Fig. 178.—Economic Range of Air Compressors.
From Eng. News, Vol. 74, p. 906.
For pressures up to about 10 pounds per square inch the positive blower seems most desirable. It has a low first cost and a relatively high efficiency of about 75 to 80 per cent of the power input. No oil or dirt is added to the air to clog the distributing plates, as in the reciprocating machine. A disadvantage is the difficulty of varying the pressure or quantity of the output of the machine. As the required pressure and volume of air increases the turbo blower becomes more and more desirable within the limits of pressure which are ordinarily used in this process. For small installations the best form of power is probably the electric drive, but when the capacity becomes such as to make turbo blowers advisable they should be driven by directly connected steam turbines.
The quantity of air required varies between 0.5 to 6.0 cubic feet per gallon of sewage, with from 3 to 6 hours of aëration. The quantity of air depends on the degree of treatment required, the strength of the sewage, the depth of the tank, and the period of aëration. The deeper the tank the less the amount of air needed because of the greater travel of the bubble in passing through the sewage, but the higher the pressure at which the air must be delivered. Shallow tanks usually require a longer period of retention. The depth of the tank then has very little to do with economy in the use of air. Hatton states:[[183]]
The purification of sewage obtained varies decidedly with the volume of air applied. Small volumes applied for 5 or 6 hours do as well as larger volumes applied for 3 or 4 hours, but the time of aëration required to obtain a like effluent does not vary directly with the volume of air applied per unit of time. For instance air applied at a rate of 2 cubic feet per minute purifies the sewage in less time than one cubic foot of air per minute, but will not accomplish an equal degree of purification in half the time.
It has been found that although a low temperature has a deleterious effect on the process, by the use of an additional quantity of air good results can be maintained. The effect of changing the quantity of air and the period of aëration are shown in Table 94 taken from Hatton.
The velocity of the air in the pipes should be about 1,000 feet per minute. There should be relatively few sharp turns in the line, and the distributing mains should be arranged without dead ends. It is desirable to use as little piping as possible and at the same time to make the travel of the sewage long in order to maintain a non-settling velocity and intimate contact with the air. The piping should be accessible and well provided with valves. It should be non-corrodible, particularly on the inside, as flakes of rust will quickly clog the air diffusers. It should drain to one point in order that it can be emptied when flooded, as occasionally happens.
| TABLE 94 | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Effect of Various Rates and Periods of Application of Air on the Results Obtained from the Treatment of Sewage by the Activated Sludge Process | |||||||||||
| (Milwaukee Results) | |||||||||||
| Time of Aëration, Hours | Cubic Feet Free Air Per Minute | Cubic Feet Air per Gallon of Sewage | Appearance of Settled Liquid | Per Cent Removal Bacteria | Parts per Million | Stability, Hours | |||||
| Nitrogen as | Dissolved Oxygen | Suspended Matter | |||||||||
| Free Ammonia | Nitrites | Nitrates | Organic | ||||||||
| 0 | 0 | 0.0 | Turbid | 0 | 22 | 0.08 | 0.08 | 0.00 | 000 | ||
| 1 | 160 | 0.67 | Clear | 52 | 17 | 0.00 | 0.04 | 0.30 | 2 | ||
| 2 | 160 | 1.32 | Clear | 81 | 15 | 0.95 | 0.70 | 1.90 | 33 | ||
| 3 | 160 | 1.98 | Clear | 92 | 11 | 1.75 | 2.80 | 4.30 | 120 | ||
| 4 | 160 | 2.64 | Clear | 94 | 7 | 2.20 | 5.60 | 5.90 | 120 | ||
| 5 | 160 | 3.31 | Clear | 98 | 5 | 2.50 | 8.20 | 6.70 | 120 | ||
| 2.5 | 90 | 1.07 | 92 | 11 | 0.05 | 2.00 | 69 | ||||
| 3 | 90 | 1.28 | 96 | 9.9 | 0.12 | 2.9 | 95 | ||||
| 4 | 90 | 1.71 | 98 | 1.8 | 0.14 | 5.2 | 120 | ||||
| 4 | 80 | 1.82 | 97.7 | 1.95 | 0.08 | 8.5 | 120 | ||||
| 4 | 70 | 1.60 | 99.6 | 5.79 | 0.14 | 9.0 | 120 | ||||
| 4 | 46 | 1.67 | 88.3 | 7.90 | 0.02 | 2.0 | 61 | ||||
| 4 | 105 | 1.75 | 92.7 | 4.86 | 0.36 | 4.9 | 120 | ||||
| 3 | 140 | 1.75 | 91.2 | 9.39 | 0.60 | 3.0 | 120 | ||||
| 2.5 | 168 | 1.74 | 96.7 | 11.2 | 0.36 | 1.1 | 84 | ||||
| 1.80 | 98.1 | 8.5 | 4 | 11 | 120 | ||||||
| 1.53 | 99 | 5.79 | 9.0 | 8 | 9 | 120 | |||||
| 1.12 | 91 | 10.1 | 2.3 | 14 | 42 | 73 | |||||
| TABLE 95 | ||||||
|---|---|---|---|---|---|---|
| Comparative Results from the Aëration of Sewage in the Presence of Activated Sludge with the Use of Different Distributing Media | ||||||
| (T. C. Hatton, Eng. Record, Vol. 73, p. 255) | ||||||
| Diffusers | Months in 1915 | Pounds per Square Inch | Air, Cubic Feet per Gallon | Per Cent Bacteria Removed | Nitrates, Parts per Million | Stability Effluent in Hours |
| Filtros plate | June 1 to Aug. 15 | 4.3 | 2.06 | 91 | 3.4 | 78 |
| Air jet | June 1 to Aug. 15 | 3.5 | 1.94 | 91 | 2.2 | 52 |
| Filtros plate | Nov. 18 to Dec. 7 | 4.6 | 1.71 | 90 | 0.3 | 113 |
| Monel metal | Nov. 18 to Dec. 7 | 3.0 | 1.71 | 80 | 0.2 | 63 |
It is desirable to diffuse the air in small bubbles as by this means the greatest efficiency seems to be obtained from the amount of air added. A diameter 1
16 to ⅛ of an inch is approximately the maximum limit for the size of an effective bubble. Monel metal cloth, porous wood blocks, open jets, paddles, and other forms of diffusers have been tried, but none have given the satisfaction of the filtros plate. The relative value of different types of diffusers is shown in Table 95 taken from Hatton.[[184]] The Filtros plates are a proprietary article manufactured by the General Filtration Company of Rochester, N. Y. They are made of a quartz sand firmly cemented together and can be obtained with practically any degree of porosity, size of pore opening or dimension of plate, but they are made in a standard size 12 inches square by 1½ inches thick. The frictional loss through the plate is not very great for the amount of air ordinarily used. The plates are classified in accordance with the volume of air which will pass through them, when dry, per minute when under a pressure of 2 inches of water. These classes run from ½ to 12 cubic feet of air per minute. The type usually specified passes about 2 cubic feet of air per minute. The loss of head through these plates as tested at Milwaukee showed an initial loss of ¾ of a pound and an additional loss of about ¼ of a pound for every cubic foot of air per minute per square foot of surface. It is necessary to screen and wash the air before blowing it through the filtros plate as ordinary air is so filled with dirt as to clog the pores of the diffuser quite rapidly.
The area of filtros plates required in the bottom of the tank is usually expressed in terms of the free surface of the tank or as a ratio thereto. In the Urbana tests the best ratio was found to be less than 1 : 3 and more than 1 : 9. In Milwaukee[[185]] the ratio adopted is in the neighborhood of 1 : 4 or 1 : 5. At Fort Worth the ratio will be about 1 : 7 and at Chicago it will be 1 : 8. The exact ratio should be determined by experiment and will depend on the construction of the tank and the character of the raw sewage and the desired effluent. It is essential that the filtros plates be placed level and at the same elevation as otherwise the distribution of air will be uneven.