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Again, we calculate the marginal effect of alum on the removal efficiency.
 For example, under capture velocity of 0.05 mm/s and raw water turbidity
 50 NTU, 
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Alum dose(mg/L)
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PC*
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/
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Marginal effect of increasing alum dose from 0.5 mg/L to 1 mg/L is calculated
 as 
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.
 We can see that under this capture velocity, the largest marginal effect
 occurs when we increase the alum dose from 1 mg/L to 1.5 mg/L.
 This means that 1 mg/L alum dose is the most economical and effective.
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So we can get the optimal alum dose under different capture velocity at
 raw water turbidity equals to 50 NTU:
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