Among the principal objections to the combined system when the sewage is treated are the increase it causes in the volume of liquid which necessarily requires a larger plant and expenditure, the changes it causes in the character of the sewage which complicates operation of the plant, and the frequency with which it causes the flow of sewage to exceed the maximum of the plant, thereby making it necessary to discharge untreated sewage into the stream. With a combined system all kinds of trade wastes must be run through the disposal plant, whether they are offensive or not; automatic devices, which should be avoided whenever possible, are necessary between the combined and intercepting sewers to limit the amount of flow; a greater amount of grit is deposited at the disposal works unless in the separate system the first wash of the street is intercepted. The New York State Board of Health advocates the separate system.

In constructing, extending or reconstructing a sewerage system it is well to bear in mind that even though a city has not at present a disposal plant, the time will come in all probability when increased population will compel the treatment of its sewage by some process. It may, therefore, be more economical eventually to make present plans so that when disposal does come the sewerage system will make possible the most economical operation of the disposal works. Gregory’s conclusion as recently expressed in an address is that “other things being equal, especially as more and more attention is being given to sewage disposal, the separate system seems to offer greater advantages.”

All engineers advocate good ventilation for sewers and gradients that will develop self-cleansing velocities, so as to reduce gas trouble and to deliver the sewage as fresh as possible to the disposal works. The best practise, according to reports of the State Boards of Health, show that these velocities should be not less than two feet per second in separate systems and two and one-half feet in combined systems. In some instances where it has been necessary to reduce the gradients because of the expense of obtaining steeper ones, a velocity of one foot per second has been found to be satisfactory; but in such instances sewers must be well constructed and flushed. Most trade wastes require a higher velocity to prevent deposits.

The Degree of Purification of Sewage

Before determining the proper method of disposal the first point to be settled by a city is the degree of purification desired or needed for both the present and the future. The decision is dependent upon three factors: the self-purifying capacity of the stream or body of water into which the effluent—liquid portions of the sewage run off after treatment—is to be discharged and its utilization for water supply, bathing, etc., the character and amount of the sewage and the possible future growth not only of the city itself, but also of the communities bordering on the stream. While there have been some demands for the absolute sterilization of sewage, many sanitarians believe that any artificial method of sewage treatment will not esthetically render the final effluent fit for ingestion, and practically all authorities agree that final discharge of sewage need not be in this perfect condition. This seems to be based on logical reasoning when one considers that all waterways are necessarily polluted to some extent. John Duncan Watson, of Birmingham, England, contends that the complete elimination of bacteria is prohibitive inasmuch as it is beyond the limits of the reasonable demands on the purse. Robert Spurr Weston, member of the American Society of Civil Engineers, at one time reminded an audience that the proper place to protect the water consumers against disease is at the water works and not at the sewage disposal plant. Authorities are in general agreed that sewage should be disposed of as the stream demands, and that local conditions should determine degree of purification required. Standards of purity have been studied by many societies and various suggestions have been made. All agree that the sewage after treatment should not deteriorate the stream into which it flows. Watson advocates under certain conditions an effluent that will not putrefy on being kept for seven days at a uniform temperature of 80 degrees F. and that does not contain more than three parts per 100,000 of suspended solid matter.

Generally speaking the suspended matter should be removed, the conditions near the point of discharge be inoffensive and the water be not impaired for purposes of manufacture and pleasure. When a city is located on the seashore or near a large lake or stream the screening out of the heavy particles before the sewage is discharged together with dilution will prevent active decomposition and putrefaction of the sewage the body of water receives and the esthetic senses of the community will not be offended. On small bodies of water and when the water is used for drinking and manufacturing purposes or for bathing or shellfish the conditions usually demand not only a non-putrescible effluent but also one that is free from harmful bacteria or one that is highly purified like that from sand filters.

There seems to be a general agreement among sanitary engineers that the condition of the river below where the effluent joins it is a safe guide and should be the ruling factor in determining the degree of purification desirable. Authorities, however, are not agreed as to whether the standard of cleanliness should be based solely on chemical analysis or on a mixed standard taking into consideration the appearance of the water and its physical, chemical and bacterial conditions, as has been demonstrated by the Metropolitan Sewage Commission of New York. One expert in answer to the question propounded by the Commission based the standard solely on chemical analysis, but none of those whose views were sought was willing to accept the dissolved oxygen test as an all sufficient criterion of the condition of the water. One considered that the oxygen should be regarded as a reliable index of the cleanliness of the water only when dealing with the condition of gross pollution and only when in conjunction with observations of the appearance and physical conditions of the water. One of them would not have a standard of cleanliness based solely upon analysis of any kind and all were agreed that the standard of cleanliness should not rest upon the effect of the polluted water upon health.

After having decided on the degree of purification the next step in the solution of the problem is to select the process of treatment best adapted with local conditions to produce the results at the lowest cost and without nuisance. No specific rules can be laid down for the selection of the best process for all communities. Domestic wastes offer the least difficulty, but they are usually complicated with the presence of trade or street wastes or both. Features difficult to overcome may then be produced. Then also, the character of the sewage varies greatly with the season, days and even hours. This is due to the habits of the people, to climatic conditions and to the amount and character of trade and industrial wastes and to the amount of water used and allowed to infiltrate. A cannery, creamery, tannery, brewery, strawboard factory, wool scouring shop, dyeing and cleaning works may discharge its wastes so that during a certain period the character of the sewage be entirely changed. Knowledge of these conditions and changes are necessary to plan a successful disposal plant. Each community has its own problem, and while there are certain general conditions that should be considered, each case is more or less unique. Charles G. Hyde, consulting engineer of the California State Board of Health, has summed up the situation in this statement: “It is folly to suppose that because one town can dispose of its sewage successfully in some certain fashion, another town can adopt the same method with a certainty of securing equally satisfactory results. Sewage differs widely in character, not only as between towns but in a given town.”

Processes of Treatment

The processes for treating sewage may be divided into three main groups—the preliminary or preparatory, the main or final, and disinfection.