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Looking at the gamma PDF graph **(Figure ), the alum dosage of 20 mg/L gave a widely distributed probability of reaching different floc sizes with a comparably low probability to reach its highest sedimentation velocity. In addition, the residual turbidity graph **(Figure )showed a high turbidity for this alum dose in its lower velocity range. Thus the alum dose 20 mg/L seems to be inefficient for this particular influent turbidity and flow rate. The mean turbidity resulting from alum dose 55 mg/L seems to be out of normal range; its NTU value is significantly lower than the values given from the previous, lower alum dosages. Hence, the result from this dosage is doubtful. Overall, after the alum dose of 35 mg/L (except for 55 mg/L), the mean turbidity seemed to be settling down to a constant value around 1.4 NTU.
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On Oct 27, 2009, another experiment was conducted with the turbidity set around 500 NTU, flocculator length 2796 cm, flow rate of 5 mL/s, and alum dosage ranging from 10 to 90 mg/L. The data was run through Mathcad to get a simplified graphic overview of the result.
In both graphs **(Figure , Figure ), the alum dosage 10 mg/L gave a comparably different result from the rest; it produced a significantly higher settling turbidity and a large amount of smaller flocs. The overall mean turbidity for this dosage was almost twice the following dosages. After the alum dose 50 mg/L, the mean turbidity seemed to be settling down to a constant value around 2.5 NTU.
FIGURE 3: The graph plots normalized turbidity vs. sedimentation velocity for each Alum dose ranging 40 mg/L~110 mg/L
Alum Dosage (mg/L) | Approx. Lowest Turbidity Range (NTU) |
|---|---|
40 | ~.85 |
50 | ~.94 |
60 | ~.88 |
70 | ~.88 |
80 | ~.76 |
90 | ~.76 |
100 | .76~.98 |
110 | .78~.99 |
On November 11, 2009, an experiment was conducted with the turbidity set around 25 NTU, flocculator length 2796 cm, flow rate of 5 mL/s, and alum dosage ranged from 10 to 50 mg/L.
In the gamma PDF graph **(Figure ), the alum dosage 10 mg/L showed a larger distribution of floc sizes with more probability to produce smaller flocs. In addition, looking at the residual turbidity graph **(Figure ), the resultant turbidity this dosage gave was significantly higher than the rest. The residual turbidity graph also showed the alum doses 15 mg/L and 20 mg/L to be producing a slightly higher resultant turbidity. From the values of mean turbidity at settling state, starting from the alum dosage 35 mg/L, the mean turbidity seemed to be settling down to a constant value around 2.5 TABLE 2: _Shows the Lowest turbidity reached by specific alum dosage ranging 30 mg/L~55 mg/L during an influent turbidity of approximately 500 NTU.
(I am again doubtful of the data shown in this table. It it is calculated data, then how did the numbers end up with a squiggle in front? And how did it turn into a range? Standard analysis would report a mean and a standard deviation. It appears that all of the alum dosages tried were too high. What was the capture velocity used for this analysis?)