| TABLE 77 | |
|---|---|
| Hydraulic Values of Settling Particles in Millimeters per Second | |
| Diameter in mm. | Hydraulic Value |
| 1.00 | 100 |
| 0.80 | 83 |
| 0.60 | 63 |
| 0.50 | 53 |
| 0.40 | 42 |
| 0.30 | 32 |
| 0.20 | 21 |
| 0.15 | 15 |
| 0.10 | 8 |
| 0.08 | 6 |
| 0.06 | 3.8 |
| 0.05 | 2.9 |
| 0.04 | 2.1 |
| 0.03 | 1.3 |
| 0.02 | 0.62 |
| 0.015 | 0.35 |
| 0.010 | 0.154 |
| 0.008 | 0.098 |
| 0.006 | 0.055 |
| 0.005 | 0.0385 |
| 0.004 | 0.0247 |
| 0.003 | 0.0138 |
| 0.002 | 0.0062 |
| 0.0015 | 0.0035 |
| 0.001 | 0.00154 |
| 0.0001 | 0.0000154 |
An example will be given to illustrate the method of using the diagram and tables to determine the size of a sedimentation basin to perform certain required work.
Let it be required to determine the period of retention in a continuously operated sedimentation basin with good baffling, corresponding to two properly baffled sedimentation basins in series. The basins are to remove 60 per cent of the finest particles which are to have a size of .01 mm. The quantity to be treated daily is 3,000,000 gallons.
1st. Entering Table 77, we find that the hydraulic value of the finest particles is .154 mm. per second.
2d. Since we wish to remove 60 per cent of the finest particles, 40 per cent will remain. Since Fig. 154 shows the per cent remaining after the time a
t we enter Fig. 154 at 40 per cent on the ordinates and run horizontally until we encounter Line 4 corresponding to good baffling in Table 78. We then run down vertically from this intersection and find that the ratio of a
t is 1.0.
Then a equals t, which means that the period of retention should equal the time that it takes a particle 0.01 mm. in diameter to drop from the top to the bottom of the basin. Since this depends on the depth of the basin it is necessary to determine the depth before the other dimensions of the basin can be fixed.
Although this method is seldom used in practice for the final design of a sedimentation basin, it is a guide to judgment and can be used to supplement the data obtained from tests.
Fig. 154.—Hazen’s Diagram, Showing the Relation between the Time of Settling and the Period of Retention in Various Types of Sedimentation Basins.
Trans. Am. Society Civil Engineers, Vol. 53, 1904, p. 45.
| TABLE 78 | ||||
|---|---|---|---|---|
| Comparison of Different Arrangements of Settling Basins | ||||
| (From Hazen) | ||||
| Description of Basins | Line in Fig. 154 | Values of a t. | ||
| Per Cent of Matter Removed | ||||
| 50 | 74 | 87.5 | ||
| Theoretical maximum. Cannot be reached. | A | 0.50 | 0.75 | 0.875 |
| Surface skimming. Rockner Roth system. | B | 0.54 | 0.98 | 1.37 |
| Intermittent basins, reckoned on time of service only. | C | 0.63 | 1.26 | 1.89 |
| Continuous basin. Theoretical limit. | D | 0.69 | 1.38 | 2.08 |
| Close approximation to the above. | 16 | 0.71 | 1.45 | 2.23 |
| Very well baffled basin. | 8 | 0.73 | 1.62 | 2.37 |
| Good baffling. | 4 | 0.76 | 1.66 | 2.75 |
| Two basins, tandem. | 2 | 0.82 | 2.00 | 3.70 |
| One long basin, well controlled. | 1.5 | 0.90 | 2.34 | 4.50 |
| Intermittent basin in service half time. | E | 1.26 | 2.50 | 3.80 |
| One basin, continuous. | 1 | 1.0 | 3.00 | 7.00 |