[default - edit me]

The net change in cell concentration over a time period may be expressed as: =− dx dt growth output or µ =− dx dt xD x. (2.10) Under steady-state conditions the cell concentration remains constant, thus dx/dt = 0 and: µ =xD x (2.11) and µ = D. (2.12) Thus, under steady-state conditions the specific growth rate is controlled by the dilution rate, which is an experimental variable. It will be recalled that under batch culture conditions, an organism will grow at its maximum specific growth rate and, therefore, it is obvious that a continuous culture may be operated only at dilution rates below the maximum specific growth rate. Thus, within certain limits, the dilu- tion rate may be used to control the growth rate of the culture. The growth of the cells in a continuous culture of this type is controlled by the availability of the growth limiting chemical component of the medium and, thus, the system is described as a chemostat. The mechanism underlying the controlling effect of the dilution rate is essentially the relationship expressed in Eq. (2.5), demonstrated by Monod in 1942: µµ =+ sK s/( ) smax At steady state, µ = D, and, therefore, µ =+ Ds Ks /( ) smax where s is the steady-state concentration of substrate in the chemostat, and µ = − s KD D ()