CHAPTER I

INTRODUCTORY

The problem of sewage disposal for a single house differs from the corresponding problem for a city chiefly in two ways: first, because in the city it is becoming, if it has not, indeed, already become, a necessity, and city authorities, though somewhat reluctantly, are willing to grant the necessary appropriation to secure engineering advice which will solve the problem in a scientific as well as economic fashion. In the case of a single house, whether a farm-house or a villa, the necessity of employing competent engineering advice has not been generally recognized, and no attempt has been made to solve the problem of sewage disposal in a scientific manner.

Cesspools have been considered the only way of caring for sewage in places where a running stream was not available, or where attempts were made to protect such a stream from pollution, and while, in these last few years, crude attempts have been made to utilize the so-called septic tank, such attempts have generally been so unintelligent that the results have been anything but satisfactory. Since it has been understood that insects, such as flies and mosquitoes, play an important part in the transmission of disease, the danger of overflowing cesspools and of open ditches in which stagnant sewage is present, has been appreciated; also the higher standards of living which have made themselves felt throughout the rural community have demanded in farm-houses and country homes sanitary conveniences which have hitherto been wanting.

Gradually every house is using more and more water for various purposes, and living conditions, which in the past tolerated a scanty supply drawn from a pump, are no longer endured. The increased water supply and the demands of extended plumbing mean a greater amount of sewage—so great an amount that, in many cases, soils which could receive and digest the waste waters from houses supplied by wells are clogged and made impervious by this greater amount.

Further, the danger to wells from the infiltration of cesspools is more feared, and it is understood as never before that in order to maintain the highest degree of health in a family the drinking-water used must be above suspicion and not subject to contaminating influences in the vicinity.

Again, communities are being aroused to the intrinsic value of maintaining streams in a pure condition—partly because of the value of fish and ice coming from the streams themselves, and partly on the broad ground that watercourses belong to the country as a whole, and must be kept pure for the sake of succeeding generations, not spoiled for them on account of the selfishness of a few at the present time.

Thus it is that to-day the problem of sewage disposal, while arousing general interest, is recognized as one which requires more than the common sense of an average person, that the force and principles involved are understood to be not those in common use, and that, for successful disposal of sewage, special knowledge and judgment are required.

Whatever the character of the sewage and whatever the kind of soil available for treatment, the method of dealing with sewage most obvious to most people has been to discharge the sewage directly into the nearest watercourse. This has been the practice of cities as well as of individual houses in the past, and the practice is very difficult to check because of the economy of this method of disposal. In many cases there is no objection to this method, and where a large stream is available, where no use is made further downstream of the waters for drinking purposes, and where the volume of water in the stream is sufficient to dilute the sewage to a point where no odors or objectionable appearances result, it would seem most uneconomical to adopt any more complicated method of disposal than by simply carrying the outfall pipe into the main bed of the stream.

In New York State, and in a number of other States, the number of which is continually increasing, such direct discharge, however, is not permitted by law except under certain conditions. In New York State it is required that any house, butter or cheese factory, manufacturing establishment, or village shall obtain the permission of the State Commissioner of Health before such a method of discharge be adopted, and in order to obtain this permission it must be definitely shown that the conditions of the stream are such that no reasonable objection to this method could be urged. The policy of the various Departments of Health in the United States is gradually becoming more and more rigorous in the matter of prohibiting the discharge of crude sewage into watercourses, and it is wise to make very sure that the discharge of sewage into streams is above the suspicion of a nuisance before adopting this as a suitable method. Rather would it seem better to provide for some method of treatment and allow only purified sewage to go into the stream than to run the risk of being forced in a few years to reconstruct the entire line of outfall pipe, with perhaps an entire reconstruction of the plumbing within the house.

The problem of treatment is the question of so modifying the character of a large volume of dirty water that it shall neither injure the quality of any drinking-water into which it may be discharged, nor cause objectionable odors, nor present disagreeable appearances in any body of water into which it may be emptied.

In order to properly understand a reasonable method of treatment some consideration must be given to the composition of sewage. This is chiefly water with which is mixed a small amount of animal, vegetable, and mineral matter. Roughly speaking, the amount of mineral dirt is about one tablespoonful to a barrelful of water, and the combined amount of animal and vegetable matter amounts to another tablespoonful. It seems almost impossible that so small a quantity of organic matter as one tablespoonful in a barrel of water could cause offense in any way, and yet engineers, city officials, and householders know by bitter experience that, when spread out on the surface of the ground or when allowed to stand in pools, water so polluted will undergo putrefaction resulting in most disagreeable odors and in complete stagnation. The problem of sewage treatment, then, consists in removing from the barrelful of water, the tablespoonful of organic dirt, whether animal or vegetable, in such a way that no odors shall be occasioned by the process and at the same time so that the cost of the process may be a reasonable one.

Unfortunately, the greater part of this organic matter is in solution, dissolved, like salt in water, so that, though undeniably present, it must be removed by some process more complicated and less obvious than that of simple straining. It would be comparatively simple if the polluting substances remained floating or suspended in the water. Then they could be strained out through a fine sieve or settled out in a tank, either with or without the aid of chemicals. But for particles in solution, straining, by itself, is useless and, while in large plants frequent use is made of sieves as a complement to the main process of purification, in small plants it is of so little value as hardly to deserve consideration.

Another factor enters to lessen the value of the use of screens or sieves in an installation for a single house. A great deal of the organic matter found in sewers requires both agitation and time for its subdivision into particles small enough to be acted upon in any process of purification adopted. If a screen is used, large particles of putrescible matter are held on the screen since not enough time has existed to break down their mass, and thus the screen itself becomes a most emphatic disturbance and a most objectionable feature of the purification plant.

For efficient purification, therefore, some method of reducing and modifying the character of organic solids, particularly those in solution, must be selected. In seeking a method by which this may be accomplished, scientific men found years ago that this very process was being carried on continually by natural forces, although at a very slow rate of purification. All organic matter, however formed and wherever present, is subject to the natural forces of decay. Fruits, vegetables, and meats of all kinds, exposed to the air, rapidly lose their original character and form and in the course of time disappear entirely. Except for this provision of nature, the accumulation of organic wastes since the beginning of the earth’s occupation by human beings would be so great that the earth would be uninhabitable on account of the deposits of waste matter which would have formed by this time. Nature, then, recognizes the need of disposing of organic wastes, and her method is the one which apparently must be followed by human beings if successful treatment is to be secured.

Only a few decades ago, it was found that this process of decay was due to the activity of very small organisms known as bacteria, and their agency was proved by experiments which showed that if vegetables or meat were kept free from bacteria, no decay, fermentation, or putrefaction took place. It was proved that the air itself was not responsible because in certain experiments air was allowed to enter through a filtering medium fine enough to strain out the bacteria and no decay took place, although oxygen and air were both freely admitted. It is well understood by the housewife that fruits can be kept indefinitely if they are cooked sufficiently to kill any bacteria present and then sealed in bacteria-free, air-tight jars. When such preserves spoil, it is because some bacteria were left in the jar or have since been admitted through an imperfect top. When decay is allowed to proceed, the obvious result is, first of all, a softening of the material, as in the case of a rotten apple, a liquefaction, as it is more technically known. Following that part of the process is a gradual breaking down of the material, the residue being of an earthy character which is assimilated by the soil into which it falls.

The bacteria required for the putrefaction of organic matter are among the most widely distributed of all the micro-organisms. They are always found in the air, except on mountain tops, in deserts, and over the ocean. They are very numerous in surface waters, such as streams and ponds, and their relative number everywhere increases as the amount of organic matter increases, so that the greater the need for them the greater is their number. It has been found that the great majority of these bacteria require air for their energetic development, and this fact is most important when it comes to the practical construction of a piece of apparatus for making use of these bacteria. It has also been found that, for several reasons, these bacteria work most effectively in the soil and can take care of a larger quantity of organic matter there than elsewhere. This is partly because in the surface layers of the soil, particularly where that soil has been cultivated, a great number of the particular bacteria involved in decay are always to be found. Pure, clean sand from the desert contains almost none of these beneficent bacteria. Rich garden soil is fairly teeming with them, so that, curiously, the more organic matter and the more bacteria present in any soil, the more active that soil will be in taking care of other organic matter.

Then, again, the soil particles, particularly in sandy soil, are so separated as to allow between them a certain and appreciable amount of air, and by means of this air the activity of the bacteria is made continuous and the products of their activity utilized. Without such an admission of air, the bacteria are choked and diminish rapidly in numbers. There is, however, a definite degree of purification and a certain quantity of organic matter which can be taken care of by the bacteria incident to any particular soil. Up to that quantity purification proceeds more or less satisfactorily according to the intelligence shown in feeding the bacteria in such a way as suits their convenience. If, however, that quantity be exceeded, all purification stops, the bacteria are apparently discouraged, and no further improvements can be expected. A fine-grained soil will not be so useful as a coarse-grained soil because the former does not allow sufficient air in the interstices of its soil particles. Another practical reason for not making use of soils of fine grains is that such soils can absorb only a small amount of liquid because of the mechanical construction of the material. On the other hand, soils whose grains are too coarse are undesirable because their mechanical construction is such that the liquids containing organic matter in solution pass through so rapidly that time enough is not given for bacterial action.

As a result of the principles just enumerated, it may be said that there are three distinct and essential conditions for the successful disposal of sewage through the soil. These three conditions are, first, a rate of application suitable to the soil which it is proposed to use; second, an interrupted or intermittent delivery of the sewage so that the bacteria can obtain, between consecutive doses of sewage, the necessary amount of oxygen for their own preservation and well-being; and, third, a resting period in which is carried forward that intimate association between the partly decomposed organic matter and the oxygen or air present in the pores of the soil by which the final oxidation is obtained.

The rate of application varies, as already indicated, with the size of particles found in the soil, and it should also vary with the purification desired. The larger the particles, the higher may be the rate of application, but less efficient will be the process. With grains of sand as fine as 1/200 of an inch, and with a rate of application not greater than five gallons per square yard of surface per day, filtration through such an area has been proved to be capable of removing from the foulest sewage all the objectionable material and converting the liquid into what is an equivalent of the purest spring water. If the rate appropriate to this particular soil is exceeded, the efficiency decreases, and the unmistakable and inevitable result is to stop all purification and convert the filter into a stagnant cesspool. If, to take the other extreme, the soil particles are increased until they are as large as hen’s eggs, then, if the rate of application is not greater than 200 gallons per square yard of surface per day, and if the method and rate of application are suitable to this large amount, the resulting effluent is sufficiently freed from its objectionable matter so that the liquid can be turned into any body of water without danger of odors or other nuisance. If this rate is exceeded, or if the method of application is not carefully considered, the resulting effluent is foul in the extreme and the process itself becomes a nuisance.

It can be seen by this brief explanation that it is not possible to assign any particular rate of application to any particular kind of treatment, since in all the methods of purification which have been worked out considerable variation in the details of that particular method have been practised. It will be possible, therefore, in succeeding chapters to indicate by the size of filters recommended only limiting or average values for rates of purification, since those rates are always dependent upon other factors than the particular method being discussed. It must also be remembered that soils may exist which have no porosity whatever, and through which it is impossible for sewage to make its way. Such soils are not available for sewage purification, and, no matter how small the rate or how careful the method of application, such areas will fail to produce any practical purification. Soils like clay, peat, and fine water-deposited silt are of this sort. Clay soils may sometimes become pulverized by cultivation so that they will ultimately be able to take care of a moderate amount of sewage. In such a case it is possible to dispose of sewage successfully in the top six inches of soil which, by continual cultivation, has been made out of the stiff clay. In such cases, the difficulty is not that of oxidizing the sewage, but that of taking care of the effluent, which must be held between the cultivated soil and the raw clay underneath.

The second requirement mentioned is secured by discharging the sewage onto the soil area at intervals, the number of doses per day depending upon the size of particles in the bed. There has been a general principle established that the size of these doses ought to be smaller as the size of the particles increases, so that, whereas in the case of sand beds the total daily dose is usually divided into from one to three parts and each part delivered onto the bed with an appropriate interval, in the case of coarser materials used for sprinkling filters, the time interval between doses is much reduced and in some installations recently constructed in England that interval has been measured in seconds. The variations in the rate of flow of sewage onto any filter, however, are so great that any such requirement as designing discharging apparatus to work at intervals of a few seconds is useless, and if as small an interval as one minute is provided for the coarsest material for the maximum rate of flow at any time of the day, the installation will probably be successful for the lesser rates occurring at other times of the day. As an indication of the way in which this modification is made, it is customary, when the size of soil particles is that of peas, to make the interval between successive discharges about one hour, so that the dose applied at any one time would be equal to 1/24 of the daily volume. With gravel filling, the particles being the size of English walnuts, the interval between doses is shortened to five minutes, and the amount of any one dose is thus made 1 about 1/280 of the total daily volume. With the coarser filling, as when a size as large as hen’s eggs is used, the interval would be cut down to about one minute. It should be added that the intervals last mentioned are characteristic only of some devices used for dosing sprinkling filters and that there is a wide divergence of practice among engineers when dealing with any particular size of sand or stone particles in all kinds of filter beds.

The third requirement, namely, the occasional resting of the bed, is met by providing some additional area over that theoretically required, so that the flow may be diverted from part to part of the total area (which is usually divided into beds for this purpose), and in this way each part is allowed, in turn, a period for resting. For example, if the required area be divided into two beds and a third bed added equal in area to one of the two and a regular rotation of dosing be practised, each bed would rest not only the time between the regular twelve-hour period dosing, but might also be given a complete rest, occasionally, for an extended period. This third requirement is probably less imperative with the coarser particles and there are many examples of coarse-grained beds which have been continuously operated for a period of years. It is found, however, that with such treatment clogging is inevitable, and that such clogging is partially relieved by a period of rest somewhat proportional to the length of time the beds have been operated. It is, then, only shortsighted policy to economize at the beginning and attempt to save money by not building an additional area, since the clogging of the whole plant is bound to occur in the course of time, and then another plant must be built or the material forming the bed taken out, washed, and replaced. Otherwise the sewage must go unpurified to the outfall while the bed is recovering from the long period of overwork.

It is convenient to divide sewage purification into two processes, the preliminary process and the final, or finishing process, and, while the preliminary process, in itself, never accomplishes purification, yet it is of considerable value in facilitating and increasing the rate and efficiency of that purification. The most common preliminary treatment is sedimentation, by which the larger solids in suspension are allowed to settle in a tank or tanks so that the filter beds later used are relieved from the accumulation of those deposits. Under the name of septic tank such a receptacle for suspended solids has been exploited as a complete method of purification, and many underground tanks have been constructed in various parts of the country which have, at the time of their installation, been considered competent to furnish all the necessary purification. When it is remembered that less than one-half of the organic matter in sewage is in suspension and that the best results in any sort of a tank succeed in depositing only one-half of those suspended solids, it can readily be seen that a tank, whether called septic or settling, cannot be a complete method of treatment. In reality, such a tank does little more than take out from the sewage the greasy material and a certain proportion of the suspended matter. Whatever part of this is organic matter may, by a particular arrangement of the tank, be considerably reduced in quantity, so that the intervals of cleaning can be extended, but in every tank the removal of the deposits is necessary, and subsequent treatment is required if adequate purification is accomplished.

The final, or finishing, process may be carried out according to any one of several methods. It may be done by discharging the tank effluent into a system of agricultural drains laid just below the surface of the ground, called sub-surface irrigation. It may be done by removing the top soil from a bed of sand placed by nature, and needing little except suitable surface distribution to insure the most efficient purification.

For a small plant, instead of a sand filter, for which the sand is found naturally in a suitable location, an artificial filter may be built by preparing an enclosure and carting in sand for filling.

Where no sand is available, or where its use would be uneconomical, broken stone may be used to ensure final treatment. With stone, on account of its large voids, the enclosure must either be water-tight, and the outlet pipe must be provided with a valve or other device so that the sewage under treatment may be held in the enclosure or tank long enough to deposit the solids in suspension and to be acted on by the bacteria concerned. This method is known as the contact bed treatment. Or, finally, the desired results may be obtained by spraying the sewage onto a deep layer of broken stone, the method being called the sprinkling filter treatment.

The choice of the final treatment, in any particular case, depends on the character and slope of the ground, on the availability and cost of sand or of broken stone, and on the amount of sewage to be treated. It is hoped that the following pages will give to the reader both an intelligent appreciation of the advantages and disadvantages of each of the several methods of sewage purification discussed, and also sufficient insight into the necessary details of construction so that the method chosen can be put into successful operation.