4. Sewage Disposal by Dilution.—The large volume of water present demands that economical transportation be effected by gravity, and that it be discharged into some convenient water course. The discharge of the untreated sewage into some water course is known as disposal by dilution. It has given rise to a series of problems, arising from nuisances and the contamination of public water supplies, from which have developed the modern methods of sewage treatment and water purification.
If the body of water into which the raw sewage is discharged is of sufficient volume, disposal by dilution may be sufficient.
On the other hand, if the diluting body is of insufficient volume, or if drinking water supplies are contaminated, or oyster beds are contaminated, or a nuisance is produced, it is unsatisfac tory or even dangerous.
When the process is satisfactory the following events take place: (a) There is a sedimentation of the heavy particles.
(b) There is an oxidation of the organic matter into stable and inoffensive substances by bacterial activity through the dis solved oxygen in the water (Fig. T).
(c) There is a gradual destruction of the fecal bacteria due to their introduction into an unfavorable environment and the activity of predatory protozoa.
Where a stream receives sewage we find that for a distance of several miles below the sewer outfalls, definite evidence of the sewage contamination is observable, but that at a greater distance the stream bed and the water assume an appearance similar to that above the entrance of the sewage and its ap pearance is no longer offensive. This phenomenon, due to the above agencies, is known as the "self purification" of streams.
Where dilution is insufficient we find that the sedimented material and dissolved matter does not undergo oxidation, but, due to insufficient oxygen, undergoes anaerobic decomposition with the production of offensive gases and odors; that the dis solved oxygen is consumed and fish life cannot survive.
Factors which limit the successful operation of this method of disposal are the following: (a) The sewage should not greatly exceed i per cent. of the volume of the water into which it is discharged, though this factor varies directly with the rapidity of the stream, since re oxygenation is more speedy in rapid streams.
(b) The amount of dissolved oxygen in the water, which is greater during cold periods than in warm weather. It must be remembered that the capacity of salt water for dissolved oxy gen is about 20 per cent. less than fresh water.
(c) The rate of the dispersion of the sewage in the diluent. This must be quick.
5. Sewage Treatment.—When satisfactory disposal by dilu tion cannot be secured, the treatment of the sewage is necessary. The following results are sought: (a) A separation of the suspended matters (sludge) from the liquid sewage.
(b) The destruction of the putrescible organic matter in the liquid sewage preparatory to its final mineralization by oxida tion (non-putrescibility).
(c) The transformation of the sludge into a stable and inert condition, with or without the aid of oxidation.
(d) The destruction or removal of bacteria from the effluent.
Several methods are employed to secure the above results which are used either singly or in various combinations. Great care and judgement must be employed in selecting the method of treatment to be employed, since different methods of treat ment vary in the degree of their successfulness, depending upon the character and composition of the sewage to be treated, as well as the degree of attention paid to the care of the disposal plants. All sewage treatment plants require intelligent opera tion, together with some degree of skill. Most dissatisfaction or failure can be traced to either neglect or ignorant operation. Sewage treatment plants should be designed by competent sanitary engineers, after study and experimentation has re vealed the method of treatment best suited to handle the local sewage.
6. Preparatory Processes.—These are designed to effect a separation of the suspended matter or sludge from the liquid sewage.
(a) Screening.—Coarse or fine gratings or screens are placed across the path of the sewage to remove coarse material, which is removed from the screens at frequent intervals and either buried or burned (Fig. 12).
(b) Sedimentation.—Most of the suspended matter is removed by sedimentation. The velocity of the stream of sewage is lowered and as a consequence the fine suspended particles settle. Various means are employed to apply this principle. Among them are the following: i. Grit Chambers.—These are small chambers, in which the velocity of the sewage is but slightly reduced, so that only the heavier of the suspended particles settle out. Because of their size the period of sedimentation is brief. They are chiefly used to effect a removal of sand and gravel in combined sewerage systems. These chambers must be cleaned frequently.