Activated carbon is used in tertiary treatment with a detention time (DT) range of

• A. 60-120 minutes
• B. 5-10 minutes
• C. 30-60 minutes
• D. 15-35 minutes

Answer: (D)

Activated carbon relies on contact time to remove dissolved organics. In a tertiary treatment setup, water must dwell in the carbon bed long enough for adsorption to occur effectively, but not so long that the tank becomes oversized with little added benefit. A detention time in the mid-teens to mid-30s of minutes provides a practical balance: it allows substantial removal of tastes, odors, and many organic contaminants without requiring an excessively large system.

If the detention time is only a few minutes, adsorption efficiency drops and removals aren’t reliable. If it’s pushed well beyond two dozen minutes, the extra benefit tapers off and the system becomes larger and more costly for little gain. Therefore, the typical and most cost-effective range used for activated carbon in tertiary treatment is roughly 15 to 35 minutes.