Water Treatment > Quality Water Solutions > FAQ > How do I determine the desired alum and PACl dosage? If I dose with dry alum, how can I best put it into solution?

Use standard jar tests to determine the dosage needed for clarification. Streaming current and Zeta potential devices, which determine the efficiency of coagulant dosage by measuring minute changes in the electric charge on contaminants, can help tune the coagulant dosage to changing raw water conditions. Other aids in setting dosage include historical trends from routine water analyses, color and UV absorbance.

When dissolving dry alum, shoot for a 10% to 20% alum solution by weight for most applications. This range allows dry alum to dissolve well and eliminates the need for strong pumps. For 50 gallons of a 12% solution, mix a 50-pound bag of granular alum with 44 gallons of water. If the mixing time needed is more than 30 minutes, reduce the amount of dry alum by 10 pounds.

If the pH of the finished solution goes above 3.5, bring it back down by adding more alum or less water. This will prevent excessive hydrolysis in the stored product, which can reduce efficiency because the alum converts to aluminum hydroxide rather than remaining as an active form of aluminum. In some cases, a 1% to 5% solution can promote hydrolysis. Watch for increases in dilute coagulant solution pH, especially when using solutions made in day tanks. (This also applies to PACl coagulants.)