Mr. Hatton. We can; but the trouble is that the sludge from the Imhoff tank, as we have found it in Milwaukee, only contains about from 1 to 1½ per cent of ammoniacal nitrogen at most, and that does not pay for its recovery. I want to say another thing about this which is of interest. There is no odor about the operation of the plant at all. You can stand over one of the tanks as it is being aerated and have no odor come to you, or no odor throughout the entire plant, unless you let the sludge stay undried.
Mr. Tawney. To what do you attribute the lower percentage in the sludge taken from the Imhoff tanks?
Mr. Hatton. The fermentation process removes the ammonias from the sludge. Well, practically all the reduction of the sludge in the Imhoff tank proposition is the fermentation process, and of course that removes a large portion of fertilizer values in the sludge.
Mr. Magrath. Are there any weak features in this process that you look forward to correcting?
Mr. Hatton. There are some problems which we are investigating, with a view of getting a more economical use of the air, and thus a lower cost of operation, and that problem concerns the diffusion of the air in the tanks. We are trying out three methods of diffusion. We are not prepared to say yet which is the most economical. I am giving you the figures based upon that which we have used, and our whole efforts now are being directed to reducing the amount of air used. That is the principal point which we are investigating at the present time.
Mr. Tawney. How does the operating cost of your system compare with the operating cost of the Imhoff tanks?
Mr. Powell. The operating cost of the Imhoff tank is considerably less than the operating cost of our tanks. They have no air to pay for, and very little plant attendance to pay for.
Mr. Magrath. As I understand you, the cost to the municipality under your system is less than the cost under the other system?
Mr. Hatton. The cost of the Imhoff tank, you mean?
Mr. Magrath. Yes.