Sewerage and Sewage Treatment

by Harold E. Babbitt

The first full sized plant for the treatment of sewage by this method was erected in Milwaukee in December, 1915. This plant had a capacity of 1,600,000 gallons per day. It was used for experimental purposes and is not now in use. The Champaign, Illinois, septic tank, among the first of its kind in the country, was converted into an activated sludge tank on April 13, 1916. The changes, developments, and the results obtained from these and other plants have been reported in the technical press from time to time.

269. Aëration Tank.—The sewage on leaving the screen and grit chamber enters the aëration tank, which is usually operated on the continuous-flow principle, although in the early days of experimentation the fill and draw method was practiced. This tank should be rectangular with a depth of about 15 feet and a width of channel not to exceed 6 to 8 feet. Such proportions allow better air and current distribution than larger tanks. The bottom should be level to insure an even distribution of air. The velocity of flow of sewage through the tank is usually in the neighborhood of 5 feet per minute, dependent on the length of the tank and the period of retention. The period of retention is in turn dependent on the desired quality of the effluent. The process is flexible and the quality of the effluent can be changed by changing the period of retention or by changing the rate of application of the air, or both. The period of retention in the aëration tank is usually about 4 hours.

The bottom of the aëration tank is usually made of concrete arranged in ridges and valleys, or small shallow hoppers, at the bottom of which the air-diffusing devices are located, as shown in Fig. 177. The inlet and outlet devices are similar to those in a plain sedimentation tank.

270. Sedimentation Tank.—It is evident that as no sedimentation is permitted in the aëration tank, the settleable particles will be discharged in the effluent unless some provision is made for their detention. The effluent from the aëration tank is therefore run through a plain sedimentation tank, usually with a hopper bottom, which has been arranged to permit frequent and easy cleaning. An air lift or a centrifugal sludge pump is satisfactory for this purpose. Another type of sedimentation tank which has been used has a smooth bottom with a slight slope towards the center. A revolving scraper collects the sludge continuously, scraping it towards the center of the tank. Although this arrangement gives better results than the hopper-bottom tank, its expense has usually prevented its installation.[^[182]]

The period of sedimentation in different plants varies from 30 minutes to one hour, although the longer periods usually give the better results. Approximately 65 per cent of the sludge will settle in the first 10 minutes, 80 per cent in the first 30 minutes, and about 5 per cent more in the next half hour.

The effluent from the sedimentation tank is ready for final disposal or if desired, for further treatment by some other method. The sludge, or a portion of it, is pumped back into the influent of the aëration tank, provided the sludge is in a satisfactory state of nitrification. Otherwise it should be pumped to the reaëration tanks. The remainder of the sludge which is not to be used in the process is ready for drying and final disposal.

271. Reaëration Tank.—The purpose of the reaëration or sludge aëration tank is to reactivate the sludge which has gone through the aëration tank. During the process of the aëration of the sewage in the aëration tank the activated sludge may lose some of its qualities because of the deficiency of oxygen to maintain aërobic conditions. By blowing air through the sludge in the reaëration tank these properties are returned and the sludge made available to be pumped back into the aëration tank. The reactivation of the sludge obviates the necessity for supplying sufficient air to the entire mass of the sewage to maintain aërobic conditions, and results in an economy in the use of air. The use of mechanical agitators has also been attempted both in the reaëration and the aëration tanks with the expectation of saving in the use of air, but with indifferent success.

It is difficult to say, without experimentation, what the size of the reaëration tank should be, as the necessary amount or reactivation is uncertain. In the experimental plant at Milwaukee, there were eight units of aëration tanks, one sedimentation tank, and two reaëration tanks, all of the same capacity and general design. This represents a ration of about one reaëration tank to four aëration tanks.

272. Air Distribution.—Air is applied to the sewage at the bottom of the aëration tank at a pressure in the neighborhood of 5.5 to 6.0 pounds per square inch, dependent on the depth of the sewage, the loss of head through the distributing pipes, and the rate of application. In different experimental plants the pressure has varied from 3 to 30 pounds per square inch. Such pressures are on the line which divides the use of direct blowers for low pressures from turbo and reciprocating pressure machines for pressures above 10 pounds per square inch. Positive-pressure blowers or direct blowers operate on the principle of a centrifugal pump and because of the lighter specific gravity of air they rotate at a very high speed. The Nash Hytor Turbo Blower consists of a rotor with a large number of long teeth slightly bent in the direction of rotation. The rotor, which has a circular circumference, revolves in an elliptical casing. At the commencement of operation the rotor and casing are partially filled with water. The revolution of the rotor throws the water to the outside of the elliptical casing thus forming a partial vacuum between any two teeth as the water is thrown from near the center of the short diameter of the casing to the extremity of the long diameter of the casing. Air is allowed to enter through the inlet port to relieve the vacuum. As the teeth pass from the long diameter to the short diameter of the ellipse, the water again approaches the center of the rotor compressing the air trapped between the teeth and forcing it out under pressure into the exhaust pipe. Among the advantages of this compressor are the washing of the air, cooling, and ease in operation. Reciprocating air compressors operate similarly to direct-acting steam pumps or crank-and-fly-wheel pumps but at much higher speeds, and they require more floor space than either of the other types. Fig. 178 shows the field of serviceability of various types of air compression machinery.