CHAPTER XIV.

SEWAGE DISPOSAL.

One of the most difficult problems presented for solution to the Sanitary Engineer to day, is that of sewage disposal. How to effectually dispose of the solid and liquid wastes in any community so that they will be neither offensive or dangerous to any one is a question of growing importance. The rapid increase in population of our cities and villages swells the flood of sewage which is poured into the streams, polluting the natural sources of water supply, while the demand for pure water is of necessity rapidly increasing.

In the older countries of Europe the pollution of water courses by sewage has forced itself upon the attention of government officials, and stringent laws have been passed to protect the purity of streams. In this country the time is not far distant when the pollution of streams and lakes by sewage will need to be forbidden by law, or in many localities pure drinking water in any considerable quantities will not be obtainable.

In many cases there is no available outfall for the sewage and the question of its disposal comes up at once with the inception of sewer projects.

Of the various methods of sewage disposal those which have been tested on any considerable scale may be classed as follows: I. It may be emptied into a stream or large body of water. 2. It may first be clarified by straining, by subsidence, by filtration, by chemical processes or by a combination of these and the effluent turned into a stream or body of water.

3. It may be applied to the soil, as in intermittent downward filtration, broad irrigation, or subsurface irrigation.

Dilution. When sewage is turned into a stream of considerable size the disappearance of the sewage is due to several causes. The sewage is diluted by the large amount of water into which it is discharged, some of the organic matter becomes food for aquatic plant and animal life and some is destroyed by oxidation, and the remaining solid particles are deposited along the bed and banks of the stream. So long as the amount of sewage is small in comparison with the volume of water this method may be admissable, but it is in use in scores of cases where it ought not to be.

Rivers and lakes often become so polluted by sewage as to become a serious menace to the health of cities on their banks. The Chicago River and Lake Michigan at Chicago, and the Cuyahoga River and Lake Erie at Cleveland are examples of this.

Subsidence. When sewage is partly purified by subsidence the sewage is collected in tanks and allowed to stand until the solids are deposited and then the water is drawn off. Although somewhat less objectionable, the effluent water is charged with impurities and is still unfit to be turned into the natural water

courses.

Filtration. Filtration is sometimes resorted to and the sewage is passed through filters of various sorts. This separates more of the solids than can be obtained by subsidence, but the effluent is still unfit to be turned into the streams.

Chemical Processes.--Chemical processes have been extensively used. In these some chemical solution is mixed with the impounded sewage, which precipitates not only the solid matter but also a part of the substances held in solution. There are scores of these patented processes-too many to even name within the limits of this chapter. The solid residuum or "sludge" is used as a fertilizer. The hope that the "sludge" would be of great value as a fertilizer has not been realized and very little profit can usually be obtained from this source.

The effluent water is far from pure and frequently decomposes after being turned into a creek or river. This method

might be used advantageously where the effluent passes into the sea or large tidal rivers, where the water is not used for water supply.

Application to the Soil.-Filtration through the soil, both upward and downward has been used. Intermittent downward filtration has worked quite successfully. By this method the sewage is turned on to ground which has been thoroughly underdrained. The sewage is filtered by passing through the soil and much of the organic matter is destroyed by oxidation and nitrification.

Separate filtering beds are prepared so that they may be used alternately. The effluent water is quite pure if the filter beds are properly made and kept in good condition. The amount of ground necessary and the depth of the under drains depend upon the character of the soil.

Where sufficient suitable land can be procured broad irrigation is the most satisfactory method of sewage disposal.

The soil should be loose and thoroughly underdrained. Compact clay is not suitable for a sewage farm without special treatment for breaking up and loosening the subsoil.

The amount of sewage per acre which can be disposed of varies with the nature of the soil, and its special preparation for sewage disposal. In practice one acre of land has been used in broad irrigation for disposing of the sewage of from 50 to 500 persons.

CHAPTER XV.

THE PURIFICATION OF SEWAGE BY APPLICATION TO THE SOIL.

It is still an open question whether water which has been contaminated with sewage may be so thoroughly purified as to be entirely safe for culinary uses.

So far as chemical purification is concerned there is no doubt it can be accomplished by filtration through the soil under favorable conditions. Whether the effluent can be so purified by this means as to contain no trace of the bacteria which are supposed to incite various zymotic diseases, the water having been previously contaminated with them, is still in dispute by eminent authorities who have labored in this field. It is well known that the purest of natural waters, as regards organic matter, are those which have undergone prolonged filtration through the soil. Our knowledge of the causes which influence the purification of sewage when applied to land, either in broad irrigation, intermittent downward filtration or subsurface irrigation is rapidly extending, however, and this is a field of such promise as to justify a reference more at length to some of the later achievments and conclusions concerning this method of purification.

The impurities with which sewage is charged consist mainly of different organic compounds in various stages of decomposition. It is not practicable, or in fact desirable, to prevent the decomposition of these organic compounds. With the exception of the small portion which may be consumed as food by animal life, they must be resolved into their elementary substances before they can again be utilized by plant life or otherwise

rendered innocuous. It is, however, practicable to so control the conditions of decomposition that it shall become inoffensive and the resulting compounds shall be fixed and rendered harmless by some surrounding medium. The soil is a medium which not only renders these products innocuous but also favors a mechanical separation and aeration very conducive to the rapid disintegration and absorption of the putrescible matters contained in the sewage.

In the report of the Select Committee on the Metropolis Sewage it is stated that, “No efficient artificial method has been discovered to purify, for drinking and culinary purposes, water which has once been infected by town sewage. By no known mechanical or chemical means can such water be more than partially cleansed; it is always liable to putrefy again. Processes of filtering and deodorization cannot, therefore, be relied upon to do more than mitigate the evil. Water which appears perfectly pure to the eye is sufficient, under certain conditions, to breed serious epidemics in the population which drinks it. Soils, however, and the roots of growing plants have a great and rapid power of abstracting impurities from sewage water and rendering it again innocuous and free from contamination.”

The process of purification of sewage by filtration through the soil is similar to that of burning up or oxidizing the organic matter leaving only a harmless mineral residue, which is soluble and passes off in the effluent, leaving the filtering medium, when properly managed, undiminished in efficiency. The application of sewage intermittently serves to increase the amount of oxidation similarly to opening the draft of a furnace.

The changes produced by wet oxidation, however, are not the same as those produced by heat, there being intermediate processes. In the former the nitrogen of the organic matter first combines with the hydrogen to produce ammonia which, upon uniting with oxygen, produces nitric acid. This, in turn, combines with potash, soda, lime or some other base present in the sewage or in the soil to produce a soluble nitrate. The extent to which this sequence of combinations has proceeded is a