219. Definition.—Disposal of sewage by dilution is the discharge of raw sewage or the effluent from a treatment plant into a body of water of sufficient size to prevent offense to the senses of sight and smell, and to avoid danger to the public health.

220. Conditions Required for Success.—Among the desired conditions for successful disposal by dilution are: adequate currents to prevent sedimentation and to carry the sewage away from all habitations before putrefaction sets in, or sufficient diluting water high in dissolved oxygen to prevent putrefaction; a fresh or non-septic sewage; absence of floating or rapidly settling solids, grease or oil; and absence of back eddies or quiet pools favorable to sedimentation in the stream into which disposal is taking place. The conditions which should be prevented are: offensive odors due to sludge banks, the rise of septic gases, and unsightly floating or suspended matter. In some instances the pollution of the receiving body of water is undesirable and the sewage must be freed from pathogenic organisms and the danger of aftergrowths minimized before disposal. Such conditions are typified at Baltimore, where the sewage is discharged into Back Bay, an arm of Chesapeake Bay. One of the important industries of the state of Maryland is the cultivation of oysters. The pollution of the Bay was therefore so objectionable that careful treatment of the Baltimore sewage has been a necessary preliminary to final disposal by dilution. It is unwise to draw public water supplies, without treatment, from a stream receiving a sewage effluent, no matter how careful or thorough the treatment of the sewage. The treatment of the sewage is a safeguard, and lightens the load on the water purification plant, but under no considerations can it be depended upon to protect the community consuming the diluted effluent.

The sewer outlet should be located well out in the current of the stream, lake, or harbor. Deeply submerged outlets are usually better than an outlet at the surface, as a better mixture of the sewage and water is obtained. The discharge of sewage into a body of water of which the surface level changes, alternately covering and exposing large areas of the bottom is unwise, as the sludge which is deposited during inundation will cause offensive odors when uncovered. Such conditions must be carefully guarded against when selecting a point of disposal in tidal estuaries because of the frequent fluctuations in level.

221. Self-Purification of Running Streams.—The self-purification of running streams is due to dilution, sedimentation, and oxidation. The action is physical, chemical, and biological. When putrescible organic matter is discharged into water the offensive character of the organic matter is minimized by dilution. If the dilution is sufficiently great, it alone may be sufficient to prevent all nuisance. The oxidation of the organic matter commences immediately on its discharge into the diluting water due to the growth and activity of nitrifying and other oxidizing organisms and to a slight degree to direct chemical reaction. So long as there is sufficient oxygen present in the water septic conditions will not exist and offensive odors will be absent. When the organic matter is completely nitrified or oxidized there will be no further demand on the oxygen content of the stream and the stream will be said to have purified itself. At the same time that this oxidation is going on some of the organic matter will be settling due to the action of sedimentation. If oxidation is completed before the matter has settled on the bottom the result will be an inoffensive silting up of the river. If oxidation is not complete, however, the result will be offensive putrefying sludge banks which may send their stinks up through the superimposed layers of clean water to pollute the surrounding atmosphere.

The most important condition for the successful self-purification of a stream is an initial quantity of dissolved oxygen to oxidize all of the organic matter contributed to it, or the addition of sufficient oxygen subsequent to the contribution of sewage to complete the oxidation. Oxygen may be added through the dilution received from tributaries, through aËration over falls and rapids, or by quiescent absorption from the atmosphere. The rapidity of self-purification is dependent on the character of the organic matter, the presence of available oxygen, the rate of reaËration, temperature, sedimentation, and the velocity of the current. Sluggish streams are more likely to purify themselves in a shorter distance and rapidly flowing turbulent streams are more likely to purify themselves in a shorter time, other conditions being equal. Although the absorption of oxygen by a stream whose surface is broken is more rapid than through a smooth unbroken surface, the growth of algÆ, biological activity, the effect of sunlight, and sedimentation are more potent factors and have a greater effect in sluggish streams than the slightly more rapid absorption of oxygen in a turbulent stream. It is frequently more advantageous to discharge sewage into a swiftly moving stream, however, regardless of the conditions of self-purification, as the undesirable conditions which may result occur far from the point of disposal and may be offensive to no one.

The sewage from a population of about 3,000,000 persons residing in and about Chicago is discharged into the Chicago Drainage Canal. It ultimately reaches tide water through the Des Plaines, the Illinois, and the Mississippi Rivers. The action occurring in these channels is one of the best illustrations known of the self-purification of a stream. In Table 75 are shown the results of analyses of samples taken at various points below the mouth of the Chicago River where the diluting water from Lake Michigan enters, to Grafton, Illinois, at the junction of the Illinois and Mississippi Rivers about 40 miles above St. Louis. The effect of the physical characteristics of the stream on its chemical composition is well illustrated in this table. The rise in the chlorine content between Lake Michigan and the entrance to the Drainage Canal is a measure of the addition of sewage. Since the chlorine is an inorganic substance which is not affected by biologic action, its loss in concentration in the lower reaches of the rivers is due to dilution by tributaries and sedimentation, e.g., between the end of the canal at Lockport and the sampling point at Joliet, the entrance of the Des Plaines River reduces the concentration of chlorine from 124.5 to 41.5 parts per million. The entrance of the Kankakee River at Dresden Heights further reduces the chlorine to 24.5 p.p.m. The increase of albuminoid and ammonia nitrogen accompanied by a decrease in nitrites and nitrates, between the upper end of the canal at Bridgeport and its lower end at Lockport indicates the reducing action proceeding therein. The oxidizing action over the various dams and the effect of dilution with water containing oxygen is shown between miles 34 and 38, at mile 79, and at mile 294. The excellent effect of quiescent sedimentation and aËration in Peoria Lakes is shown between miles 145, 161 and 165.

222. Self-Purification of Lakes.—Sewage may be disposed of into lakes with as great success as into running streams if conditions exist which are favorable to self-purification. Lakes and rivers purify themselves from the same causes; oxidation, sedimentation, etc., but in the former the currents are much less pronounced and may be entirely absent. In shallow lakes (20 feet or less in depth) dependence must be placed on horizontal currents and the stirring action of the wind to keep the water in motion in order that the sewage and the diluting water may be mixed. In deeper bodies of water, currents induced by the wind are helpful but entire dependence need not be placed upon them. Vertical currents, and the seasonal turnovers in the spring and fall completely mix the waters of the lake above those layers of water whose temperature never rises higher than 4° C.

In the early winter the cold air cools the surface waters of a lake. The cooling increases the density of the surface water causing it to sink, and allowing the warmer layers below to rise and become cooled. After the temperature of the entire lake has reached 4° C. the vertical currents induced by temperature cease, as continued cooling decreases the density of the surface water maintaining the same layer at the surface. In the spring as the temperature of the surface water rises to 4° C. and above it becomes heavier and drops through the colder water below causing vertical currents. These phenomena are known as the fall and spring turnovers. The former is more pronounced. These turnovers are effective in assisting in the self-purification of lakes.

223. Dilution in Salt Water.—The oxygen content in salt water is about 20 per cent less than in fresh water at the same temperature. The greater content of matter in solution in salt water reduces its capacity to absorb many sewage solids. This, together with the chemical reaction between the constituents of the salt water and those of the sewage serve to precipitate some of the sewage solids and to form offensive sludge banks. The evidence of the action which takes place in the absorption of oxygen from the atmosphere by salt water and its effect on dissolved sewage solids is conflicting, but in general fresh water is a better diluting medium than salt water.

Black and Phelps have made valuable studies of the relative rates of absorption of oxygen from the air by fresh and salt water. The results of their experiments are published in a Report to the Board of Estimate and Apportionment of N. Y. City, made March 23, 1911.^[131] Concerning these rates they conclude:

Therefore there is no reason to believe that the reaËration of salt water follows any other laws than those we have determined mathematically and experimentally for fresh water. In the absence of fuller information on the effect of increased viscosity upon the diffusion coefficient, it can only be stated that the rate of reaËration of salt water is less than that of fresh water, in proportion to the respective solubilities of oxygen in the two waters, and still less, but to an unknown extent, by reason of the greater viscosity and consequent small value of the diffusion coefficient.

224. Quantity of Diluting Water Needed.—In a large majority of the problems of disposal of sewage by dilution it is not necessary to add sufficient diluting water to oxidize completely all organic matter present. Ordinarily it is sufficient to prevent putrefactive conditions until the flow of the stream, lake, or tidal current, has reached some large body of diluting water or where putrefaction is no longer a nuisance. It is never desirable to allow the oxygen content of a stream to be exhausted as putrescible conditions will exist locally before exhaustion is complete. The exact point to which oxygen can be reduced in safety is in some dispute. Black and Phelps have assumed 70 per cent of saturation as the allowable limit; Fuller has placed it at 30 per cent; Kinnicutt, Winslow, and Pratt have placed it at 50 per cent. Since the reaction between the oxygen and the organic matter is quantitative, others have placed the limit in terms of parts per million of oxygen. Wisner,^[132] has recommended a minimum of 2.5 p.p.m. as the limit for the sustenance of fish life, which is not far from Fuller’s limit for hot-weather conditions.

Formulas of various types have been devised to express the rate of absorption of oxygen with a given quantity of diluting water which is mixed with a given quantity and quality of sewage. The quantity of sewage is sometimes expressed in terms of the tributary population or in other ways. Knowing the rate at which oxygen is exhausted and the velocity of flow of the stream, the point at which the oxygen will be reduced to the limit allowed is easily determined. The accuracy of none of these formulas has been proven, and their use, without an understanding of the effect of local conditions, may lead to error. They may be used as a check on the bio-chemical oxygen demand determinations, which should be conclusive.

The following formula, based on the work of Black and Phelps, is a guide to the amount of sewage which can be added to a stream without causing a nuisance. It is: