gauge it becomes necessary to operate the sewage pumping

stations.

The solid materials will be separated out from the sewage by the Imhoff tanks and the residuum of the sludge finally withdrawn from the tanks and dried on the sludge beds as is customary in plants of this character.

These plants are designed to take care of the estimated population of the city up to 1930 and will be so constructed that they can be readily enlarged to take care of the future growth in population.

The tax-payers of Grand Rapids are confronted with an expenditure which will be in the neighborhood of $500,000 for the construction of the disposal plants, diversion sewers and real estate and rights of way. Of this amount about $150,000, which. is required for the extension of the East Side Trunk sewer before referred to, will probably be assessed on the property benefited. The remainder, or about $350,000 will be taxed directly upon the city at large and the total amount will probably be raised by means of a bond issue.

While sewage purification is unquestionably desirable, the general feeling in this city has been that there is no call at the present time for an expenditure of any such amount of money as the city will be compelled to raise in order to meet the requirements of the Court decree, but having the problem to meet it will undoubtedly wish to do it in the most satisfactory manner, as it has been usual for Grand Rapids to do well whatever it has undertaken in the way of public improvements. Having spent upwards of one million dollars for flood protection and with that project still in view; having put some $430,000 into a filtration plant to purify its water supply, it will undoubtedly construct sewage disposal plants of which it may be as proud as it justly is of the other engineering undertakings which have been carried to a successful completion so far.

Mr. l'aughan: Our next paper will be:

Basis of Design and Unit Costs

ALBERT ROTH

Member M. E. S.

The problem of sewage disposal is a broad one. Irrigation of land with sewage dates far back into history. A gradual development of modern methods of disposal began with the chemical precipitation treatment established in England in 1870. The most rapid advancements in the art have taken place within the past decade. The septic tank, sand filter, contact bed and sprinkling filter each marked a forward step, while within the past year the activated sludge method of oxidizing sewage has come to the front.

The Imhoff tank, developed some eight years ago in Germany was a decided improvement in the methods of settling and rendering innacuous the suspended solids in sewage. Much could be said regarding the bacterial processes involved, the relative advantages of this type of tank and the position which the tank holds in the general field of sewage treatment. This paper, however, will be limited to a discussion of the essential principles of design and the costs of Imhoff tanks.

In connection with the *Detroit sewage investigation, the writer has analyzed and tabulated the basis of design and unit costs of several of the typical Imhoff tank installations in this country. A summary of the data is given in tabular form under the discussion of unit cost data.

HOW CHARACTER AND QUANTITY OF SEWAGE AFFECTS DESIGN

One of the first factors in the design of a sewage disposal works is to determine the quantity and character of the sewage to be treated.

In general the character of the sewage depends upon the type of sewerage system, whether combined or separate.

The combined system of sewers receives both the storm water runoff or surface wash and the sanitary or house sewage.

As

An investigation of the Detroit sewage disposal problem has been carried on during the past year by C. W. Hubbell, Consulting Engineer for the city, and by H. C. McRae, District Engineer for the International Joint Commission.

can be expected, the sewage contains considerable amounts of detritus. Grit chambers and coarse screens should be built to care for this feature. More sludge usually results from the combined system.

In the separate plan, the sanitary or house sewage is collected by an individual system of sewers. The sludge quantities are usually smaller and the grit chambers can be omitted.

The character of sewage is further determined by the type of industrial and commercial establishments discharging their wastes into the sewers. The amount of sludge to be handled is often greatly increased by these wastes.

The quantity of sewage flow per day and its hourly variation

determines the size of the settling devices. The population contributing sewage and the type of system largely determines the quantity of sludge to be expected.

ESSENTIAL PRINCIPLES OF THE IMHOFF TANK

In the design of the so-called two story or Emscher tank, Dr. Imhoff embodied essentially two features which were different from the septic or plain sedimentation tank.

First: The settling sewage was separated from the sludge digestion action by means of curtain walls, thus forming two compartments, termed settling chamber and sludge chamber. As a result of this division, a shorter period of detention was found to give efficient settling of the solids, also the absence of obnoxious odors is attributed to this change.

Second: The sludge chamber was deepened, consequently contracted. This gave a much better digested sludge with a lower moisture content, and the drawing off of the sludge was facilitated.

DISTINCT TYPES OF TANKS

Since the two-compartment idea was established, the principle has been embodied in the design of many varied installations designed to meet the particular needs of the locality and sewage.

Essentially, however, two distinct types of tanks have resulted, namely the horizontal flow and the radial flow type. The horizontal flow tank was the first type developed and is used more extensively at present. It has the advantage of permitting the use of larger individual units, which is an important factor from the standpoint of distribution if a large number of tanks are required. Contractors also generally favor the straight form work.

Advocates of the radial flow type point to the fact that individual units are desirable from the standpoint of sludge digestion, also that with radial flow the flow line is short and consequently the velocities very low, which permits of greater overloading to take care of storm water. Baltimore recently installed twentyeight individual units of this type.

DESIGN OF SETTLING CHAMBER

The object sought for in the design of the settling chamber is to remove the settleable solids with a minimum volume of chamber. It should be noted that the measure of its efficiency is not the proportion of total solids as removed as determined by chemical analysis, but the proportion of solids removed which are capable of settling.

Tests have shown that for ordinary sewage approximately

90% of the settleable solids are removed during the first hour of retention. In general a retention of the sewage from 2 to 4 hours will give the maximum efficiency in settling.

The above is true if the settling chamber is properly designed. In a recent article Leslie Frank states "that the result of German experience has shown that an average velocity of from 0.1 to 0.13 ft. per sec. is a reasonable maximum limit above which whirling is liable to occur, but in the design of Imhoff tanks it has been the practice not to exceed for dry weather flow 0.03 ft. per sec."

The velocities are controlled by length of flow line and detention period. On the basis of .03 F. P. S. which equals 108 ft. per hour, the length of a tank for one hour detention of sewage should not exceed 108 ft. For two hours detention, the tank could be made 200 ft. long. From this it is seen that velocities govern to a limited extent. Theoretically the shorter the flow line, the slower the velocities. For efficient settling, however, the length of flow line for the horizontal flow type should not be less than about 25 to 30 ft., for the radial flow type 10 to 15 ft., should be the minimum.

Present practice is to design on detention period, keeping in mind the velocities and length of flow line.

The usual American practice is to design detention period for 18 hour runoff of sewage which equals 1.33% average flow. For small installations the detention period ranges from 2 to 4 hours.

For large installations, detention periods of from 1 hour to 3 hours for 1.33% average flow have been used.

In general a 2 hour detention for 1.33% average flow is good practice.

DESIGN OF SLUDGE CHAMBER

The size of the sludge chamber depends upon the quantity of sludge in the sewage, length of time to be held and in a measure upon the activity of the sludge digestion action.

The quantity of sludge per million gallons is much less for dilute American sewages than for the more concentrated European sewages. The ordinary measure of the quantity of sludge is by the population tributary and the type of sewer system.