Ravensford Chlorine Dosing: The Pounds Formula in Action

Hook (5 min): Ask students why adding "just enough" chlorine to kill germs at the plant might leave none at the far end of town. Take a few ideas, then introduce chlorine demand as the chlorine used up along the way.

Direct instruction (12 min): State the two tools for the fictional plant: feed in pounds per day = dose in mg/L, times flow in MGD, times 8.34, and chlorine demand = dose minus residual. Model one worked example on the board: at a 4 MGD flow and a 2.5 mg/L dose, feed = 2.5 times 4 times 8.34 = 83.4 pounds per day. This example is teacher-modeled only; students solve different numbers on the handout.

Guided practice (11 min): With a partner, students work handout items 1 through 3, computing a feed, finding a demand, and solving for a dose that leaves a target residual. Circulate and confirm they add demand and residual to get the dose.

Independent practice (12 min): Students complete handout items 4 through 7 on their own: recomputing feed at the new dose, scaling feed to a higher flow, checking the residual against the chlorine MRDL, and explaining why a residual is kept.

Closure and formative check (5 min): As an exit ticket, each student answers on a card: if demand is 1.0 mg/L and the operator wants a 0.5 mg/L residual, what dose is needed? Collect and scan for 1.0 plus 0.5 = 1.5 mg/L. If more than a third miss it, reteach demand plus residual before the next class.