| TABLE 79 | |||||
|---|---|---|---|---|---|
| Critical Velocities for the Transportation of Debris | |||||
| Sedimentation will not Occur at Higher Velocities | |||||
| Diameter of Particle in Millimeters | Critical Velocity, Feet per Second. | Size of Screen or Number of Meshes per Inch | |||
| Specific Gravity | |||||
| 1.5 | 2.0 | 3.0 | 5.0 | ||
| 0.010 | 0.13 | 0.20 | 0.22 | 0.28 | |
| 0.050 | 0.23 | 0.34 | 0.39 | 0.50 | More than 200 |
| 0.100 | 0.30 | 0.42 | 0.50 | 0.65 | More than 150 |
| 0.500 | 0.55 | 0.73 | 0.91 | 1.15 | More than 28 |
| 1.0 | 0.71 | 0.92 | 1.18 | 1.50 | More than 14 |
| 1.25 | 0.77 | 1.00 | 1.30 | 1.60 | |
| 2.0 | 0.92 | 1.20 | 1.50 | 1.90 | More than 10 |
| 5.0 | 1.30 | 1.70 | 2.20 | 2.60 | More than 4 |
| 10 | 1.70 | 2.20 | 2.8 | 3.4 | |
| Diameter in Millimeters for a Velocity of 1 Foot per Second | |||||
| 2.5 | 1.25 | 0.65 | 0.32 | ||
234. Limiting Velocities.—Sand, clay, bits of metal and other particles of mineral matter will commence to deposit in appreciable quantities when the velocity of flow falls below 3 feet per second. The amount deposited will increase as the velocity decreases. In Table 79 are given the approximate horizontal velocities at which certain size particles of mineral matter will deposit. At a velocity of about one foot per second organic matter will commence to deposit. It will be noticed by interpolation in Table 79,[[145]] that particles with the same specific gravity as sand (2.6), larger than one mm. in diameter will deposit at a velocity of about one foot per second or less, and that smaller and lighter particles will not deposit at velocity of one foot per second or greater. It will also be noticed that a velocity of one foot per minute is sufficiently slow to permit the deposit of the smallest and lightest particles. For this reason velocities of 1 or 2 or even 3 feet per second have been adopted as the velocities in grit chambers and velocities less than 1 foot per minute in plain sedimentation basins.
235. Quantity and Character of Grit.—The amount of material deposited in grit chambers varies approximately between 0.10 and 0.50 cubic yard per million gallons. It is to be noted that grit chambers are used only for combined and storm sewage and for certain industrial wastes. They are unnecessary for ordinary domestic sewage. The material deposited in grit chambers operating with a velocity greater than one foot per second is non-putrescible, inorganic, and inoffensive. It can be used for filling, for making paths and roadways, or as a filtering material for sludge drying beds. An analysis of a typical grit chamber sludge is shown in Table 80.
| TABLE 80 | |||||
|---|---|---|---|---|---|
| Analysis of Grit Chamber Sludge | |||||
| Velocity Feet per Second | Specific Gravity | Per Cent Moisture | Calculated to Dry Weight, Per Cent | ||
| Nitrogen | Fixed Matter | Miscellaneous | |||
| 1.0 | 1.5 | 45 | 20 | 78 | 2 |
236. Dimensions of Grit Chambers.—The quantity of sewage to be treated and the amount and character of the settling solids which it contains should be determined by measurement and analysis, and the amount of settling solids to be removed should be determined by a study of the desired conditions of disposal, in order that a grit chamber that will accomplish the desired results may be designed. The period of retention and the velocity of flow are the controlling features in the successful operation of any grit chamber. These should be determined by experiment or as the result of experience. Where neither are available, Hazen’s method can be followed or a decision made based on a study of other grit chambers. In general, the period of retention in grit chambers is from 30 to 90 seconds, and the velocity of flow is about one foot per second.
After having determined the quantity of sewage to be treated, the quantity of grit to be stored between cleanings, the period of retention, the arrangement of the chambers, and the velocity of flow to be used, the overall dimensions of the chambers are computed. The capacity of the chamber is fixed as the sum of the quantity of sewage to be treated during the period of retention and the required storage capacity for grit accumulated between cleanings. The length of the chamber is fixed as the product of the velocity of flow and the period of retention. The cross-sectional area of the portion of the chamber devoted to sedimentation is fixed as the quotient of the quantity of flow of sewage per unit time and the velocity of flow. Only the relation between the width and depth of the portion devoted to sedimentation and the portion devoted to the storage of grit remain to be determined. These should be so designed as to give the greatest economy of construction commensurate with the required results. They will be affected by the local conditions such as topography, available space, difficulties of excavation, etc. Common depths in use lie between 8 and 12 feet, although wide variations can be found. A study of the proportions of existing grit chambers will be of assistance in the design of other basins.
237. Existing Grit Chambers.—The details of some typical grit chambers are shown in Figs. 155 and 156. The grit chamber at the foot of 58th Street, in Cleveland, Ohio, is shown in Fig. 155. The special feature of this structure is the shape of the sedimentation basin, the bottom of which is formed by sloping steel plates forming a 6–inch longitudinal slot above the grit storage chamber. Flows between 8,000,000 and 16,000,000 gallons per day are controlled by the outlet weir so that the velocity of flow remains at one foot per second. This is accomplished by increasing the depth of flow in the same ratio as the increase in the rate of flow. The bottoms of the two chambers differ, one having a special hopper for grit and the other a flat bottom. This is due to the method of cleaning the chambers, it being necessary in the one with a flat bottom to shut off the flow when removing the grit while in the one with the hopper bottom it is hoped to remove the grit by the use of sand ejectors without stopping the sewage flow. The details of the chamber at Hamilton, Ontario, are shown in Fig. 156. In studying these drawings the following features should be noted: 1st, the smooth curves in the channel to prevent eddies, undue deposition of organic matter, and difficulties in cleaning; 2nd, the hopper in the upper end of the grit storage chamber and the slope of the bottom of at least 1:20; and 3rd, the simplicity of the inlet and outlet devices which may be either stop planks or cast-iron sluice gates.
Fig. 155.—Grit Chamber at Cleveland, Ohio.
Eng. Record, Vol. 73, 1916, p. 409.
