The two big measures
BOD = oxygen microbes use to break down organic matter. Strength of the waste.
TSS = weight of suspended solids (mg/L).
★Lowering BOD and TSS is the plant's job; both are common permit limits.
Primary vs secondary
Primary = physical settling; heavy solids sink, grease floats and is skimmed.
Secondary = biological; microbes eat the dissolved organic matter.
Then disinfection before discharge.
Activated sludge
RAS (return activated sludge) = pumped back to aeration to keep the microbes.
WAS (waste activated sludge) = the surplus removed to keep the balance.
Return to keep it alive; waste to keep it balanced.
The math
Detention time = volume / flow
Loading (lbs/day) = flow (MGD) x concentration (mg/L) x 8.34
Percent removed = (in - out) / in x 100