Science of Water : Concepts and Applications

Water Treatment Calculations 357
Example 10.58
Problem:
A total of 44 mg/L alkalinity is required to react with alum and ensure proper precipitation. If the
raw water has an alkalinity of 30 mg/L as bicarbonate, how many mg/L alkalinity should be added to
the water?
Solution:
Alkalinity to be added, mg/L total alkalinity required, mg/Lalkalinity
present in the water, mgL 44 mgL30mgL 14 mgL alkalinity to be added
In Step 3, after determining the amount of alkalinity to be added to the water, we determine
how much lime (the source of alkalinity) needs to be added. We accomplish this by using the ratio
shown in Example 10.59.
Example 10.59
Problem:
It has been calculated that 16 mg/L alkalinity must be added to a raw water. How much mg/L lime will
be required to provide this amount of alkalinity? (1 mg/L alum reacts with 0.45 mg/L and 1 mg/L alum
reacts with 0.35 mg/L lime.)
Solution:
First, determine the mg/L lime required by using a proportion that relates bicarbonate alkalinity
to lime:
Next, we cross-multiply:
0.45 mgL alkalinity 16 mgL alkalinity
0.35 mgL lime x mgL lime
0.45 x (16)(0.35)
(16)(0.35)
x
0.45
x 12. 4 mgL lime
In Example 10.60, we use all three steps to determine lime dosage (mg/L) required.
Example 10.60
Problem:
Given the following data, calculate the lime dose required, in mg/L:
Alum dose required (determined by jar tests)—52 mg/L
Residual alkalinity required for precipitation—30 mg/L