The Wisconsin State Board of Health is conducting an educational campaign which has brought about a better co-operation between industries contributing wastes of this character and city officials in charge of the operation of Disposal Plants.

With reference to the supernatant liquid, we return this liquid directly through the digester through an overflow pipe which is properly baffled at the digester, thus preventing the solids coming over to the clarifier. The fact that the sludge pump operates only thirty minutes a day minimizes the effect of disagreeable odors, for we did find that the supernatant liquid gives off odors at the point where this liquid mixes with the fresh sewage, so that on the revised design we dropped this entrance pipe below the level of liquid in the clarifier.

There is still opportunity for study along this line, and I expect to have additional data showing the effect of the supernatant liquid on settlement in the clarifier, after we have operated the new plant at Antigo for a year.

MR. CRAMER: Referring to what Mr. Donohue has just said about this subject, the operation of Imhoff tanks in Wisconsin, as in all northern parts of the United States, has presented great difficulties. I have heard it stated that sludge drying beds cannot be operated successfully in winter. It appears that that is not the only cause of the difficulty. The real cause is, as Mr. Donohue says, that, during a great part of the year, the temperature of the sludge is too low to support digestion. If I recall correctly, the scientists in the laboratory at Essen have shown that the action of the bacterial organisms causing digestion commenced at about 40 degrees F. and reached its maximum intensity at 80 degrees F., and within that range the rate of sludge digestion, as measured in this case by the generation of gas, ranges from nothing to the maximum. Undigested sludge should not be discharged. If during a period of some months sludge is not digested, such sludge must be stored and that is the reason Imhoff tanks and other digestion apparatus must be made unduly large and costly, and present great difficulties in operation during a large part of the year. This matter is not only serious from standpoint of operation difficulties, but also from the standpoint of cost. As Mr. Donohue has pointed out very clearly, this is especially serious when you have to build tanks in costly underground construction. If the thoughts which are clearly the basis of Mr. Donohue's work are fully carried out we will finally arrive at a plant which practices continuous sludge digestion and continuous sludge disposal irrespective of outside temperature. It would mean that the temperature of sludge in the digestion chamber could be kept practically uniform at the optimum value. At regular periods throughout the whole year sludge can then be discharged on drying beds. It means that if you could discharge sludge every three or four days you could make provisions to dry it and discharge it quickly in a closed room sheltered from weather and kept at suitable temperature. This would not be possible if it had to be done under present conditions. In plants as

usually designed that cost of the necessary enclosure would be prohibitive. The cost of a building or shelter only large enough to dry the accumulated sludge resulting from three or four days operation would be low enough so that the community could afford it.

There is certainly opened up a field for important development and I am sure that Mr. Donohue's work is only the beginning of this important development.

MR. MILLER: What is the population of the city of Hartford? MR. DONOHUE: About 5,000.

RESULTS ON SEPARATE SLUDGE DIGESTION
EXPERIMENTS MADE DURING THE
LAST YEAR'

By William Rudolfs, New Jersey Agricultural Experiment Station,
Department of Sewage Disposal, New Brunswick, N. J.
INTRODUCTION

Considerable experimentation, both in the laboratory and at the plant, promised to show a way out of difficulties existing at Plainfield, and the superintendent of the Plainfield sewage disposal plant, Mr. John R. Downes, built a square, earthen separate sludge digestion tank with a capacity of 25,000 cubic feet, covered with a floating wooden cover. The cover was constructed in such a way that it fitted rather closely to the walls, the object being to keep the scum. formed submerged to insure better anaerobic digestion and to prevent gases and odors from escaping. In the middle of the cover an opening was left over which a square sheet iron box was placed for the collection of gases. This tank was preliminary in character and after sufficient data on the digestion of the sludge was obtained, the superintendent built two 16-sided concrete tanks, supplied with floating covers and facilities for adjusting the fresh solids with lime and heating the sludge.

PROCEDURE

Previous to August 20, 1925, an amount of 5,500 cubic feet of ripe sludge was pumped into the tank from the Imhoff tanks for seeding. Fresh solids were added every 2 or 3 days up to October 16 and our experiments were begun at the latter date. The fresh solids were obtained from an Imhoff tank which was used as a preliminary settling tank. These fresh solids were adjusted with dry hydrated lime to a pH value of 7.3.

The amount of gas, composition of the gas, pH values, total acidity, total alkalinity, NH-Nitrogen, solids and ash were determined. Bacteriological and zoological counts were made and an automatic record was kept of the temperature in the tank and of the air.

RESULTS

Our laboratory determinations were begun after the tank had been in operation for 60 days. During the two months preceding the laboratory determinations the tank had a chance to come to a certain. equilibrium and our determinations started at a time when possible initial difficulties in operation were adjusted, although the time of the year was least fitted to obtain maximum results. All our determinations were made on the liquid between sludge and scum and on

'N. J. Agr. Expt. Sta.

the sludge itself. We had found previously that in Imhoff tanks the liquid gave a true indication of the activities in the tank and the same held true for the liquid in the separate sludge digestion tank. If in a separate sludge tank provisions are made to draw sludge not too near to the bottom, I would favor determinations on the sludge, but in case the sludge outlet is at the bottom or reaches only partly

Fig. 1-Methods of construction of floating cover used in the new separate digestion tanks at Plainfield, N.J. The method of support and placement of a heating coil is seen under the cover.

Fig. 2-Floating cover nearing completion. Three of the four sludge inlets are shown. The open center portion is covered by an iron gas collector.

down into the tank, sampling from the liquid layer would give a better index of activities. When new separate sludge digestion tanks are built it might be well to have outlets at different depths. This would allow for the withdrawal of liquid (sludge concentration) and sampling at different levels. When it is desired to determine the trend of activities in the tank, samples of the liquid should be taken about 1-1% feet above the sludge level.

The laboratory determinations begun October 16, lasted for a period of about 180 days (6 months), samples being taken three times a week. The total addition of fresh solids was 96,300 cubic feet (95% moisture). The amount of material present in the tank when the experiment was started was about 15,000 cubic feet (92.3% moisture), comparable to 23,000 cubic feet of the 5% solids sludge. The rate of addition of fresh solids on a daily basis was 2.3% (dry solids).

At the beginning of the experiment the total acidity was generally higher than during the latter part, the pH values of the tank material reached only a few times 7.3 (usually 7.1-7.2), but during the second half the pH value was practically always 7.3 or slightly above. The trend of the alkalinity curves could naturally directly be related to the pH values, but the curve for the carbondioxide content of the gas was in general inversely to the pH values. With a low pH value carbondioxide was high and with a higher pH value this component of the gas was low.

The curves constructed for the ammonia nitrogen content was in general directly related to the additions of fresh solids. However, the increases in ammonia content was usually slightly behind the additions of fresh solids and with the accumulation of fresh solids ammonia increased. This was to be expected, since decomposition of nitrogenous materials follows the breaking down of soluble and easily destroyed carbohydrate materials.

The ash content of sludge in the tank was at the beginning 37.7%. The ash content of the fresh solids varied between 22% and 24%, while the average ash content of the material in the tank during the whole period was 37.4%. There was, therefore, an average percentage ash increase of the fresh solids in the course of digestion of at least 50%.

Assuming that the sludge in the tank at the end of the gas measurements was in the same stage of decomposition as at the beginning of the experiment (all figures bear out this assumption), and taking the fresh solids added as a basis for comparison, it was found that 3.74 cubic feet was was produced per cubic foot fresh solids (95% moisture). Calculating the gas production on a basis of dry fresh solids added, a total of 72.5 cubic feet gas was obtained per cubic foot dry solids. Only part of the organic matter was destroyed since