The moisture content of sludge to be used in the manufacture of fertilizer must be reduced to 10 per cent or less. None of the methods of drying described so far can be relied upon for such a product and it becomes necessary to use direct or indirect heat dryers. There are various types of dryers on the market. The details of a Buckeye dryer are shown in Fig. 181. In the operation of this machine moist sludge is fed in at the left end at the point marked “feed.” The hot gases pass from the fire box up and around the cylinder which revolves at about eight r.p.m. The gases are drawn into the inner cylinder through the openings marked A which revolve with the two cylinders. The gases escape from the inner cylinder through the openings to the right and flow towards the left in the outer cylinder. They come in contact with the sludge at this point. The gases then pass off through the fan at the left. The sludge is lifted by the small longitudinal baffles fastened to the outer cylinder, as the drying cylinders revolve. The right end of the cylinder is placed lower than the left so that the drying sludge is lifted and dropped through the cylinder at the same time that it moves slowly toward the right hand end of the cylinder. These dryers require about one pound of fuel for 10 pounds of water evaporated. The odors from the dryer can be suppressed by passing the gases through a dust chamber and washer.

A summary of the results from methods of sludge drying at Milwaukee[[206]] follows:

Excess sludge produced, 12,100 gallons, having 97.5 per cent moisture, per million gallons of sewage treated.

Sludge cake produced (by presses), 10,083 pounds having 80.3 per cent moisture, per million gallons of sewage treated.

Dried sludge (from heat driers) produced, 2,521 pounds having 10 per cent moisture, per million gallons of sewage treated.

Press will produce 3 pounds of cake per square foot of filter cloth in four and a half hours, or five operations per twenty-four hours.

Dryers will reduce 6,700 pounds of sludge cake at 80 per cent moisture to 10 per cent moisture, and will evaporate 8 pounds of water per pound of combustible.

Thickening devices known as Dorr thickeners, patented and manufactured by the Dorr Co. and originally intended for metallurgical purposes, have been adapted to the thickening of sewage sludge. These thickeners are circular sedimentation tanks, from 8 to 12 feet deep, more or less, and are made in any diameter up to 200 feet or more. An arm, pivoted in the center and extending to the circumference, is provided at the bottom with a number of baffles or squeegees set at an angle with the arm. The arm revolves at from one to fifteen revolutions per hour, and the squeegees, in contact with the bottom of the tank, scrape the deposited sludge towards a central sump, from which it is removed by a pump or by gravity, without interrupting the operation of the thickener. The sludge so thickened may be reduced to 95 or 96 per cent moisture. These devices are ordinarily used only in the activated sludge process in which they have been a pronounced success.

CHAPTER XXI
AUTOMATIC DOSING DEVICES

283. Types.—Automatic dosing devices are used to apply sewage to contact beds, trickling filters, and intermittent sand filters. These devices can be separated into two classes; those with moving parts and those without moving parts. The latter are better known as air-locked dosing devices. Simple devices without moving parts are less liable to disorders and are nearer “fool-proof” than any device depending on moving parts for its operation.