Chemostats

In this section we return to mass equations on the cells [Equation (7-75)] and substrate (Equation (7-76)] and consider the case where the volumetric flow CSTR rates in and out are the same and that no live (i.e.. viable) cells enter the chemostat. We next define a parameter common to bioreactors called the dilution rate. D. The dilution rate is 

Using the Monod equaiion. ihe growth rate is delermined to be 

We now neglect the death rate, rj, and combine Equations (7-51) (7-83) for steady-state operation to obtain the mass flow rate of cells out oi system, f,.. 

An inspection of Equation (7-86) reveals that the specific growth rate of cells can be ctmtmlfed by the operator by controlling the dilution rate Using Equation (7-52) to substitute for p. in terms of the substrate concer tion and then solving for the steady-state substrate concentration yields 

Assuming that a single nutrient is limiting, cdl growth is the only process c tributing to substrate utilization, and that cell maintenance can be neglec the stoichiometry is 

Reaction Mechanisms, Pathways. Bioreacaions, and Bioreactors Chapter 9 

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Chemostat A bioreaetor in whieh steady-state growth of miero-organisms is maintained over prolonged periods of time under sterile eonditions by providing the eells with eonstant input of nutrients and eontinuously removing effluent with eells as output. 

The multi-stream multi-stage system is a valuable means for obtaining steady-state growth when, in a simple chemostat, the steady-state is unstable eg when the growth-limiting substrate is also a growth inhibitor. This system can also be used to achieve stable conditions with maximum growth rate, an achievement that is impossible in a simple chemostat (substrate-limited continuous culture). 

We can see that for type 1 processes, high growth rate is obligately linked to a high rate of product formation. Indeed, this is the case for all products produced by a fermentative mode of metabolism, eg ethanol, lactic add, acetone. Chemostat studies have shown that for most aerobic processes when growth is limited by some nutrient other than the carbon source, the yield of product decreases with increase in spedfic growth rate (p or D p = dilution rate (D) in chemostat culture). Conversely, both the biomass yield and the spedfic rate of substrate utilisation (qs g substrate g biomass-1 h-1) increase with spedfic growth rate. 

Figure 3.2 Theoretical relationships for (a) qs against dilution rate and for (b) Yp/S and Yx/s against dilution rate. The micro-organism is grown aerobically in a nitrogen limited chemostat culture.

This relationship is very useful experimentally because it can be used to determine both Y x and m. In practice, carbon limited chemostats are used and Yx/s is measured at... 

A bacterium was grown as a glucose-limited chemostat culture and steady state growth yield (Yx/J was measured at different dilution rates. 

In practice, carbon limited chemostat cultures are used to estimate the P/O quotient These conditions are used because they favour the most efficient conversion of the carbon substrate into cellular material, ie the highest efficiency of energy conservation. The steady state respiration rate (qo,) is measured as a function of dilution rate (specific growth rate) and Yq can be obtained from the reciprocal of the slope of the plot. qo, is also known as the metabolic quotient for oxygen or the specific rate of oxygen consumption. 

In this section we will consider the energetics of exopolysaccharide production in some detail. We will see how chemostat (substrate limited) derived yield coefficients and slfbstrate elemental balances can be used to determine how the nature of the substrate influences... 

The energetic requirements of exopolysaccharide production from various carbon sources can be calculated if the P/O quotient during growth on the carbon substrate is known. Table 3.1 shows molar growth yields measured during carbon limited growth in chemostat culture. 

Table 3.1 Parameters of growth and exopolysaccharide production for Agrobacterium radiobacter grown in chemostat culture on various carbon sources. Data obtained from Linton J. D. et al (1987) Journal of General Microbiology 133, 2979-2987.

The production-scale fermentation unit, with a projected annual capacity of over50,000 tonnes was fully commissioned in 1980. The bioreactor (Figure 4.8) is 60 m high, with a 7 m base diameter and working volume 1,500 m3. There are two downcomers and cooling bundles at the base. Initial sterilisation is with saturated steam at 140°C followed by displacement with heat sterilised water. Air and ammonia are filter sterilised as a mixture, methanol filter sterilised and other nutrients heat sterilised. Methanol is added through many nozzles, placed two per square metre. For start-up, 20 litres of inoculum is used and the system is operated as a batch culture for about 30 h. After this time the system is operated as a chemostat continuous culture, with methanol limitation, at 37°C and pH 6.7. Run lengths are normally 100 days, with contamination the usual cause of failure. 

Dilution rate of 0.5 h 1 produces the fastest growth rate (since p = D), but gives poor productivity and substrate utilisation. This is a feature of chemostat cultures, when p. approaches w. 

Fig. 5.3. Schematic diagram of continuous culture with control units in a constant volume chemostat.

Fig. 5.4. Chemostat without pumps maintained at constant level.

In a chemostat and biostat or turbidostat, even with differences in the supply of nutrients and/or fresh media, constant cell density is obtained. The utilisation of substrate and the kinetic expressions for all the fermentation vessels are quite similar. It is possibile to have slight differences in the kinetic constants and the specific rate constants.3,4 Figure 5.9 shows a turbidostat with light sources. The system can be adapted for photosynthetic bacteria. 

The continuous cultures of chemostat and biostat systems have the following criteria ... 

Fig. 5.5. Chemostat with feed pump overflow drainage maintained at constant level.

Fig. 5.6. Chemostat using single medium inlet feed and outlet pumps.

Fig. 5.7. Chemostat with inlet and outlet control loops, feed and product pump with cell loading and recycling.

At steady-state condition for chemostat operation, change of concentration is independent of time. Material balance for the fermentation vessel is ... 

The material balance for cells in a continuous culture chemostat is defined as ... 

With cell recycling, chemostat efficiency is improved. To maintain a high cell density the cells in the outlet stream are recycled back to the fermentation vessel. Figure 5.10 represents a chemostat unit with a cell harvesting system. The separation unit is used for harvesting the cells and recycling then to the culture vessel to increase the cell concentration. 

The material balance in a constant volume chemostat is derived based on cell balance as shown in the following equations. Material balance in a chemostat with recycle, pcell ... 

The ICR flow rate was five to eight times faster than the CSTR. The overall conversion of sugars in the ICR at a 12 hour retention time was 60%, At this retention time, the ICR was eight times faster than CSTR, but in the CSTR an overall conversion rate of 89% was obtained. At the washout rate for the chemostat, the ICR resulted in a 38% conversion of total sugars. Also, the organic acid production rate in the ICR was about four times that of the CSTR. At a higher retention time of 28 hours, the conversion of glucose in the ICR and CSTR are about the same, but the conversion of xylose reached 75% in the ICR and 86% in the CSTR. 

CSTR CHEMOSTAT VERSUS TUBULAR PLUG FLOW... 

A special CSTR fermentation known as a chemostat bioreactor is used for microbial cell growth. The rate of biomass generation is given by ... 

We wish to compare the performance of two reactor types plug flow versus CSTR with a substrate concentration of Csf = 60g-m 3 and a biomass yield of Y = 0.1. In a plug flow bioreactor with volume of 1 m3 and volumetric flow rate of 2.5 m -li what would be the recycle ratio for maximum qx compared with corresponding results and rate models proposed for the chemostat ... 

The performance data for plug versus mix reactor were obtained. The data were collected as the inverse of qx vs inverse of substrate concentration. Table E.1.1 shows the data based on obtained kinetic data. From the data plotted in Figure E.1.1, we can minimise the volume of the chemostat. A CSTR works better than a plug flow reactor for the production of biomass. Maximum qx is obtained with a substrate concentration in the leaving stream of 12g m 3. 

A tubular bioreactor design with operational may lead to a CSTR, having sufficient recycle ratio for plug flow that behave like chemostat. The recirculation plug flow reactor is better than a chemostat, with maximum productivity at C, 3 g-m 3. Combination of plug flow with CSTR which behave like chemostat was obtained from the illustration minimised curve with maximum rate at CSf = 3 g-m-3. 

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