A process engineer performed jar tests for a water in order to determine the optimal pH and dose using alum. A test was conducted by first dosing each jar with the same alum dose of 10 mg/L and varying pH in each jar from 5.0 to 7.5 with an increment of 0.5 unit of pH. After the first test, he/she plotted the results of remaining turbidity versus pH and found the optimal pH was 6.25. He/she continuously perform a second set of jar tests by holding the optimal pH of 6.25 constant and varying alum doses from 10 to 15 mg/L. Here are the results:
Results of Jar Tests for raw water at optimal pH of 6.25
Turbidity of raw water = 15 NTU
Alkalinity of raw water = 5 mg/L expressed as CaCO3.
Alum Dose 10 11 12 13 14 15
(mg/L)
Turbidity 5.0 4.6 4.5 3.0 5.0 6.0
Remaining
(NTU)
Determine:
1) Plot turbidity versus dose and find the optimal dose of alum (mg/L) with the water lowest remaining turbidity.
2) The theoretical amount of alkalinity consumed at the optimal dose expressed as CaCO3, mg/L.
3) Compared with the theoretical alkalinity from the above results, is the raw water alkalinity sufficient for the coagulation? If not, what kinds of chemical do you need in order to enhance the alkalinity?