Objective: To study the growth and product formation kinetics of A. suboxydans under continuous cultivation (steady state) conditions.
Theory: Basic concepts and need of continuous cultivation
Continuous Cultivation of microorganism –
Continuous cultivation of microorganism are open systems which features addition of nutrients at a constant rate and simultaneous with drawl at the same rate. This mode of cultivation is particularly useful as it results in significant improvement in productivity of fermentation. Also it is rather easy to implement process control for these systems. However some disadvantages of this cultivation e.g, development of mutants and contamination free cultivation for longer time limits its common usage. However it is a best tool to study the physiology of cultivation as there is a perfect steady state cultivation condition at a particular dilution rate (= sp. growth rate) in the bioreactor.
Transients in Chemostats –
The overall response of any continuous cultivation can be simulated by the mathematical model however it is rather interesting to see the culture behavior in transients in cultivation (Shift up / Shift down in dilution rates) It has been observed that Monod model is unable to perfectly simulate the transients in Continuous cultivations because the model assumes dependence of growth on the instantaneous value of substrate concentration. In shift up of dilution rates (i.e., increasing the feeding rate of nutrient) the metabolism switches from “famine” condition to “feast” condition, meaning suddenly the substrate concentration see a significant change against the nutrient limiting / dying culture and there is no proportionate increase in the cell growth as proposed by Monod’s model. It is therefore necessary to incorporate the “physiological state marker” in the model which can not only quantitatively describe the metabolic reactions of the cells but adequately graduate changes in nutrient limiting and nutrient rich situations in transient conditions of continuous cultivations. The transients are particularly important in microbial cultivations as it may lead to significant increase in the product concentrations which are not available elsewhere. This is particularly important because the culture experiences a major shift in nutrient availability and thereby it leads to quick change over of the culture metabolism.
Wash out condition –
In any cultivation it is always necessary to devise strategies which might result in high productivity. Productivity in continuous cultivation is dependent on not only the concentration of biomass /product but also on its dilution rate (Productivity = DP or DX). It is therefore necessary to increase both D and X to increase the productivity of fermentation. However in actual practice, it is observed that at lower dilution rate, unconverted substrate is low & biomass concentration is high (Productivity = High Biomass x Low D) and on the contrary at high dilution rates, unconverted substrate is high & biomass concentration is low. (Productivity = Low Biomass x High D). Therefore in either of the two cases the multiplication of DX results in lower productivity. Also if dilution rate of the bioreactor is further enhanced then it may lead to “wash out” of the biomass from the reactor. The substrate concentration in the reactor will then be equal to the inlet concentration of feed substrate. For optimization of the productivity, suitable Dilution rate (Dmax out put) is identified which when used during cultivation conditions gives best value of steady state biomass accumulation in the bio reactor there by increasing the productivity. It is also possible to improve the process productivity by using cell retention / recycle system which allow build up of biomass in the bioreactor and it becomes possible to operate the reactor at higher dilution rates (D=Feed rate/Volume) with-out wash out.