CHAPTER VII.

THE LIQUEFACTION OF SEWAGE.

The evolution of the septic tank-Pioneers: Scott-Moncrieff, Cameron, Dibdin, and Colonel Ducat-The Exeter septic tank-Leeds and Manchester experiments prove tanks necessary-Necessity for detritus tanks-Closed and open septic tanks-Liquefaction by lateral bacteria beds-Conclusions.

DURING the last seven years the attempts of the earlier experimenters to avoid the necessity for precipitation by causing the suspended organic matter in sewage to become liquefied by the action of bacteria, have been brought to a successful issue.

It was well known that an ordinary cesspool did not require emptying as often as it should if the solid matters poured into it accumulated; and as long ago as 1881, M. Mouras invented a closed tank in which fæcal matter and kitchen refuse disappeared as gases, or were transformed into a homogeneous, slightly turbid fluid, this result being due to the fact that animal dejecta contains the ferments necessary to liquefy them.

M. Mouras and his system, in this country at any rate, attracted little attention, and it was not until the nineties that Scott-Moncrieff brought forward his scheme to liquefy sewage by the aid of the bacteria natural to it. It was left, however, to Cameron, of Exeter, to be the first to successfully deal with the sewage of a town, by first liquefying the sewage and then oxidizing or nitrifying it on bacteria beds. Cameron did more to rivet public attention on the new aspect of the sewage problem than any one else, by boldly calling his tank "Septic" tank, thereby calling attention to the fact that purification was effected by encouraging the growth of microbes rather than by killing them. There is no doubt that the word "septic," in connection with the process of

purification, took hold of the public imagination, and extravagant ideas prevailed of the purification which was effected in septic tanks.

The exaggerated popular notion was that "the microbes ate each other up, and nothing but innocuous fluid remained."

Mr. Adeney, of Dublin-to whom we owe the word "bacteriolysis"-has suggested that the purification also of sewage could be effected by the action of micro-organisms by adding a sufficient quantity of nitrate of soda to the sewage.

Dr. Sims Woodhead found that in 1 c.c. of Exeter crude sewage there were one million organisms which were anaerobic, or did not grow in the presence of air, and 51⁄2 million organisms which were aerobic, or did live in the presence of air. Of the one million anaerobic, 300,000 were found to be liquefying organisms; and of the 53 million aerobic, 500,000 were also found to be liquefying: so that the proportion of liquefying organisms was found to be greater among the anaerobic than among the aerobic.

The credit for first applying, on a practical scale, the knowledge of the bacteriologists, that certain organisms had this power of liquefying organic matters,* belongs to ScottMoncrieff, who, in 1891, liquefied the sewage from a household of ten persons by means of a continuous upward-flow tank filled with coarse flints. It was five years later that Mr. Cameron, of Exeter, introduced his septic tank. Cameron, having come to the conclusion that the organisms which have the power of liquefying organic matters are largely anaerobic, or thrive best in the absence of air, conducted his sewage into elongated cemented watertight covered tanks, capable of holding 14 day's flow. The tanks are covered with concrete arches, carried by brick piers, and levelled over with soil; they are designed to promote the growth of liquefying organisms present in the sewage, by whose action the organic solids are broken down into simpler substances. The flow through the tanks is uniform and continuous, the inlet and outlet being submerged so as to minimize the disturbance by

* It is well known that many bacteria have the power of liquefying solid albuminous matter; in fact, one of the ordinary methods of distinguishing different species of bacteria is to ascertain their action upon such substances as gelatine. As many as 196 varieties out of 440 well-known bacteria have this property of liquefying gelatine.

the currents of the incoming and outgoing streams, and to prevent the admission of air, and the velocity through the tank should be from inch to 1 inch per minute.

In the course of time a thick tough scum formed on the top of the tank, and it was found that the sludge which settled at the bottom of the tank underwent decomposition with the evolution of carbonic acid gas, marsh gas, hydrogen and ammoniacal compounds, the resultant mixture being an inflammable gas which Cameron utilized on the sewage works. After the sewage had passed through the septic tank Cameron oxidized it by means of contact beds. As we shall see, contact beds are filters, the outlets of which can be closed to allow the filters to fill with the effluent from the septic tank, the liquid being left "in contact" with the filtering medium for an hour or so; when the outlet from the filter is opened and the sewage allowed to escape, air is drawn into the interstices of the filter as the effluent rushes out.

Following on Cameron's Exeter experiments came the investigations of Colonel Ducat and Mr. Dibdin. Their experiments were very similar. Colonel Ducat filtered crude sewage through a percolating filter eight feet deep, the sewage being uniformly distributed over the surface of the filter by means of narrow iron girders, in the sides of which V-shaped notches were filed, and through which the sewage continuously trickled. This filter had other special points, which will be described hereafter. Dibdin, on the other hand, purified crude sewage by directly applying it to contact beds. This was first done at Sutton, Surrey, and the process is often called the Sutton process.

In either case, solids in suspension are arrested in the upper layers of the filters, where they are gradually liquefied.

The question at once, therefore, arises, Is any form of septic tank necessary? If it is necessary, do any advantages attach to the closed form connected with the name of Cameron, of Exeter ?

Perhaps the best and most authoritative experiments on purifying crude sewage, both before and after screening, with and without septic tanks, are those conducted by the Corporations of Leeds and Manchester.

The Leeds experiments consisted in applying crude sewage to a coarse contact bed, and filtering the effluent through a

fine bed.

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The coarse bed was filled with coke not less than 3 inches in diameter, the fine bed with coke from inch to 1 inches in diameter. The beds were filled three times in the twenty-four hours. The gross capacity of the rough bed was 174,800 gallons, the net liquid capacity after filling with coke 83,300 gallons.

The following table shows the rate at which the working capacity diminished when no septic tank was used:

Colonel Harding, the chairman of the Committee, and Mr. Hewson, the city engineer, estimate that about 80 per cent. of the sludge is liquefied, 20 per cent. of the solids accumulating; but this quantity they found might be reduced by allowing the beds to rest from two to five weeks. Whereever crude sewage is applied to the contact beds or filters, the surface becomes clogged with fibre, bits of cellulose, and with a kind of papier mâché, from paper which has disintegrated in the sewage. Subsequently, the Leeds sewage was screened and then settled for one hour, and the filters were used at the rate of two fillings a day.

Even with this modification it was found that without a septic tank the process could not be relied upon to purify more than 80 gallons per square yard per day. It is found that the capacity of contact beds, even when supplied with thoroughly clarified sewage, decreases rapidly at first but more slowly afterwards, and becomes constant in about three months, when their normal capacity can only be calculated at two-thirds of the original water capacity.

In 1899, the Corporation of Manchester appointed Professor Percy Frankland, Professor Perkins, and Mr. Baldwin Latham to report upon a scheme of sewage disposal for Manchester.

These experts came to the following conclusions, amongst others:

1. "That suspended matter must be removed as far as possible by sedimentation."

2. "That any suspended matter not so removed should be retained as far as possible on the surface of the bed."

Necessity for Detritus Tanks.—If septic tanks are used, the liquefaction of the sewage takes place in the tank. If tanks are not used, the solid matter in suspension in the sewage is deposited upon the surface of the filter or contact bed and in its interstices, where it is gradually liquefied, but not until the bed has lost some 40 or 50 per cent. of its working capacity.

From the analysis of average sewage given on p. 11, it will be seen that some 10 grains per gallon of the solid matter in the sewage consists of insoluble mineral matter, the great bulk of which cannot be liquefied by the aid of bacteria. Where a precipitation process is adopted, such insoluble mineral matter, as road detritus, is useful in helping precipitation, but when a purely biological process is to be adopted, it is necessary to separate the detritus before the sewage is conducted to the septic tank, or is applied to contact beds or percolating filters; otherwise the tank will be unnecessarily silted up, or the contact beds or filters permanently plugged.

In every case, therefore, when a biological process is adopted, a detritus tank must be constructed.

Of course it is quite impossible for the bacteria to dissolve the road detritus, and reference to pp. 26 and 27 shows that, while the mineral matter in suspension in the American sewage was about 5 parts per 100,000, and in the Exeter sewage 10 parts, that in a mining district, like Alfreton-where the quantity of water used is less than 10 gallons per head per day, and there are no water-closets-the mineral matter in suspension is 32.5 parts per 100,000. This suspended matter will have to be got rid of, either by simple subsidence before the sewage enters the septic tank, or else be precipitated in an ordinary Dortmund tank. At Exeter it is proposed to let this mineral matter subside in three grit chambers, each fifteen feet square and fifteen feet deep.

The usual system is to have two shallow tanks of such a size that the velocity of sewage passing through them is reduced to about thirty feet a minute, when the coarse mineral matter falls to the bottom of the tank. When about a foot of detritus has settled, the other tank is used and the first one is emptied.

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