Which factors are used to calculate the loading rate in lbs/day/ft² for a secondary clarifier?

• A. Influent pH and temperature
• B. Influent flow rate only
• C. MLSS concentration only
• D. Influent flow rate and MLSS concentration

Answer: (D)

Loading rate for a secondary clarifier is about how much solid mass arrives per unit area each day. To calculate lbs per day per square foot, you need the daily mass of solids entering the clarifier and the clarifier’s surface area. The daily mass of solids comes from multiplying the influent flow rate (how much water is coming in each day) by the MLSS concentration (how much solids are in that water). Dividing that solids mass by the clarifier’s surface area gives the lbs/day/ft² loading rate.

If you only use flow rate, you’d ignore how dirty the water is; if you only use MLSS concentration, you’d ignore how much water is moving through. The influent pH and temperature don’t determine the solids loading rate.