Question

Jar testing was performed using alum on a raw drinking-water source that contained an initial turbidity of 20 NTU and an alkalinity of 35 mg/L as CaCO3. The optimum coagulant dosage was determined as 18 mg/L with a final turbidity of 0.25 NTU. Determine the quantity of alkalinity consumed as CaCO3.

Answer 1

suspended particle of finer nature can not be settle down easily in plane sedimentation tank but such particle can be remove easily by using optimum coagulant dosage which is obtain by jar testing. coagulant remove suspended particle(turbidity) by increasing particle size and weight.

ALUM is generally used to remove turbidity. To remove turbidity 1 g of alum consumed 0.45 g of alkalinity as CaCO3.

In our case raw water alkalinity is 35 mg/l and 18 mg/l coagulant is required to remove turbidity from 20 NTU to 0.25 NTU

So, we knows that 1 g alum consumed 0.45 g of alkalinity as CaCO3

Hence, 18 mg of alum will consume

0.45*18 mg/l of alkalinity as CaCO3

= 8.1 mg/l as CaCO3