of this question, it may be said that, as to the agencies at work, we know now they are of a mechanical, chemical and biological nature; but as to the processes and products brought about by these agencies we know very little beyond the initial and terminal stages, as will be pointed out in some of the subsequent observations.

Directing now attention to the practical side of the question, it has already been stated that the only known sewage treatment at the commencement of last century was land irrigation. Then about the middle of the century chemistry seems to have taken the matter in hand and tried to make a lucrative business out of it. It is on record, however, that it did not succeed in this attempt, and the financial loss which this endeavour has caused is a dismal subject to investigate.

There is before my mind’s eye the case of a gallant officer of His Majesty’s land forces who, after having reached very near the summit of his career, retired and employed his time in trying to make a fortune out of sewage. So enamoured was he of the subject, that—so the story goes—he commuted his pension to have all the more ready money; but fortune did not smile on him, and his last days were spent under the lengthening shadows of the sorrow of financial difficulties, having practically lost all he possessed.

The emphatic verdict of the first Sewage Commission of 1857, the first and second Rivers Pollution Commission, and, indeed, of all other authoritative investigations, was in favour of land treatment; and it cannot, therefore, be surprising to find that the Local Government Board insisted, save in exceptional cases, that “any scheme of sewage disposal, for which money is to be borrowed with their sanction, should provide for the application of the sewage or effluent to an adequate area of suitable land before it is discharged into a stream.” Indeed, had this body taken any different view and neglected the findings of practically all authoritative inquiries, it would have been singularly deficient in the discharge of its duties to the ratepayers of this country.

But the best of land cannot go on for ever doing its duty if by systematic neglect and ignorance the essential conditions for successful purification are year after year violated; and the great pity is that the Local Government Board, after deciding in favour of land treatment, did not systematically superintend this operation. It may not have had the power, but it is quite evident that had it done so, things would not have drifted from bad to worse, until local authorities, driven to despair by the apparent failure of land and not discerning the right cause, refused altogether to be ruled by what seemed to them a very unfair and absurd restriction.

It was at this time that Mr. Dibdin, who, on behalf of the London County Council, had been carrying out a set of valuable experiments, came forward with his application of well known theories to sewage operations on a large scale. As I pointed out at the time, Mr. Dibdin’s experiments proved beyond a doubt that the application of sewage to suitable land was right in principle and that the failures were brought about by the non-observance of the rules laid down by this gentleman—that, in fact, sewage irrigation was the only natural method of sewage purification and that all the other methods were artificial. I described land treatment as the natural self-purification of sewage and the oxidation or contact bed system as the artificial self-purification of sewage.

But the swift current of public opinion had set very strongly against sewage farms, and nothing but the contact bed treatment would do. A large number of experimental plants on this system grew up like mushrooms all over the country, and the waves of enthusiasm seemed at one time to engulf even the Local Government Board itself with its “antiquated notions,” until Parliament came to the rescue and appointed on May 7, 1898, a new Royal Commission to study the question of sewage purification.

This Commission consists of nine members,[4] i.e. six professional men and three laymen. Of the professional men, one is a biologist, one a chemist, two are medical men in administrative positions, and two are engineers likewise in administrative positions. Of the laymen two are members of special boards for the prevention of the pollution of rivers.

So far the Commissioners have issued an Interim Report dated July 12, 1901, a volume of evidence and a volume of appendices. Quite lately, it is stated, they have issued a further Interim Report, to which are attached separate reports on some special subjects by their officers, but this report has not yet come to hand.[5]

At the time of their first Interim Report, July 12, 1901, the Commissioners had held altogether thirty-five sittings, the first of which was on June 22, 1898, and the last on May 22, 1901. The period thus covered is nearly two years, and out of the thirty-five sittings thirty took place in London, and five in the provinces, viz. at Leeds, Ripon, Manchester, Accrington and Reigate.

On these occasions, all in all, fifty-eight witnesses were examined, who may be grouped as follows:

1 Zoologist
1 Botanist
2 Laymen
3 Bacteriologists
5 Lawyers
7 Medical men
11 Patentees
14 Chemists
14 Engineers

58 witnesses in all.

Out of this number twenty-five were officials, viz. five lawyers, six medical men, six chemists and eight engineers. Four officials were further managers of artificial sewage purification works, but not one single manager of natural purification works, i.e. a sewage farm manager, was called, the term “sewage farm manager” being used here to indicate an official whose sole duty it is to manage a sewage farm.

The entire absence of this latter class of official is so striking that it cannot be due to accident, but must be the outcome of a settled policy not to reopen questions conclusively settled by previous inquiries.

Another point that strikes the observer is that the Commission only called one zoologist and one botanist, as it is to these scientists that belongs in the first instance the question of studying the fauna and flora of sewage before the subject is taken up by other branches of natural science.

Speaking on the whole, the evidence taken by the Commissioners forms very interesting reading, and ought to be carefully studied by those who have to deal with the subject. When now and again opinions are expressed, which seem directly opposed to each other, it must be borne in mind that here, as in other things human, unanimity of opinion, though much desired, is apparently unobtainable.

To understand the conclusions fully, at which the Commissioners in their Interim Report have arrived, it ought to be pointed out that they had either to accept the recommendations in favour of land passed by all previous Royal Commissions and authoritative inquiries, or they had to show by incontestable evidence that their predecessors had made grievous mistakes, and where!

Of these two courses, the present Commissioners have adopted, no doubt for very good reasons of their own, the first, and they have started therefore, in the conclusions to which they have come, at the point where previous inquiries had left off, viz. that land treatment is a very proper method of sewage purification.

But before referring more in particular to their observations on land treatment, it will be necessary to point out that the Commissioners evidently divide all methods of sewage purification into two main classes, viz. natural and artificial methods. Into the former they only place land treatment, whilst they call all other methods artificial.

This division seems to have given a great deal of offence to all those who have expressed decided and frequently very one-sided views in favour of the “bacterial” treatment of sewage; but on closer examination it cannot be denied that the Commissioners were quite right in forming this view, as the following remarks will show.

For main divisions of all methods of sewage treatment two factors seem to be of primary importance, viz. the agencies which bring about this purification, and the way in which these agencies are employed. Now, it will not be denied that all agencies are natural ones, whether the process employed is a purely chemical one, a purely “bacterial” one, land treatment pure and simple, or a combination of these, and, at the present time no such thing as an artificial agency is known; indeed, it is perhaps not too much to say that there cannot be such a thing as an artificial agency. Hence it is impossible to divide sewage purification methods in this respect by the agencies employed, and one is bound to fall back upon the way in which these agencies are employed. Here it is no longer open to argument whether a chemical process or the contact bed system—oxidation bed system—is artificial, or whether the land treatment is natural! For who would deny that masonry or concrete tanks and the materials contained in the same are artificial products—i.e. products formed by man—and that land is a natural product—i.e. formed by nature—and that further the soil is the natural home of bacteria. Hence it must be perfectly clear, even to a casual observer, that the line of demarcation drawn by the Commissioners between all known systems of sewage purification is a correct and legitimate one, and that all objections to such a division are based on misconceptions.

Concerning land treatment, the Commissioners observe, “We doubt if any land is entirely useless,” but further on they observe that peat and stiff clay lands are generally unsuitable for the purification of sewage. Concerning peat, nobody acquainted with the subject would probably differ from their conclusions owing to the great amount of moisture contained in this material; but as to clay soils, the Commissioners when making this statement must have known that there are several successful sewage farms on this kind of land in existence, such as the sewage farms at South Norwood, Wimbledon, Warwick and Leicester, not to mention others. In the case of Leicester, although the land is a very dense boulder clay, the Corporation of this town have just purchased the freehold of the farm for about 160,000_l._

Dealing with the artificial processes from a chemical point of view, the Commissioners are of opinion that it is practicable to produce by these processes alone, either from sewage or from certain mixtures of sewage and trade refuse, effluents which might be discharged without fear of creating a nuisance, and that in consequence the Local Government Board would be justified in modifying, under proper safeguards, the present rule as regards the application of sewage to land.

The artificial processes referred to in the observations appear to be the following:—

Closed septic tanks and contact beds.
Open septic tanks and contact beds.
Chemical treatment, subsidence[6] tanks and contact beds.
Subsidence tanks and contact beds.
Contact beds alone.
Closed septic tank followed by continuous filtration.
Open septic tank followed by continuous filtration.
Chemical treatment, subsidence tanks, and continuous filtration.
Subsidence tanks followed by continuous filtration.
Continuous filtration alone.

The Commissioners do not say what these safeguards are, in fact they state that no general rules concerning them can be laid down, and that in the case of these artificial processes it is necessary to consider every case on its own merits.

The next point dealt with is the bacteriological quality of effluents, and here the Commissioners observe: “We find that, while in the case of effluents from land of a kind suitable for the purification of sewage there are fewer micro-organisms than in the effluents from most artificial processes, yet both classes of effluents usually contain large numbers of organisms, many of which appear to be of intestinal derivation, and some of which are of a kind liable under certain circumstances at least to give rise to disease.”

No particulars of effluents from sewage farms are given, and later on it will be shown that this conclusion of the Commissioners is not in accord with the results published up to now and available concerning the bacterial purity of effluents from land treatment.

The report concludes with some remarks on rivers pollution. The Commissioners state that it is of the utmost importance to provide the simplest possible means for adequately protecting all rivers, and they think that this subject is of such grave importance “as to demand the creation of a separate Commission or a new department of the Local Government Board, which shall be a supreme Rivers Authority, dealing with matters relating to rivers and their purification, and which, when appeal is made to them, shall have power to take action in cases where the local authorities have failed to do so.”

Summing up the observations on the practice of sewage treatment, it may be said that as a result of their extended inquiries, the present Royal Commissioners have at the end of the century re-established land in its position as the first and only natural method of sewage purification, beside which they have recognised artificial (biological) treatments as being under proper safeguards admissible for the purification of sewage.

Before concluding this portion of the observations, it is necessary to mention the valuable work done by Mr. Scott-Moncrieff and Mr. Cameron, who, contemporaneous with Mr. Dibdin, but quite independently, had experimented with sewage and evolved their own artificial methods of sewage treatment.

These remarks must suffice for the more historic portion of the subject, viz. the progress of sewage purification during the last century, and it is time now to direct attention first to natural and afterwards to artificial sewage treatments.

[4] Two of these have since retired.

[5] This report has just been issued (August 18, 1902), and
although the special reports it contains are of the greatest
interest, it is not necessary to refer to it again in these
observations.

[6] The expression “subsidence tanks” is intended to denote
tanks which are used in such way that little or no septic
action is produced.

III. THE SUBSOIL.

[Sidenote: General remarks on subsoil and its properties.]

Before dealing more in detail with the processes taking place in the pores of the subsoil of sewage farms, it may not be out of place to make here a few general observations on the mechanical structure of soil, its permeability, water capacity, retentive power, the capillary movements in the same, its temperature, the subsoil air, the movement of water in and through the same, the micro-organic life in soil, and its absorbing powers.

1. MECHANICAL STRUCTURE OF SOIL.

[Sidenote: Size of grain and pores.]

Here is of interest the size of the grains or particles composing the soil, the size of the pores and their collective capacity.

According to the character of the soil, its grains or particles will vary from very large in coarse gravel to very fine in fine sand and clay.

[Sidenote: Variable size of pores.]

[Sidenote: Surface attraction.]

The size of the pores will vary as the size of its grains from large to small, but frequently a certain kind of soil will contain a mixture of large and small pores. The finer the pores the more energetic will, as a rule, be the surface attraction of the grains composing the soil.

[Sidenote: Pore-volume.]

[Sidenote: With particles of equal size pore-volume amounts to about 38 per cent. of the total space, and sinks down to 10 or 15 per cent. with particles of unequal size.]

[Sidenote: With equally sized particles the pore-volume is the same whether the particles are small or large.]

The collective capacity of the pores or the pore-volume mainly depends on the equal or unequal sizes of the particles. When the same are of equal size the pore-volume amounts to about 38 per cent. of the total space occupied by the soil, but when this is not the case it may sink to as low as from 10 to 15 per cent. of this space. With equally sized particles the pore-volume is the same whether the individual particles are large or small. In nature it will be the exception to find all the particles of equal size, such a condition of things prevails only when careful sorting by sifting or riddling has taken place, and in the majority of cases the larger pores will be partly filled up by the smaller particles of the soil.

2. PERMEABILITY OF SOIL.

[Sidenote: Permeability depends first on the size of the pores, and secondly on the pore-volume.]

The permeability of a soil for the passage of air and water depends, in the first instance, on the size of the pores, and is further to some extent influenced by the pore-volume.

[Sidenote: Effect of large and small pores.]

Soil with large pores will offer but little resistance to the passage of air and water, but when the pores are small these movements will be greatly impeded.

[Sidenote: Permeability is proportional to the fourth power of the pore-diameter.]

It has been ascertained that the permeability of soils is proportional to the fourth power of the diameter of the pores, so that it decreases very rapidly with the diminishing size of the pores.

[Sidenote: In frozen soil permeability decreases rapidly.]

In subsoil with small pores all movements of air practically cease when it is half full of water, and in frozen soil the decrease of the permeability is still more marked.

3. WATER CAPACITY OF SOIL.

[Sidenote: Water capacity is equal to the pore-volume.]

[Sidenote: Air can never be wholly driven out of the pores.]

The water capacity of a soil is that quantity of water which can be stored in its pores; it is therefore equal to the pore-volume. For very accurate measurements allowance must be made for a small amount of air, which even after filling remains in the pores and cannot be dislodged, but for practical purposes this can be overlooked.

[Sidenote: 1 cubic yard of soil with particles of equal size will hold about 85 gallons of water.]

As has already been stated, the pore-volume of a soil consisting of equal particles throughout, amounts to about 38 per cent. of the space occupied by it, and 1 cubic yard of such a soil—whether we have to deal with coarse gravel or fine sand—will hold about 85 gallons of water.

4. WATER-RETENTIVE POWER OF SOIL.

[Sidenote: The water-retentive power of soil is a percentage of its water capacity.]

The water-retentive power of a soil is expressed by that quantity of water which can be retained by it; it will always be a percentage or portion of the water capacity of this soil.

[Sidenote: Soil with a large pore-volume and a large percentage of fine pores retains more water than soil with a small pore-volume and large pores.]

[Sidenote: Clean gravel retains about 10 gal. and clean sand about 70 gal.]

Soil with a large pore-volume and with a large percentage of fine pores will retain more water than soil with a small pore-volume and few fine pores. Clean gravel will retain about 12 per cent. of its water capacity, i.e. 10 gallons per cubic yard, whereas fine sand may retain as much as 84 per cent. of its water capacity, i.e. about 70 gallons per cubic yard.

[Sidenote: Organically polluted soil retains more water than clean soil.]

This will explain why a polluted subsoil containing a large amount of organic substances will retain more water than the same soil in a clean condition.

[Sidenote: The retentive power of a soil is due to its surface attractions.]

The retentive power of a soil is due to the surface attraction of its particles, and when the space between them is small, or when, in other words, the pores are small, this attractive power will be all the greater.

[Sidenote: When, after the limit of the retentive power has been reached, of water are poured upon the soil, a portion of the previously stored water is driven out, and its place in the pores taken up by the fresh supply.]

It is further of interest to observe here, that if after the limit of the retentive power has been reached further quantities of water are poured upon the soil, the water retained in the lower layers will commence to drain away. This means that the water freshly poured upon the soil will drive out a portion of the water previously stored in the pores. It is important to bear this in mind when dealing with polluted water, as owing to this action the water penetrating into deeper layers will to some extent at least have become purified in the upper layers.

5. CAPILLARY MOVEMENTS OF WATER IN SOIL.

[Sidenote: Capillary attraction causes an upward movement of the water.]

Through capillary attraction an ascending movement of the water is caused in direct opposition to the laws of gravity, and the height to which water will thus ascend depends mainly on the smallness of the pores; large pores do not assist in this movement. As the same, however, extends over the whole pore-volume the quantity of water thus raised may exceed the water-retentive power of soil.

[Sidenote: Capillary attraction also causes lateral and downward movements.]

In addition to the upward movement brought about by capillary attraction, this power is also continually at work in a lateral and downward direction; but for the present purposes only the upward movement will be noticed.

[Sidenote: Time occupied by upward movement. Height reached by upward movement.]

In observing the upward movement, it is interesting to notice the time occupied by it and the total height reached. As to the time occupied, it has been established that the upward movement in gravel and coarse sand is much quicker than in fine and loamy sand, but the heights attained are reversed. For whereas the height in a material consisting of coarse or large pores amounts to from 2 inches to 4 inches; a height of about 4 feet after thirty to thirty-five days has been recorded in fine or loamy sand; in peaty soil one observer states that the upward movement of the water may reach a height of 20 feet.

6. TEMPERATURE OF SOIL.

[Sidenote: Three principal sources of heat.]

The earth’s crust receives its supply of heat from three principal sources, viz.:

1. From the sun through its rays;

2. From the interior of the earth through conduction; and

3. From various physical and chemical processes which take place in it and create heat.

[Sidenote: Heat through sun’s rays.]

[Sidenote: Dark soils absorb more heat than light-coloured soils.]

[Sidenote: Capacity for heat is greater in damp and fine-grained soils.]

[Sidenote: Evaporation and condensation of aqueous vapour produce the greatest effect in fine-grained soils.]

Dealing with the upper layers of the crust, it may be said that, besides the intensity of the sun’s rays, the temperature also depends on a variety of properties possessed by various kinds of soil, amongst which latter may be mentioned the absorption of heat, which is much greater in dark than in light-coloured soils; the heat conductivity and the capacity for heat, which lead to higher temperatures in damp and fine-grained soils; and finally the evaporation and condensation of aqueous vapour, which tend to prevent extremes of heat and cold and which likewise produce the greatest effects in fine-grained soils.

[Sidenote: A fine-grained damp soil does not get so hot, but retains the heat better.]

It follows from these observations that a coarse-grained, dark coloured and dry soil will show the highest and lowest temperatures, whereas a fine-grained damp soil does not get so hot but retains the heat better.

[Sidenote: The temperature of the surface of the soil may exceed that of the air.]

It ought to be pointed out in this place that a variety of circumstances may bring about very high temperatures on the surface of the ground which considerably exceed the average temperatures of the air at the same time.

[Sidenote: Laws regulating the subsoil temperatures.]

Concerning the laws that have been deduced from careful and long continued observations of subsoil temperatures, it will not be necessary at this point to deal minutely with them; it must on the contrary suffice to summarise only the more important ones.

With the distance from the surface of the ground,

1. The differences of temperature become less,

2. The temperatures are retarded, and

3. The variations of short durations gradually disappear.

[Sidenote: Subsoil temperatures 18 in. below surface.]

[Sidenote: Subsoil temperatures at depths of 4 ft. 6 in. and 9 ft.]

At a depth of 18 inches below the surface the daily fluctuations are hardly observable, the temperature differences of various days become obscured, the differences between the monthly mean temperatures are less by several degrees, and the yearly fluctuation amounts only to about 10° C. At a depth of 4 feet 6 inches the latter is only 4° C., and at a depth of 9 feet it is only 1°C.

[Sidenote: Subsoil temperatures at depths from 9 ft. to 33 ft.]

Between 9 and 33 feet, according to the yearly mean of the surface, the yearly fluctuation ceases and the temperature remains the same throughout the year.

Below this point an increase of temperature is observable towards the earth’s centre, which amounts to about 1° C. for every 40 feet.

[Sidenote: Retardation of temperatures with increase in depth.]

Concerning the retardation of the temperatures with an increase in depth below the surface, it is interesting to point out that this, according to Fodor, amounts to about three weeks for every yard, so that the yearly maximum at a depth of 1 yard will take place in August, at a depth of 2 yards in the beginning of September, and at a depth of 4 yards in October. This is on the assumption that the maximum temperature of the atmospheric air is reached in July.

[Sidenote: Frost depth about 3 ft.]

The depth to which frost under ordinary conditions penetrates is about 3 feet, but there are cases on record where water pipes at depths of from 4 to 5 feet have been frozen up during long continued severe frost.

7. SUBSOIL AIR.

[Sidenote: Subsoil air is saturated with aqueous vapour and contains large quantities of carbonic acid.]

The pores of soil are either partly or wholly filled with air, which as a rule is saturated with aqueous vapour. This air consists very largely of carbonic acid (from 0·2 to 14 per cent., on an average from 2 to 3 per cent.) and to a small extent of oxygen, which has been used up for the formation of carbonic acid. It also contains traces of ammonia and gases of decomposition.

The movements of subsoil air need not be considered here, and beyond these few general observations it will not be necessary to deal with the subject.

8. MOVEMENTS OF WATER IN SOIL.

[Sidenote: Strata above level of subsoil water.]

Two main strata may here be distinguished in subsoil, one above the level of the subsoil water and one below this level. The latter strata do not interest us, and those above the level of the subsoil[7] water may again be subdivided into three zones, which in descending order are as follows:—

The evaporation zone;
The passage zone; and
The capillary zone.

[Sidenote: One-third of the rain-water evaporates. One-third flows off the surface. One-third percolates.]

All these three zones must be passed by the water in its descent from the surface of the ground to the subsoil water level, and the quantity of water retained by them will depend on their state of dryness. Speaking quite generally and within wide limits, one-third of the rain-water flows off the surface, one-third evaporates, and one-third percolates into the subsoil.

[Sidenote: Evaporation zone.]

The evaporation zone reaches from the surface of the soil to that point below, which marks the extent of the drying influence of the atmospheric air. In the same the quantity of water stored in the pores may at times sink below the retentive power of the soil, i.e. below that quantity which can be retained in the pores owing to the mechanical powers of adhesion, etc. When it has become very dry through evaporation and other causes the zone, especially when it extends some way down, may retain large quantities of water. In a depth of 10 inches, 1 square yard of soil, with fine pores, may retain about 10 gallons of water, and as a rainfall of ½ inch produces only 2·3 gallons per square yard, it is clear that subsoil of this nature may retain a number of successive showers. During the height of summer fine porous soil may become so dry that practically no water finds its way into deeper zones; in this state the evaporation zone can be compared to a large sponge.

[Sidenote: Passage zone.]

The next zone traversed by the water in its downward movement is the passage zone, which lies beyond the drying influence of atmospheric air. When too far removed from the level of the subsoil water, its pores will not be completely filled with water, but will only contain that amount which is due to the retentive powers of the soil. By direct measurement it has been found that on an average a cubic yard of fine porous soil will retain from 30 to 80 gallons of water, and it can easily be calculated that in a layer from 1 to 2 yards in thickness the rainfall of a whole year may be retained. The passage zone, especially if it is of considerable thickness, represents a very large storage reservoir.

[Sidenote: Capillary zone.]

The last zone before the level of the subsoil water is reached is the capillary zone, in which the pores are partially or wholly filled by the upward movement—due to capillary attraction—from the subsoil water. The extent of this filling will depend on the size of the pores.

[Sidenote: Springs.]

When the descending water has finally reached the subsoil water it either comes to a standstill altogether on the impervious layer or moves along the same, if the latter is not horizontal, until it may eventually leave the subsoil again by issuing therefrom in the form of visible or invisible springs.

[Sidenote: Rate of downward movement governed by pores.]

The rate of movement of any liquid—rain-water, sewage or other polluting liquid—is largely governed by the size of the pores. Where these are large, as for instance in coarse gravel, the descent of the water will be comparatively rapid, but when they are small it may take a very long time before the water reaches the level of the subsoil water, and in that case it will have undergone material changes as regards its chemical or bacterial composition.

[Sidenote: With a high level of subsoil water zones become indistinguishable.]

With a high level of subsoil water the zones may become indistinguishable, one zone reaching into the other, with the result that the whole of the soil becomes very wet.

When subsoil has been artificially drained the amount of water reaching the subsoil water below the general level of the drains will depend on the size of the latter and the distance between them. In such a case the downward movement of the water through undrained soil, previously described, may be further interfered with through the ventilation of the subsoil by drains, and the drying up action caused thereby.

[7] The term subsoil water is here used to denote that
portion of the water in the pores of the soil, which is
either at rest on or moves along the inclined plane of an
impervious layer.

9. THE MICRO-ORGANIC LIFE IN SOIL.

[Sidenote: Soil probably original home of micro-organisms.]

[Sidenote: Distribution of micro-organisms in soil.]

The soil is probably the original home of all micro-organisms, from which they have emigrated into other media. It contains vast numbers, and, according to some observers, 1 ccm. may hold 100,000 germs. By far the greater number is found on or near the surface, and in lower layers the numbers gradually diminish, until at last a depth is reached, which depends on local conditions, where the soil is perfectly sterile. The aerobes live near the surface and carry on their work in this region, whereas the anaerobes are at work lower down in the soil.

[Sidenote: Cycle of micro-organic activity during the year.]

The picture of the cycle of micro-organic activity in the upper layers of the soil during the various seasons of the year is probably the following. In winter, especially during that period when frost and ice bind the earth, micro-organic life is apparently at its lowest ebb, and may in some very cold climates come to a standstill altogether, when micro-organisms may be said to hold their vegetative winter sleep. With the return of life and the awakening of nature in spring—especially with the approach of higher temperatures and the formation of moisture—micro-organic activity once more makes itself felt all round. During the summer months it is exposed to some injurious influences such as the heating and drying up of the upper layers of the soil, but, still gradually increasing, micro-organisms reach the climax of their activity during the autumnal rains, to remain in this state until with the advent of the cold season their activity gradually declines again.

[Sidenote: Micro-organic life in layers from 3 ft. to 6 ft. in depth.]

In the lower layers of the soil, down to 3 feet and 6 feet, micro-organisms are more protected against the injurious influences of the atmosphere, sunlight and drying up, but the want of oxygen, together with the greater difficulty of removing such products as carbonic acid, has an injurious influence. As the temperature in these layers is considerably more uniform, it may be inferred that the micro-organic activity is there of a more uniform kind, less influenced by sudden changes, probably also less intense, but without pronounced periods of rest.

[Sidenote: Micro-organisms probably quickly perish in depths greater than 6 ft.]

In depths greater than 6 feet micro-organisms probably perish very quickly owing to unfavourable conditions, and if found their presence must be explained by emigration from higher layers, not by actual growth at these depths.

On sewage farms the micro-organic activity is without doubt greatly modified, and proceeds all the year round at a more uniform rate than on ordinary land, as the sewage always contains the necessary warmth and moisture so beneficial for it.

10. THE ABSORBING POWERS OF SOIL.

[Sidenote: Absorbing powers due to surface attraction of the particles of the soil.]

[Sidenote: The finer the pores the greater the absorption.]

The absorbing powers of soil are due to the surface attraction of its particles or grains, and these, as has already been pointed out, will be all the greater the finer the pores are; they extend on the one hand to aqueous and other vapours and gases, and on the other to matters in solution.

[Sidenote: 1 cub. yd. of coarse gravel may contain 50 sq. yds. of surface and 1 cub. yd. of fine sand 9200 sq. yds.]

That the attractive force of the surface of the particles is pretty considerable will be at once apparent when it is stated that 1 cubic yard of coarse gravel may contain about 140,000 grains with a combined surface of 50 square yards, and 1 cubic yard of fine sand 40 million grains with a combined surface of 9200 square yards, which is a little under 2 acres.

[Sidenote: Deodorising action of soil absorption of gases.]

Concerning the absorption by soil of aqueous vapour and gases (apart from condensation through a fall in temperature), dry soil with fine pores acts most energetically. The almost instantaneous deodorisation of foul-smelling gases, such as are formed by decomposing fæcal matters (earth closet) or coal gas, through a thin layer of fine dry soil is well known, and is to be explained in this way.

[Sidenote: Absorption of dissolved substances by soil.]

More interesting still, and also more important, is the absorption of dissolved substances by soil. In this way is to be explained the decolorising effect and the retention of dissolved polluting substances such as are contained in sewage. In the same way soil has the power of destroying such poisons as strychnine, nicotine, coniine, etc., and the experiments of Falk and others go to show that ptomaines and toxines are likewise retained and rendered harmless by it. This absorbing power of soil is of the utmost importance in agriculture, and without it soil could not possess purifying powers for polluting liquids. It is quite true that in this process of purification other factors play an important part, but they could not come into play if this absorption did not exist.

The absorbing powers of soil are in some way dependent on the presence of micro-organisms and air, and in the absence of these they will soon come to a standstill.

IV. SELF-PURIFYING POWERS OF SOIL. NATURAL SELF-PURIFICATION OF SEWAGE.

[Sidenote: Self-purifying powers of soil.]

After these preliminary remarks it becomes necessary now to examine into the self-purifying powers of soil with special reference to sewage farms. Generally speaking, the term “self-purifying powers 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.”

[Sidenote: Self-purifying powers vary with local conditions.]

[Sidenote: 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.

[Sidenote: 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.

[Sidenote: Suspended matters retained on the surface, soil acts like a sieve.]

[Sidenote: Coating of surface of the land.]

[Sidenote: Removal of suspended matters generally an advantage.]

Portion of the suspended matters will be retained on the surface and the rest will be strained out in a mechanical manner in the pores, the soil acting as a sieve more or less fine according to its character. If the suspended matters are present in very large quantities it may happen that they will gradually form a coat on the surface of the land and choke the pores to the exclusion of air, and as this is a thing to be avoided in sewage farming it is in most cases advisable to remove them out of the liquid before it is poured upon the land.

[Sidenote: 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.

[Sidenote: 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.

[Sidenote: 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.

[Sidenote: 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.

[Sidenote: Depths to which polluting substances may penetrate into soil.]

The depth to which polluting substances may penetrate into soil will probably differ in each case, but the following factors may be said to influence it, viz. the velocity of the downward flow, the nature and degree of the polluting liquid, and the character of the soil. Where, therefore, the powers of the soil are over-taxed the polluting substances may reach the level of the underdrains and pass out through them, in which case the effluent will be but little better than the raw liquid. It must be the aim of careful management to avoid this.

[Sidenote: 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.

[Sidenote: 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.”

[Sidenote: 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.

[Sidenote: 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.

[Sidenote: Action of lime.]

When the soil of a sewage farm has got into this state, owing to having received heavy doses of sewage, the application of lime has proved very beneficial by accelerating the process of nitrification, and in this respect interesting experiments have been made on the Berlin sewage farms. The action of lime is said to be a twofold one.

1. It quickly attacks and splits up the organic matters and accelerates afterwards their decomposition and their utilisation by plants; and

2. It neutralises the excess of acid in the soil, and causes the latter to part with its carbonic acid.

[Sidenote: 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.

[Sidenote: 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.

[Sidenote: 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.

[Sidenote: Greater depths than 4 ft. will be rarely necessary.]

On some farms a depth of 3 feet on an average has proved sufficient, and on others the drains have been laid at depths ranging between 3 and 6 feet, but very special reasons ought to be shown for all depths over 4 feet.

[Sidenote: 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.

[Sidenote: 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.

[Sidenote: 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.

[Sidenote: 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.

[Sidenote: 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.

[Sidenote: Displacement of sewage held by the pores of the land by the fresh discharge of sewage upon the surface of the land.]