What does FAC stand for in water quality testing?

• A. Fixed available chlorine
• B. Free available chlorine
• C. Fully activated copper
• D. Free applied calcium

Answer: (B)

In water quality testing, FAC is the amount of chlorine that remains in the water in a form capable of disinfecting. It’s the free portion, not bound to ammonia or other compounds, and it represents the active residual disinfectant in the water. This distinguishes it from combined chlorine, which forms when chlorine bonds with nitrogenous compounds and is less effective for disinfection. The best-fitting term is Free Available Chlorine because it uniquely describes this active, residual disinfectant form.

Other options don’t fit because they describe concepts not used in chlorine residual testing: “Fixed available chlorine” isn’t a recognized term; “Fully activated copper” has nothing to do with chlorine; and “Free applied calcium” is unrelated to chlorine residuals. In practice, you’d measure FAC with a colorimetric test (like DPD) to ensure there’s enough residual disinfectant.