Calculators / MCRT / Sludge Age
MCRT / Sludge Age calculator
MCRT is how many days, on average, solids stay in the activated sludge system. You divide the solids in the system by the solids leaving it each day, through wasting and in the effluent.
MCRT (days) = lbs solids in aeration + clarifier ÷ (lbs/day WAS + lbs/day effluent TSS)
Worked example
Aeration volume is 1.2 MG and clarifier volume is 0.3 MG. MLSS is 3,000 mg/L. You waste 0.05 MGD at 8,000 mg/L. Plant flow is 3 MGD with effluent TSS of 10 mg/L. What is the MCRT?
- Solids in system = (1.2 + 0.3) × 3,000 × 8.34 = 37,530 lbs
- Wasted = 0.05 × 8,000 × 8.34 = 3,336 lbs/day
- Lost in effluent = 3 × 10 × 8.34 = 250.2 lbs/day
- MCRT = 37,530 ÷ (3,336 + 250.2) = 10.47 days
Answer: 10.47 days
Common mistakes
- Leaving out effluent solids. Some questions include them and some don't; follow the formula your question gives.
- Some textbooks count only aeration solids (no clarifier). Set clarifier volume to 0 if your question does.
Check your exam's reference sheet for its exact constants. Some use 3.14 instead of 3.1416, which changes answers in the last digit.