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Activated Sludge F/M & MCRT Sizing Calculator environmental
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Activated Sludge F/M & MCRT Sizing Calculator

Determine Food-to-Microorganism (F/M) ratio, Mean Cell Residence Time (MCRT sludge age), hydraulic detention time, and aeration basin volatile solids inventory.

Plant Flow & Aeration Basin

Typically 0.70 to 0.80 for municipal activated sludge

Sludge Wasting & Effluent

Process Operating Parameters

Food-to-Microorganism (F/M) Ratio
--
-- mode
MCRT / Sludge Age
-- Days
-- hrs HRT
Daily BOD Loading
-- lbs/day
-- lb/1000 ft³·day
Aeration MLVSS Biomass: -- lbs MLVSS
Total Solids Wasted Daily: -- lbs TSS/day
WAS Solids Wasted: -- lbs/day
Conventional activated sludge operates optimally with F/M between 0.20 to 0.40 lb BOD/lb MLVSS-day and MCRT between 5 to 15 days.

Recommended Tools & Equipment

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Frequently Asked Questions

What is the Food-to-Microorganism (F/M) ratio in activated sludge?

The F/M ratio is a primary biological process control parameter representing the pounds of organic food (BOD5) fed into the aeration basin daily divided by the total pounds of active microbial biomass (MLVSS) residing in the aeration tank: F/M = (Q × BOD × 8.34) / (V × MLVSS × 8.34).

What is Mean Cell Residence Time (MCRT) or Sludge Age?

MCRT represents the average length of time in days that biological sludge organisms remain inside the activated sludge treatment system before being removed via waste activated sludge (WAS) or escaping in secondary clarifier effluent: MCRT = Total System MLSS Inventory / Total Daily TSS Wasted.

How does MCRT control biological nitrification of ammonia?

Autotrophic nitrifying bacteria (Nitrosomonas and Nitrobacter) reproduce much slower than carbon-eating heterotrophs. If MCRT is less than 5 to 7 days in warm water (or 10 to 15 days in cold winter water), nitrifiers are washed out of the system faster than they can multiply, causing total loss of ammonia removal.