AMERICAN SOCIETY OF CIVIL ENGINEERS.

INSTITUTED 1852.

PAPERS AND DISCUSSIONS.

This Society is not responsible, as a body, for the facts and opinions advanced in any of its publications.

SEWAGE PURIFICATION.

An Informal Discussion at the Annual Convention, June 10th, 1903. *

By MESSRS. RUDOLPH HERING and GEORGE W. RAFTER.

RUDOLPH HERING, M. Am. Soc. C. E. (by letter).-The writer's com- Mr. Hering. pulsory absence prevents him from opening this discussion in person, as he had intended, namely, to outline the entire subject in its essential parts, dwelling upon those methods which are now more or less on trialand concerning which there is a lack of enough experience to eliminate all differences of opinion. Instead, he sends the following brief remarks:

This subject stands to-day, in a totally different light from what it did less than twenty-five years ago. A complete purification of sewage now resolves itself into the necessity of establishing the best conditions for the most suitable bacterial life to become most effective, because only in recent years have we recognized this minute life to be the essential feature in the process.

Bacteria may be active in flowing, or in large bodies of comparatively quiet, water. To get the desired effect, the bacteria contained

*The discussion of this subject, for which no formal paper was presented, is printed in Proceedings in order that the views expressed may be brought before all members of the Society for further discussion. (See Rules for Publication, Proceedings, Vol. XXV, p. 71.)

Communications on this subject received prior to October 23d, 1903, will be published subsequently.

Mr. Hering. in the sewage itself, its proper dilution, accomplished in an inoffensive manner, is therefore one of the important questions, and it is not yet completely answered.

Mr. Rafter.

The purifying bacteria may also be active in porous soil, notably in sand, and in imitation of Nature's production of spring water, the result may be completely satisfactory under suitable conditions, which are now fairly well understood.

Under still other conditions, a different class of bacteria, also contained in sewage, is brought into service, or the same bacteria may be brought to act under modified conditions, and, with the aid of the liquefying process of putrefaction caused by them, we may get rid of much of the suspended matter, which is the most troublesome

element in all phases of sewage purification. But such aid only partially solves the problem. Bacteria must again be set to work to convert the liquid sewage finally into mineral and inoffensive solutions. In order to gain this end, a number of practical ways of accomplishing these different processes are now in use

Numerous local conditions have been the cause of a variety of solutions, some of which are still on trial and await further investigations and study from actual experience on a large scale. In America and in Europe this experience is now being gathered, and a general discussion of the recent results gained should be instructive and valuable.

GEORGE W. RAFTER, M. Am. Soc. C. E. (by letter).-Sewage disposal has always been a simple enough matter to the non-technical people who mostly control the affairs of municipalities, but to those charged with its administration, it has been, and still continues to be, exceedingly complex. The latter class are conservative in estimates and statements, while the former are positive that they, at any rate, have struck the final solution.

When the matter is examined historically, it is found that sewage disposal, pendulum-like, has swung back and forth between certain extremes. From fifty to sixty years ago the sewage nuisance had so far forced itself upon public attention that a considerable number of works were constructed in England, and in the report of Henry Austin, "On the Means of Deodorizing and Utilizing the Sewage of Towns," published in 1857, a general résumé of the subject is given. At that time, chemical precipitation was coming into use, and extravagant notions of the value of the manure to be obtained were indulged in. It is unnecessary to state that none of these methods has justified the hopes of its inventors. A great many patents were taken out for various chemical processes, from about 1835 to 1860 or 1870, but the majority of these patents were so impractical as never to go into operation. Indeed, sewage disposal, owing perhaps to its complex nature, is not properly subject to patents. The useful developments,

thus far, have been mostly the work of scientific men, and ordinary Mr. Rafter. inventors have scarcely contributed anything thereto, nor are they likely to, because the complexity of the operations is such as to be beyond them.

The separation of the solid matters in suspension by sedimentation, straining or filtration, however, is an interesting process, which was tried in England about the year 1850, and which, from its developing on lines quite similar to what is now known as the septic process, may be considered a little at length. Mr. Austin describes the works at Cheltenham, England, for what he calls a mechanical process for separating the solid matter of sewage, as follows:

"The building is divided longitudinally, forming below ground two sets of reservoirs or tanks, which are employed alternately. The sewage passes through vertical filters in the upper and lower tanks, whereby the great bulk of the matters in suspension is separated and retained. These filters are 5 ft. deep and 2 ft. thick, and consist of coarse gravel inclosed within 2-in. perforated boards, these being protected with basket work to prevent clogging.

"The heavier matters of the sewage deposit themselves at the bottom of the tanks, but a large proportion of the solid forms itself into a floating body, and accumulates to about 18 ins. thick on the surface. The liquid is conveyed from the angular filters in the upper tanks by a line of pipes in each division.

"A weir, or rather division, in the third or lining tank, causes the water, then partially clear, to flow through a channel at each end, during its passage through which a certain proportion of cream of lime, mixed on the floor above, falls into it, and occasions a further precipitation to take place. The effluent water then passes through another filter with gravel finer than before, and then through a third, finer still, to the outfall.

"When either reservoir contains a certain amount of solid matter, the flow of sewage is cut off and turned into the other. This takes place about every eight weeks, and the filtering medium of gravel is removed at the same time and washed. The contents of the tanks which are in a state of 'slush' are then hoisted in buckets through the trap lids on to the floor above, and wheeled out and mixed with the scavenger's refuse of the town, the ashes, street sweepings, etc. These are brought to the yard, and a kind of embanked reservoir is formed of them immediately outside the building, and as the semi-solid sewage is wheeled into the midst, the dry refuse outside is turned on to it. The liquid is at once absorbed, and after being turned over and thoroughly intermixed, the solid mass is fit for immediate removal and use.

"The ashes and dry refuse of the town are said to be just sufficient for the purpose. They absorb about two-thirds of their bulk of sewage; that is to say, one cubic yard of the ashes, etc., and two-thirds of a cubic yard of the semi-fluid sewage, make only one cubic yard of solid manure.

The foregoing quotation is especially interesting as indicating that Mr. Austin observed an operation in the Cheltenham tanks exactly as is now known to exist in a septic tank; that is, the heavier matters deposit themselves at the bottom, while a considerable portion of the

Mr. Rafter. solids forms itself into a floating body, accumulating to the thickness of about 18 ins., on the surface. In view of this fact, it seems fair to say that in a practical way Mr. Austin was in reality the discoverer of the septic tank. Nevertheless, the real significance of the septic tank -the fact that the changes going on are biological-was not appreciated by him, as is shown by his calling it a "mechanical process for separating the solid matter of sewage." Nor, indeed, does anyone seem to have appreciated the significance of these tanks, although a number of them were built and operated in England, with such success as is possible for that kind of a tank, but the so-called chemical purification process forged rapidly to the front, and mechanical separation-which is now called septic action-was lost sight of for a number of years, and chemical purification became for a time the leading form in England.

In the meantime, land purification processes, where the proper conditions obtained, were slowly gaining ground. These processes have always labored under this disadvantage, that not being in any way subject to patent, they have never had the booming which chemical precipitation and purification by septic action have received, although whether this is so great an advantage may, perhaps, be an open question. At any rate, the progress which such processes have made, while slow, has nevertheless been sure, until we are justified in saying that where proper conditions obtain, broad irrigation and intermittent filtration are, all things considered, the most satisfactory systems yet worked out. As a broad proposition, in consideration of the value of the crops produced, broad irrigation is the cheapest system yet devised. Experiments at Berlin, Paris, Croyden, Doncaster, Birmingham and other places, show that large crops may be produced and eaten without prejudice to health.

Let us now consider the recent development of sewage purification by means of bacterial action. This has been a favorite subject for consideration by a number of biologists, scientific writers, etc., for a good many years. The history of the matter will not be gone into at length at this time, any further than to point out two or three significant guide posts along the line. Like all other discoveries, its development has been gradual, and is the work of many men instead of one.

As far as can be learned, Dr. Alexander Mueller was the first person to appreciate clearly that bacteria were the prime agents in sewage purification. About 1880, Dr. Mueller took out a patent, endeavoring to utilize the micro-organisms in sewage for purposes of purification. Works were built under this patent, and were in operation for some time, to purify the effluent of a manufacturing establishment for beet sugar. Dr. Mueller says:

"The contents of sewage are chiefly of organic origin, and in consequence of this, an active process of decomposition takes place in

sewage, through which the organic matters are gradually dissolved into Mr. Rafter. mineral matters, or, in short, are mineralized, and thus become fit to serve as food for plants. To the superficial observer this process appears to be a chemical self-reduction; in reality, however, it is chiefly a process of digestion, in which the various, mostly microscopically small, animal and vegetable organisms utilize the organically fixed power for their life purposes.

"The decomposition of sewage in its various stages is characterized by the appearance of enormous numbers of spirilla, then of vibrios (swarming spores), and, finally, of moulds. At this stage commences the reformation of organic substance, with the appearance of the chlorophyl-holding protococcus, etc."

In the meantime, in December, 1881, and January, 1882, an account of Mouras' automatic scavenger was published in France. This apparatus consists of a closed vault with a water-seal, which it is stated rapidly transformed excrementitious matters into a homogeneous fluid, only slightly turbid, and holding the solid matters in suspension in the form of scarcely visible filaments. The principle upon which the automatic scavenger acts is that animal dejecta contain within themselves all the principles of fermentation necessary to liquify them and to render them useful. Later observations show that Mouras' view, that animal dejecta contain within themselves the fermentative principle, is fundamentally true, and, in this particular, Mouras, with Dr. Mueller, was in advance of the scientific workers of that day.

A number of investigators were at work upon the general problem, and in May, 1886, Dr. Dupré, in discussing a paper of Dr. Tidy's, at the Society of Arts of England, proposed to cultivate bacteria on a large scale, discharging them with an effluent into a river where they might be expected to further purify sewage.

In January, 1887, Mr. Dibdin read a paper, before the Institution of Civil Engineers, in which he discussed the propriety of using large quantities of lime. At that time many chemists claimed that lime was especially valuable in sewage purification because it destroyed living organisms, such as bacteria, which give rise to putrefaction, the idea being that, if all the organisms could be destroyed, sewage would be rendered innocuous. Mr. Dibdin took the ground that the very essence of the sewage purification is not the destruction of bacterial life, but is the resolution of organic matter into other combinations. Hence, it follows that an antiseptic process is the very reverse of the object to be aimed at. That is to say, a septic process is required, and hence the name septic sewerage applied later on to designate this process.

In November, 1887, the Massachusetts State Board of Health commenced their classical experiments on the purification of water and sewage by filters. The first two volumes of the reports, published in 1890, laid the foundations of the modern development of bacterial