Analysis of buffering capacity
Buffering capacity is defined as the equivalents of titrant required to
change the medium pH to the reactor pH. The
additional governing equation is the mass balance on the H+ ion concentration in the bioreactor.
By using fresh medium for pH
control, the limiting nutrient level in the reactor can be manipulated
through adjustments in buffering capacity and/or limiting nutrient
concentration in the feed. Assuming
steady state and that the difference between feed and bioreactor H+ is very small (pH 7 means a very tiny amount of H+), simplification gives the following:
and
An extensive study of buffering capacity by Rice and
Hempfling (1985) showed that the specific growth rate was roughly ten times greater with low buffering capacity than with a well-buffered system.
The pH-auxostat is the most widely applied type of auxostat. Oltmann, et al., (1978) evaluated it for efficient
high-rate biomass production. Their improvement was
separating the buffer from the nutrient medium to overcome inhibition
of high buffer concentrations. Fraleigh, (1988) used a pH-auxostat to study aerobic production of ethanol by yeast operating near the
maximum specific growth rate where specific ethanol productivity was
highest. It also permitted an investigation of the coupling of energy and acid production, which was not possible in a chemostat.
System crashes
Minkevich, it et al., (1988) improved the pH-auxostat
to overcome buffering capacity mismatch. If the buffering capacity is set too high, there will be insufficient substrate to maintain the biomass concentration determined by the buffering capacity. In this
situation the pH will not drop below the setpoint (assuming that acids are produced) before depleting the reactor of the limiting nutrient. Therefore the media pumps will not turn on because without nutrient to metabolize, the pH will not change. To overcome this problem, a fixed feed rate component was added to the overall feed rate.