Which statement about HRT units is true?

Which statement about HRT units is true?

• A. HRT is expressed in hours; volume divided by flow rate
• B. HRT is expressed in minutes; volume divided by flow rate
• C. HRT is expressed in meters per second
• D. HRT is expressed in gallons per minute

Answer: (A)

Hydraulic Retention Time is the time the wastewater spends in a tank. It’s found by dividing the tank volume by the influent flow rate (HRT = V / Q). Since you’re taking a volume and dividing by a flow rate, you end up with a time unit. When you use consistent units like cubic meters for volume and cubic meters per hour for flow, the result is hours. That’s why expressing HRT in hours is considered correct. If you used different unit pairings, you could get minutes or seconds, but the fundamental idea remains that it’s a time value derived from volume divided by flow rate. Units like meters per second or gallons per minute don’t describe a time duration, so they don’t fit as HRT units.

Hydraulic Retention Time is the time the wastewater spends in a tank. It’s found by dividing the tank volume by the influent flow rate (HRT = V / Q). Since you’re taking a volume and dividing by a flow rate, you end up with a time unit. When you use consistent units like cubic meters for volume and cubic meters per hour for flow, the result is hours. That’s why expressing HRT in hours is considered correct. If you used different unit pairings, you could get minutes or seconds, but the fundamental idea remains that it’s a time value derived from volume divided by flow rate. Units like meters per second or gallons per minute don’t describe a time duration, so they don’t fit as HRT units.