The term D is the reciprocal of the mean residence
time or hydraulic retention time, as used in waste-water
treatment (see Chapter 15). Addition of fresh medium
into the reactor can be controlled at a fixed value, therefore
the rate of addition of the rate-limiting nutrient is
constant. Within certain limits, the growth rate and the
rate of loss of cells from the fermenter will be determined
by the rate of medium input. Therefore, under steadystate
conditions the net biomass balance can be described
as