Define Hydraulic Retention Time (HRT) and state its typical range in a secondary clarifier.

• A. HRT is the time needed to replace all the water in the tank; typical range is 24 to 48 hours.
• B. HRT is the average time wastewater spends in a tank (volume divided by influent flow); typical secondary clarifier HRT ranges from about 1 to 4 hours.
• C. HRT is the ratio of influent flow to tank volume; typical range is 15 to 60 minutes.
• D. HRT is only relevant for primary clarifier and not for secondary clarifier.

Answer: (B)