If you are studying for a wastewater treatment exam at Grade 2 or above, the activated sludge process is where the hardest questions live. The good news: they almost always come down to three control parameters. Learn what each one means, how to calculate it, and what a "good" number looks like.
Food-to-microorganism (F:M) ratio
F:M = BOD applied (lbs/day) ÷ MLVSS under aeration (lbs)
F:M describes how much food (organic load) you have relative to the bugs doing the work. Conventional plants typically run 0.2–0.5. Too high and the bugs are overloaded; too low and you risk old, pin-floc sludge.
Mean cell residence time (MCRT / SRT)
MCRT = solids in the system (lbs) ÷ solids leaving (lbs/day)
Also called sludge age, MCRT is the average time a microorganism stays in the system. Nitrifying plants often target a longer MCRT (roughly 10–20 days) because nitrifiers grow slowly.
Sludge volume index (SVI)
SVI = (settled sludge volume, mL/L × 1000) ÷ MLSS (mg/L)
SVI measures how well your sludge settles. Below about 120 mL/g is good; above 150 mL/g usually signals filamentous bulking and poor settling.
How the exam frames it
Exam questions rarely ask you to define these. They give you plant data — flows, concentrations, tank volumes — and ask you to calculate the parameter or decide what to do when it drifts out of range. Practice with real numbers, not just definitions.
Work these with plant data, not just definitions. Our free wastewater practice questions give you the calculation reps the exam actually tests.