The disposal of sewage by dilution is subject to statutory limitations in many states. The enforcement of these laws is usually in the hands of the state board of health, which is frequently given discretionary powers to recommend and sometimes to enforce measures for the abatement of an actual or potential nuisance. Such recommendations usually take the form of a specification of certain forms of treatment preliminary to disposal by dilution. No project for the disposal of sewage by dilution should be consummated until the local, state, national, and in the case of boundary waters, international laws have been complied with. The attitude of the courts in different states has not been uniform. Little guidance can be taken from the personal feeling of the persons immediately interested. The opinion of the riparian owner 5 miles down stream may differ materially from the popular will of the voters of a city, and it is likely to receive a more favorable hearing from the court. Statutes and legal precedents are the safest guides.

226. Preliminary Treatment.—If the sewage to be disposed of by dilution contains unsightly floating matter, oil, or grease, no amount of oxygen in the diluting water will prevent a nuisance to sight, or the formation of putrefying sludge banks. Under such conditions it will be necessary to introduce screens or sedimentation basins, or both, in order to remove the floating and the settling solids. Biologic tanks, filtration, or other methods of treatment may be necessary for the removal of other undesirable constituents.

227. Preliminary Investigations.—Before adopting disposal of sewage by dilution without preliminary treatment, or before considering the proper form of treatment necessary to render disposal by dilution successful, a study should be made of the character of the body of water into which the sewage or effluent is to be discharged. This study should include: measurements of the quantity of water available at all seasons of the year; analyses of the diluting water to determine particularly the available dissolved oxygen; observations of the velocity and direction of currents, and the effect of winds thereon; a study of the effect on public water supplies, bathing beaches, fish life, etc. Good judgment, aided by the proper interpretation of such information should lead to the most desirable location for the sewer outlet. If preliminary treatment is found to be necessary tests should be made to determine the necessary extent and thoroughness of the treatment.

CHAPTER XV
SCREENING AND SEDIMENTATION

228. Purpose.—The first step in the treatment of sewage is usually that of coarse screening in order to remove the larger particles of floating or suspended matter. Screens and sedimentation basins are used to prevent the clogging of sewers, channels, and treatment plants; to avoid clogging of and injuries to machinery; to overcome the accumulation of putrefying sludge banks; to minimize the absorption of oxygen in diluting water; and to intercept unsightly floating matter.

By the plain sedimentation of sewage is meant the removal of suspended matter by quiescent subsidence unaffected by septic action or the addition of chemicals or other precipitants. In order to prevent septic action plain sedimentation tanks must be cleaned as frequently as once or twice a week in warm weather but not quite so often in cold weather.

Fine screening may take the place of sedimentation where insufficient space is available for sedimentation tanks, and it is desired to remove only a small portion of the suspended matter. Recent American practice has tended to restrict the field of fine screening to treatment requiring less than 10 per cent removal of suspended matter, thus eliminating screens from the field covered by plain sedimentation tanks. The practice is well expressed by Potter, who states:[[135]]

Where a high degree of purification is sought, the use of fine screens is of doubtful value. A modern settling tank will give better results and at a less cost for a given degree of purification. A settled liquid is also superior to a screened liquid for subsequent biological treatment in filters.... Again the storing of large quantities of screenings must necessarily be more objectionable than the storing of the digested sludge of a modern settling tank.

Fig. 150.—Types of Moving Screens.
Trans. Am. Society Civil Engineers, Vol. 78, 1915, p. 893.