Operator calculator
Mean Cell Residence Time (MCRT/SRT) calculator
Divide the solids in the system by the solids removed each day.
Calculator
MCRT
9.08 days
- 1. Solids in system = 2,500 × (1 + 0.25) × 8.34 = 26,063 lb
- 2. Solids removed = (0.04 × 8,000 + 2 × 12) × 8.34 = 2,869 lb/day
- 3. MCRT = 26,063 ÷ 2,869 = 9.08 days
For instructional use. Results assume the standard conversion factors shown on this page. Actual field calculations may need chemical strength, density, temperature, and other adjustments, so check your plant’s procedures.
Formula and inputs
MCRT (days) = [MLSS (mg/L) × (aeration + clarifier volume, MG) × 8.34] ÷ [(WAS flow × WAS TSS + effluent flow × effluent TSS) × 8.34]
| Input | Unit |
|---|---|
| MLSS | mg/L |
| Aeration tank volume | MG |
| Clarifier volume | MG |
| WAS flow | MGD |
| WAS TSS | mg/L |
| Effluent flow | MGD |
| Effluent TSS | mg/L |
| Result: MCRT | days |
Conversion factors used: 8.34 lb per gallon of water (cancels in the ratio).
When to use it
MCRT, also called sludge retention time, is the average number of days a solids particle stays in the activated sludge system. Operators adjust wasting to hold it steady.
Count solids in both the aeration tank and clarifier, and count solids leaving by wasting and in the effluent.
Applies to: Wastewater Treatment.
Exam tip: Write two lines: solids in the system, solids leaving per day. Then divide. Do not skip the effluent.
Study guides
Build the math skill behind the calculator
Water Operator Math Made Simple walks through the formulas step by step.
Water Operator Math Made Simple
A Step-by-Step Guide to the Math Every Water Operator Needs to Know