uniformly stable for a period in excess of ten days, and in appearance is substantially equal to the effluent from intermittent sand filters.

At present the sewage is very dilute, averaging about 1,500,000 gallons per day with a contributing population of about 2,000. The liquor between the sludge and scum in the digestion chamber is periodically drawn to a low level reservoir into which the filter wash water also discharges, both being pumped back to pass through the settling tanks as sewage. Sludge can only be withdrawn through the pipe at the bottom of the digestion tank hopper. No sludge has been withdrawn for over two years and at the present time approximately 10,000 c. f. of sludge and scum is in the digestion chamber, i.e., about onethird of the sludge storage capacity.

HARTFORD, WIS.

At Hartford, Wis., a separate sludge digestion tank plant was built in 1924. The plant has now been in operation a little over a year. It consists of two tanks, one a settling tank and the other a sludge digestion tank, each equipped with Dorr clarifier mechanism.

The plant does not have the benefit of laboratory control, but some infrequent observations have been made by Mr. Frank Bachman of the Dorr Company. These observations have been checked visually by a visit to the plant.

The settling tank having a storage period of approximately 21⁄2 hours is provided with a Dorr clarifier mechanism which operates continuously at slow speed. Sludge is pumped twice a day for about one hour at each pumping. There is no scum on the settling tank and no bubbles are visible. Visually the clarification seems to be entirely satisfactory.

The digestion tank has a storage capacity equivalent to 3.07 c. f. per capita. Sludge beds equivalent to .58 sq. ft. per capita are available. The digestion tank operates free from odor and produces an excellent sludge. The revolving mechanism in the digestor is operated only about 2 hours each day and at a speed of 3 revolutions per hour. Both the scum and sludge are agitated.

The sludge dries readily, is porous and spongy, and has all the characteristics of a properly well-digested sludge. No ordors are apparent.

CONCLUSIONS

From observations of the construction and operation of these plants and a comparison with other plants of other types it is believed some general conclusions may be drawn, as follows:

1st. Separate sludge digestion tanks have demonstrated their ability to accomplish efficient clarification and sludge reduction and to produce a good sludge which can be readily dried.

2nd. Adequate sludge digestion capacity is needed with any type of tank. The separate sludge digestion type of tank, by having its entire sludge capacity available through the withdrawal of the liquor lying between the sludge and scum layers, can probably be built with somewhat less total sludge digestion capacity than tanks of the two-story type where the withdrawal of the liquor is not practicable.

3rd. In difficult excavation, such as quicksand or rock, tanks of the separate sludge digestion type can be built at less cost than those of the two-story type. The construction of shallow settling tanks and the pumping of the sludge only to a deep sludge digestion tank located with its flow level at a higher elevation may result either in the saving of great construction costs, or obviate the necessity of pumping the entire sewage flow as would be necessary with a two-story tank.

4th. The separate sludge digestion type of tank is flexible in that the first construction does not limit the relation between settling and sludge storage capacity. Either may be enlarged without the expense incident to enlarging both. It would seem further that separate sludge digestion probably offers an economical means of caring for part of the sludge in some plants of the two-story type in which the sludge capacity has been found deficient, but the flow capacity adequate.

5th. It is possible that the digestion of sludge in separate sludge digestion plants may be speeded up by applying heat artifically, and the sludge storage capacity correspondingly reduced. This procedure has been suggested for experimental observation at the Lincoln plant.

6th. The efficiency of separate sludge digestion tank plants, as of all others, depends as much upon the operator as the designer. Plants of this type may require even more intelligent operation than plants of the two-story type.

7th. With sludge digestion tanks of the separate type it is possible to mix the sludge by pumping as at Madison or by mechanical agitators as at Hartford, and thus facilitate uniform and more complete digestion.

It is the writer's opinion that the difficulties experienced with separate sludge digestion have largely arisen from the intermittency and incompleteness of sludge removal from the settling to the digestion tanks. It is believed that the installation of mechanical means to accomplish this more readily and more positively will become more prevalent in the future.

It has been the writer's effort to point out some of the operating difficulties as well as advantages of separate sludge digestion. So far as he knows no means of separating sewage solids from the liquid and reducing them to an inoffensive condition has been entirely free from operating difficulties.

EXPERIENCE WITH SLUDGE DIGESTION AT THE BALTIMORE SEWAGE WORKS

By C. E. Keefer, Engineer of Sewage Disposal, Bureau of Sewers, Baltimore, Md.

The digestion of sewage sludge in tanks is not a new process. It has been in use at Birmingham, England; Toronto, Canada; Birmingham, Alabama, and at Baltimore from 1912 to the present time. There are also a number of smaller sewage works in this country where this method of sludge treatment is in vogue. Furthermore, several sewage works in Germany have been provided with these tanks.

Within the past few years there has been a renewed interest in this country in this method of sludge treatment. Probably one reason has been the difficulties which some operators of sewage works have had with Imhoff tanks. One advantage of separate digestion tanks over Imhoff tanks is that the sludge is entirely separated from the sewage, and, therefore, the excessive foaming and other difficulties encountered in Imhoff tanks are not present. Perhaps one other reason why more sludge digestion tanks are being installed is that mechanical means have been devised for removing daily the sludge from sedimentation tanks. This procedure has naturally meant the installing of digestion units. Moreover, the use of separate digestion tanks is one of the older processes of sewage treatment, and the number of its advocates for its use are gradually increasing.

ODORS FROM SLUDGE TANKS

One of the objections to the use of sludge digestion tanks has been the fear that considerable odors would come from them. The claim is made that when large areas of this material are exposed to the atmosphere, considerable odor arises, which travels for some distance. On the face of it such a condition would seem to be true. The observations made over a number of years by those who have been in close touch with the Baltimore Sewage Works do not bear out the above conclusion. The odors coming from the above plant are extremely variable.

In operating the sludge tanks the fresh and undigested material is introduced at a point below the surface of the sludge. The reason for this procedure is to keep the undigested sludge from coming in contact with the air, for if a relatively small stream of this material is allowed to pass through the atmosphere, there will be some odor.

If comparisons are made between the sludge exposed to the atmosphere in an Imhoff tank sewage works and in a separate sludge digestion tank installation for the same flow of sewage, it would naturally be found that the exposed area in the latter case is considerably in excess of the former. The surface material in each case, however, is frequently very much alike in character. After being exposed to the atmosphere for several days it usually forms into a hard crust which has a low moisture content and little odor. The upper six or eight inches of this crust is usually a porous and dry fibrous material which is well digested. As long as the crust remains intact it acts as a blanket and helps to keep the odors from escaping to any great extent.

SLUDGE IN IMHOFF TANKS AND IN SEPARATE DIGESTION TANKS

An opportunity has been afforded to observe and study the sludge from Imhoff tanks and from sludge digestion tanks at the Baltimore Works where both kinds of tanks have been used. So far as can be determined there is no difference in the digested sludge from either of these types of tanks. In each case the digested sludge has the same tarry odor, cracks well, and dries readily on sludge beds. The sludge from the digestion tanks, however, has a higher moisture content than from Imhoff tanks. The average moisture of the sludge put on the drying beds from 1920 to 1924 has been 91.8 per cent. With such a sludge, more drying bed area is needed than if deep Imhoff tanks were used.

DEGREE OF DIGESTION AT THE BALTIMORE WORKS

During the winter months the digestion process slows up considerably, so it is necessary to provide for sufficient sludge capacity to store all of the sludge removed from the sedimentation. tanks until it can be dried in the spring. As a general rule little sludge can be drawn from the digestion tanks from the 1st of November until the 1st of April. In the summer and early au