F/M is defined as the amount of BOD entering the system to mass of biomas in the aeration tank.

$\frac{F}{M}ratio=\frac{Q_0S_0}{V_x}(kg/kg/day) or \frac{1}{d}$

2)Sludge age:-

Sludge age represents the time for which biomass remain in the system.

It is defined as the ratio of mass of biomass in the aeration tank to the mass of biomass leaving the system per day.

Note:-Tendency of nitrification in ASP is controlled by sludge age.