Worksheet 1b – Calculate the required level of alkalinity
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When using metal based coagulants, alkalinity should be at least half the expected alum dose and equal
to the highest expected ferric chloride dose.
The following is a guide for calculating alkalinity required when using alum.
Additional alkalinity for the flash mixing stage required when using alum can be estimated by:
1.
Divide existing raw water alkalinity by 0.45 (0.45 mg/L of alkalinity reacts with 1 mg/L of alum).
2.
Subtract the answer from the expected highest alum dose.
3.
Multiply this answer by 0.33 to give the required additional lime, or 0.48 for soda ash, required to
react with the alum.
Example
Values
Alkalinity in the raw water
15 mg/L
Expected highest alum dose
40 mg/L
Divide existing raw water alkalinity by 0.45
33.3 mg/L
Subtract the answer from the expected highest alum dose.
6.7 mg/L
Multiply this answer by 0.33 to give the required additional lime
2.2 mg/L of lime
Worksheet 1b – Calculate the required level of alkalinity
Page 1 of 2
How is alkalinity calculated in your workplace? Include the equations that are used.
Description
Values
Alkalinity in the raw water
Expected highest alum dose
Divide existing raw water alkalinity by 0.45
Subtract the answer from the expected highest alum dose.
Multiply this answer by 0.33 to give the required additional lime
Worksheet 1b – Calculate the required level of alkalinity
Page 2 of 2