Question 1

How much alum should I feed?

Accepted Answer

Typical doses run 10–60 mg/L depending on raw water turbidity and color. Always confirm with a jar test — over- and under-dosing both increase finished water turbidity.

Question 2

Does alum lower pH?

Accepted Answer

Yes. Alum hydrolyzes to release H⁺ and consumes ~0.45 mg/L alkalinity per mg/L alum. Plants with soft water often need supplemental lime, soda ash, or caustic.

Question 3

What is the difference between dry and liquid alum?

Accepted Answer

Dry alum is ~17% Al₂O₃ powder. Liquid alum (48.5% w/w) is the same compound dissolved in water, 11.1 lb/gal, 5.38 lb dry alum per gallon.

Question 4

Why isn't ferric chloride covered here?

Accepted Answer

The formula is identical — substitute the ferric chloride dose in mg/L. Liquid FeCl₃ (38–40%) weighs 11.8 lb/gal; pounds of dry FeCl₃ per gallon ≈ 4.6.

Alum Coagulant Dose Calculator – lbs/day & Gallons/Day