This agreement with the Ingrim-Rutledge Co. lasted from April, 1917, to Jan., 1920, when the company changed hands and its successors notified us of their desire to give up the contract. Mr. Ingrim, who had been with the Ingrim-Rutledge Co., had transferred his business connections to the H. S. Crocker Co., and he endeavored to get that company to take over the publication of the magazine but without success. In the dilemma we again turned to A. Carlisle & Co., who agreed to resume publication of the magazine for the advertising receipts and the right to publish all handbooks, street laws, forms or other documents issued by the League or its secretaries. The contract with Carlisle, which is still in effect, contains numerous details regarding the size of the publication, number of copies, etc., the original of which I have with me at this time. It also contains a provision that the publisher should contribute $61.25 per month toward defraying the office expense of the League.

In 1916 the executive secretary compiled a handbook for officials of the fifth and sixth class cities. This handbook contained the essential provisions of the constitution, general laws and codes relating to the government of municipalities. From the comments received it apparently filled a long felt want. It was published by A. Carlisle & Co. at a cost of something over $500.00, paid for out of the general treasury of the league, and was distributed to all the officials of the league cities free of charge. In 1920 a second and larger edition of the handbook was issued. However, it was found that the cost of paper and printing had increased over 100 per cent since 1916, and therefore it was impossible to publish and distribute the second edition without extra charge. However, one free copy was distributed to each city of the fifth and sixth class and additional copies sold by the publisher at a price which was not regarded as unreasonable in view of the

limited sale of the book. Since our last meeting in Long Beach another and more elaborate edition of the handbook has been issued by this office, one copy of which was distributed free to all the cities of the fifth and sixth class, the same as was done in the case of the second edition. In the publication of the handbook and also in the preparing and editing of the street improvement laws and forms for their use, which are published by A. Carlisle & Co., the executive secretary has refrained from accepting any financial compensation from the publishers but has construed the work as part of his official duties.

The last edition of the handbook contains a model city manager charter for small cities which is based on the charters recently adopted by San Mateo, Santa Rosa and Monterey. The reason for inserting a form of city manager charter in the handbook is due to the fact that the Governor had turned down our bill for putting such a provision in the general laws. At the present time the general laws provide for the council plan of government and also provide a plan for the commission form of government, but no provision is made for the city manager form of government.

Among the other activities of the League during the past year has been the preparation and issuance of a model zone ordinance for small cities, a model setback ordinance, and a model plumbing ordinance. A committee was appointed by the department of city attorneys on the preparation of a model building code for small cities but a number of legal questions have arisen in connection with the matter which it was thought desirable to submit to the department of city attorneys at this session before going ahead with the work.

It may be said that the past year has been one of activity and progress.

Respectfully submitted,

WM. J. LOCKE, Executive Sec'y.

The early history of the activated sludge method of sewage disposal credits two American investigators, Black and Phelps, with doing the first experimental work on the aeration of sewage in 1910. In 1912 the Massachusetts State Board of Health at Lawrence, Mass., conducted laboratory experiments by aeration of sewage in gallon bottles, and later in a small tank. The chief result of these early experiments was to prove that to prove that aeration of sewage had a marked clarifying effect and to greatly stimulate scientific interest in this method.

Dr. Fowler in Manchester, England, began experiments in 1913 and was the first to show the real value of mixing previously aerated sludge with fresh sewage. Bartow and Mohlman at the University of Illinois in 1915 conducted somewhat similar experiments. The City of Milwaukee started investigations in 1914. under direction of Hatton and Copeland. The subsequent work at Milwaukee has contributed much to the present knowledge of the activated sludge process.

A few years later the City of Chicago started on a 250 million dollar sewage disposal program and established a large experimental laboratory. A million dollar experimental plant was built where different types of equipment and methods of sewage disposal could be studied under actual working conditions.

Two other eastern cities have carried on extensive experimental work in connection with their activated sludge plants, Indianapolis, Indiana, and Houston, Texas.

Several European countries have car

ried on investigations on sewage disposal but in the field of activated sludge studies, England has the lead of other foreign countries. The English investigators have evolved new types of plants, three of which will be described later. They also use types of screens and other equipment not seen in this country. The cities of Manchester, Birmingham and Sheffield have conducted the most extensive investigations.

In Canada, particularly in the Province. of Ontario, there have been several installations of activated sludge plants, mostly in small cities. These plants are reported as operating successfully but there is a scarcity of data on their efficiency and operation costs that makes comparison with other plants difficult.

In California, the City of Pasadena, together with adjoining territory, built an activated sludge plant which was largely modeled after the Milwaukee plant. Lodi has an activated sludge plant of a similar type to the Pasadena plant. The principal difference is in the method of sludge disposal. Another plant has recently been constructed at Pomona. The only other plant in operation in California is at the Folsom Prison.

The U. S. Public Health Service has just completed a plant at the Grand Canyon, which is a very complete small plant. After filtering and chlorinating, the effluent meets drinking water standards.

Taking up now the different types of construction, we find that the usual design for activated sludge plants consists primarily of four units. First, some

method for removing the coarser materials in the sewage; second, aeration tanks; third, a clarifier; and fourth, some arrangement for sludge disposal. Provision is also made for the return of a certain amount of sludge from the clarifier to the raw sewage entering the aeration tank.

The removal of the coarser materials eliminates substances which would require long periods of aeration and gives an influent entering the aeration tank which contains only finely divided substances. This removal is accomplished by the use of bar screens and grit chambers followed either by the use of fine screens or by sedimentation basins. Bar screens consist of iron bars placed from a half inch to an inch apart and remove the large solid particles from the sewage. The grit chambers consist of broad channels in which the rate of flow of the sewage is reduced to about one cubic foot per second. These grit chambers follow the bar screens and remove gravel and other heavy solids.

Several types of self cleaning fine screens are in use which will give a satisfactory influent for the aeration tanks. It has been found that rotating cylindrical screens with short bevelled slots of not less than one-sixteenth of an inch in width give good results. This type of screen is preferred to a mesh screen. There is a tendency toward the choice of screens with a minimum of mechanical devices. Rotating cylindrical screens are used at Pasadena and Lodi. The use of settling basins for removing coarse solids is gaining favor. These have a smaller operating cost but larger initial cost than screening plants and take care of maximum flow conditions more easily. The use of Imhoff tanks as settling basins provides for the digestion of the settleable material and offers an opportunity to return the excess activated sludge there for final digestion and de-watering. Dorr clarifiers are used as settling basins at the

new North Side plant at Chicago. An Imhoff tank is to be used at the new Pomona plant.

The period of detention in the settling basins and in the Imhoff tanks is the same and varies with the nature of the sewage. It is usually about one hour.

In the aeration tanks there are two principal methods of maintaining aerobic conditions in the sewage, one by agitation with air and the other by agitation with mechanical devices.

The aeration tanks for air agitation are rectangular with wide variations in dimensions, being built from 12-20 feet in width, from 70-300 feet in length and from 7-16 feet in depth. Air agitation is usually accomplished by forcing air under pressure through porous plates laid in rows at the bottoms of the tanks. The compressed air is furnished by power blowers capable of about 10 pounds pressure. The pressure necessary is but little more than that sufficient to force the air through the plates. The plates occasionally become partially clogged, requiring greater air pressure and may eventually need to be replaced. The air for the blower is filtered through air filters to remove any substances that would clog the plates. There are two general types of aeration tanks, the "ridge and furrow" and the "Manchester or circulating." The ridge and furrow type has the rows of plates perpendicular to the length of the tanks, each row being separated from the next by a ridge. The ridges prevent the settling of the sludge other than over the plates, where it is caught again in the rising current. The area of the porous plates is from one-fifth to one-tenth of the surface area of the liquid. This type of tank is in use at Pasadena and Lodi.

The Manchester or circulating type of tank has the plates arranged only along one longitudinal side of the tank which arrangement, with the aid of sloping baffles at the top and bottom, produces a

rotating motion of the liquid. This method requires fewer plates, slightly less air, and mixes the sewage and sludge more thoroughly than in the ridge and furrow type. There is also the advantages of cheaper construction, lower power costs, and cheaper upkeep. The circulating type of tank is used at Manchester, England, Indianapolis, Indiana, and will be used in the huge new North Side plant at Chicago, and in the new Pomona, California, plant. The amount of air required depends on several variable factors, but is usually about 1.5 cubic feet of air per gallon of liquid treated. The Lodi plant has an enviable record in being able to use what is probably the lowest amount of air per gallon of sewage of any plant keeping authentic records. During most of the year the air consumption is reported at from .6 to 1.0 cubic feet of air per gallon of sewage.

The detention period for the sewage in the aeration tanks is about six hours for both the "ridge and furrow" and the Manchester tanks.

The sludge which is returned to the incoming sewage, is frequently reaerated or "reconditioned" in separate aeration tanks before it is used. These reconditioning tanks are usually similar in construction to the other aeration tanks. All conveying channels for both fresh and treated sewage are usually built with porous plates in the bottoms to permit a constant aeration and to prevent the settling out of solids which might start septic action.

Experiments with the air diffusion type of plants indicate that oxygen is most rapidly dissolved from the air as the bubbles break on the surface of the tank and dissolves much more slowly as the bubbles rise through the liquid. The main function of the rising bubbles is therefore to agitate and mix the sewage and the added sludge. It seemed possible that some mechanical method of mixing and surface aeration could be found that

would be as successful as air diffusion and easier and cheaper to operate. This has resulted in the manufacturing of different devices for this purpose. At least two of these are worthy of descrip

tion.

A plant at Sheffield, England, circulates the sewage through narrow and comparatively shallow parallel channels arranged like cross baffles. Paddle wheels in each channel give the sewage an undulating motion similar to that seen in the water behind a stern wheel steamer, the wheels also give the liquid the same frothy appearance. The speed of the current is sufficient to prevent settlement and the length of the channels such that the undulating motion is continuous. This type of tank is claimed to be economical and successful but has not been widely adopted. There is no plant of this type in operation in the United. States.

The "Simplex" type of mechanical aerator consists of an upright iron cylinder approximately twelve feet long and two and a half feet in diameter. This is set on legs in the bottom of a hopper bottom tank with about a six inch clearance, which permits a free circulation of the liquid into the cylinder. Over the cylinder but not attached to it, is a revolving inverted cone about five feet across, the upper side of which is fitted with curved blades on edge. An opening is cut in the inverted apex of the cone the size of the cylinder. The cone is about the level of the liquid and throws it out in a spray drawing it up from the bottom of the tank through the cylinder. When the spray strikes the surface of the liquid, additional agitation is secured. The cone is driven from a power unit by a shaft which can be extended to run a series of aeration units. This type of plant has given satisfactory results experimentally in Chicago and Decatur, Illinois, and under operating conditions. in a small city in Texas, and a unit is to

be installed in California this year. The Simplex system has been installed in several British disposal plants and is being installed in a few American cities. The clarifier following digestion makes it possible to decant the clear liquid and collect the sludge. Plain basins with steep hopper bottoms are used, but flatter bottoms with a mechanical device for scraping or squeegeeing the sludge to the center such as used in the Dorr clarifier, are more commonly found. The period of detention is from one to four hours.

The collected sludge contains 98-99.5 per cent of water and is easily handled by air lifts, pumps, or some type of ejector. A portion is returned to the sewage influent and the remainder to the sludge disposal system. The amount of sludge found necessary to return to the aeration tanks for best results varies from 12-25 per cent of sludge by volume. This amount can only be determined by experiment and is usually about 20 per cent.

The satisfactory disposal of the sludge produced by the activated sludge process offers, perhaps, the biggest problem found in such plants. The sludge from the clarifiers is a brown flocculent mass of organic matter of high bacterial content. It is slow to dry on sand beds, does not filter easily, and is difficult to handle in artificial driers. Efforts to get it into a form that can be readily sold as fertilizer, have not met with much success without excessive costs. A large amount of work has been done at Milwaukee, Chicago, Houston, and elsewhere in an effort to make this an economic possibility. Various mechanical filters have been tried in an effort to get sludge in a condition in which it can readily be dried. The treatment of the sludge with various chemicals such as sulphur dioxide, sulphuric acid, alum and ferrous sulphate has been tried in hopes of increasing its rate of filtration and has met with varying success. It is found that sludges from

different sewages react differently under the same treatment. In the arid and semi-arid regions of the Pacific Coast it is likely that sand drying beds will be successful. This method has been used with good results at Lodi. The sludge must be placed in much shallower depths than would be used with Imhoff sludge. The method of sludge disposal in use at Indianapolis and Houston is to pump the sludge to large lagoons. The lagoons are usually under-drained. The sludge decomposes and partially dries with some odor and is flushed into a nearby stream during high water. Another method is to use the sludge for direct irrigation on agricultural land. Perhaps the most promising method is to digest the sludge, after mixing it with the coarse material taken out by preliminary treatment in Imhoff tanks. This not only offers a method for disposal of the coarse material but dewaters the sludge so that it can be more readily dried on sand beds. This causes the loss of some of the fertilizing value and greatly increases the initial cost of the plant. Researches have indicated that two or three times as much sludge capacity is required as is necessary in an Imhoff installation without activation.

The use of activated sludge plants as a preliminary treatment for sprinkling filters offers a promising future. The effect is to relieve the load of the filters, thereby increasing the filter capacity. Surprisingly large increases are possible in the dosage applied to the filters by the use of only limited detention periods in the aeration tanks. This makes it possible to supplement sprinkling filter systems with standby activated sludge units for use during peak periods. These periods of high flow are common in California as in resort cities having a large seasonal fluctuation in population, and in cities

with seasonal industries such as canneries and packing houses.

(Continued on page 375.)