## External Feedback

A diagrammatic representation of an external feedback system is shown in Fig. 2.8b. The effluent froni -the fermenter is fed through a separator, such as a continuous centrifuge or filter, which produces two effluent streams — a concentrated biomass stream and a dilute one. A fraction of the concentrated stream is then returned to the vessel. The flow rate from the ^ medium reservoir is F (dm3 h 1); the flow rate of the effluent upstream of the separator is Fs (dm3 h ') and the concentration of biomass in the stream (and in the . fermenter) is x; a is the proportion of the flow which is fed back to the fermenter and g is the factor by which the separator concentrates the biomass. Biomass balance in the system will be:

Change = growth - output + feedback or Ax/At = fix - F,x/V + aFsgx/V. (2.21)

The culture outflow (before separation), Fs, from the chemostat is:

FS=F + aFs or Fs = F/{ 1 - a), substituting F/( 1 - a) for Fs in equation (2.21) and remembering that D = F/V:

If all the cells are returned to the fermenter then biomass will continue to accumulate in the vessel. I Unvever. if the feedback is partial then a steady state may be achieved, Ax/At = 0 and:

Stirrer

Separator

Stirrer

Separator

. uK-snu- substrate and biomass concentrations

■llK' mrnU'r^ with feedback are then given by the 111 »1 ' 1

lolloping equal ions:

i.-,„in ||,e equations describing ¡x, s and x (2.18, 2.19, ■.■.,) \:-t. 2.25) in a fermenter with either exter-

n!Ii ^internal leedback it can be appreciated that:

(i) Dilution rate is greater than growth rate.

mi Hi(.m,r.-. concentration in the vessel is increased.

(,ii i 1 lie mcieased biomass concentration results in a decrease in the residual substrate compared with a simple chemostat.

11- i The maximum output of biomass and products is increased.

i vi Because D is less than the critical dilution rate (the dilution rate at which washout occurs) is increased.

ISmmass feedback is applied widely in effluent treatment -vstems where the advantages of feedback contribute significantly to the process efficiency. The outlet substrate concentration is considerably less and the feedback of biomass may improve stability in effluent treatment systems where mixed substrates of varying conccni ration are used. The system will also result in inciL-.^ed productivity of microbial products as illus-ti.itcJ by Major and Bull (1989) who reported very high lactic acid productivities in laboratory biomass recycle fermentations. Anaerobic processes are particularly suited to feedback continuous culture because the elevated biomass is not susceptible to oxygen limitation.

Feedback systems seem particularly attractive for animal cell culture where low growth rates and low cell densities limit productivity. A number of internal feedback systems have been developed based on immobilized cells on either hollow fibres or microcarriers isee .iKo Chapter 7) and it is claimed that with the rapid developments in centrifuge design, centrifugal separation and feedback in suspension cultures may be scaled-up (Griffiths, 1992). The potential for continuous animal-cell processes is considered in the next section of this Chapter.

Comparison of batch and continuous culture in industrial processes

HK>M \SS PRODUCTIVITY

rlhe productivity of a culture system may be de scribed as the output of biomass per unit time of the fermentation. Thus, the productivity of a batch culture may be represented as:

where R is the output of the culture (g biomass dm^3 h-1), xmax is the maximum cell concentration achieved at stationary phase, x0 is the initial cell concentration at inoculation, tt is the time during which the organism grows at and is the time during which the organism is not growing at /¿max and includes the lag phase, the deceleration phase and the periods of batching, sterilizing and harvesting.

The productivity of a continuous culture may be represented as:

where Rcont is the output of the culture (g biomass dm~3 h-1),

/,„ is the time period prior to the establishment of a steady state and includes vessel preparation, sterilization and operation in batch culture prior to continuous operation, and T is the time period during which steady-state conditions prevail.

The term Dx increases with increasing dilution rate until it reaches a maximum value, after which any further increase in D results in a decrease in Dx, as illustrated in Fig. 2.9. Thus, maximum productivity of biomass may be achieved by the use of the dilution rate giving the highest value of Dx.

Dilution rate

Fig. 2.9. The effect of dilution rate on biomass productivity in steady-state continuous culture.

Dilution rate

Fig. 2.9. The effect of dilution rate on biomass productivity in steady-state continuous culture.

The output of a batch fermentation described by equation (2.26) is an average over the period of the fermentation and, because the rate of biomass production is dependent on initial biomass (dx/dt = fix), the vast proportion of the biomass is produced towards the end of the fermentation. Thus, productivity in batch culture is at its maximum only towards the end of the process. For a continuous culture operating at the optimum dilution rate, under steady-state conditions, the productivity will be constant and always maximum. Thus, the productivity of the continuous system must be greater than the batch. A continuous system may be operated for a very long time period (several weeks or months) so that the negative contribution of the unproductive time, tm, to productivity would be minimal. However, a batch culture may be operated for only a limited time period so that the negative contribution of the time, tu, would be very significant, especially when it is remembered that the batch culture would have to be re-established many times during the time-course of a continuous run. Thus, the superior productivity of biomass by a continuous culture, compared with a batch culture, is due to the maintenance of maximum output conditions throughout the fermentation and the insignificance of the non-productive period associated with a long-running continuous process.

The steady state achievable in a continuous process also adds to the advantage of improved biomass productivity, as discussed by Hospodka (1966). Cell concentration, substrate concentration, product concentration and toxin concentration should remain constant throughout the fermentation. Thus, once the culture is established the demands of the fermentation, in terms of process control, should be constant. In a batch fermentation, the demands of the culture vary during the fermentation — at the beginning, the oxygen demand is low but towards the end the demand is high, due to the high biomass and the increased viscosity of the broth. Also, the amount of cooling required will increase during the process, as will the degree of pH control. In a continuous process oxygen demand, cooling requirements and pH control should remain constant. Thus, the use of continuous culture should allow for the easier introduction of process automation.

A batch process requires periods of intensive labour during medium preparation, sterilization, batching and harvesting but relatively little during the fermentation itself. However, a continuous process results in a more constant labour demand in that medium is supplied continuously sterilized (see Chapter 5), the product is continuously extracted and the relative time spent on equipment preparation and sterilization is very small.

The argument against continuous biomass pro, is that the duration of a continuous fermentai very much longer than a batch one so that theu-greater probability of a contaminating organism en/ " ing the continuous process and a greater probability^ equipment failure. However, problems of contamina tion and equipment reliability are related to equipmem design, construction and operation and, provided suf ficiently rigorous standards are applied, these problems can be overcome. In fact, the fermentation industry has recognized the superiority of continuous culture for the production of biomass and several large-scale processes have been established. This aspect is considered ln more detail in a later section of this Chapter.