Chemostat operation

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

Operate the chemostat initially as a batch reactor with D = 0, and then switch to chemostat operation with D [Pg.541]

Continuous fermentation processes are primarily used in the research and development stage. However, more chemostat operations are being used at the production level as the understanding of this reactor increases. Examples include ethanol fermentation for the production of fuel grade ethanol and single-cell protein production from methanol substrates. 

Some representative results are shown in Figures 1 and 2 for the cases of a linear decrease of y with time and that of a sudden increase of D in a chemostat, respectively. Clearly, the agreement is very satisfactory, and similar behavior was obtained for a variety of other situations typical of a chemostat operation. 

Most fermenter vessels suitable for suspension cell culture can be readily adapted for chemostat culture. A device for maintaining a constant culture volume will be required, the simplest and most reliable of which is a weir that allows the culture to overflow to a collection vessel. Alternatively, a dip tube that draws medium from the surface of the culture can be mounted in the headplate of the vessel and the medium pumped to the collection vessel. Most manufacturers supply modified tubes for chemostat operation of their fermenters. 

Except for the presence of the cell death/maintenance metabolism term in the present analysis and the need to identify the factor P in Figure 13.8 with the ratio of the concentrations in equation (13.3.15), the analysis above gives results similar to those for a chemostat operating in a biomass recycle mode. 

Microalgae can also be grown continuously meaning that the microalgal culture is continuously harvested and the liquid volume removed is continuously replaced with fresh water with nutrients. In the case of chemostat operation the dilution rate is fixed. Assuming a constant photon flux density a steady state will be reached where the biomass concentration does not change anymore and is constant. The influent, water with nutrients, usually does not contain any microalgae. Furthermore, the liquid voltune is usually maintained constant, so Tm = Tout=fr- Finally, it wiU be assumed that the liquid inside the photobioreactor is perfectly mixed, so C om = Cx. Then the biomass balance over the photobioreactor can be simplified as follows ... 

Under outdoor conditions under day/night cycles biomass accumulation inside the photobioreactor cannot be neglected. Especially in case of chemostat operation considerable changes in biomass concentration will occur. Even in a turbidostat in the ri ht biomass concentrations will decrease because of respiration. The biomass balance then must be written as follows ... 

Despite the fact that productivity of chemostat operation is similar to turbidostat operation the turbidostat is preferred especially at locations with more day-to-day variation in irradiance. Under chemostat operation biomass concentration will slowly decrease on consecutive days with cloud cover. If followed by a clear-sky day, the culture is vulnerable to photoinhibition because of its low biomass concentration. Turbidostat operation allows for a direct control of biomass concentration. Washout of a microalgae culture in a turbidostat is by definition not possible. In a chemostat this could occur in situations the dilution rate is not adjusted in time. Turbidostat operation is thus more robust and allows for rehable automation of outdoor microalgae production (Bosma et al, 2014). 

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. 

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. 

In this case we assume that we know the dilution rate (D-F/V) precisely as a function of time. In a chemostat D is often constant since the feed flowrate and the volume are kept constant. In a fed-batch culture the volume is continuously increasing. The dilution rate generally varies with respect to time although it could also be kept constant if the operator provides an exponentially varying feeding rate. 

Figure 17.6 Dialyzed Chemostat Monoclonal antibody concentration (raw and smoothed measurements) during initial batch start-up and subsequent dialyzed continuous operation with a dialysis flow rate of 5 L/d. [reprinted from the Journal of Biotechnology Bioengineering with permission from J. Wiley],...

Figure 17.13 Dialyzed Chemostat Estimated values of specific MAb production rate versus time during the period of continuous operation. A 5% standard error in the raw data was assumed for data smoothing.

The term chemostat refers to a tank fermentation which is operated continuously. This bioreactor mode of operation normally involves sterile feed (Xo=0), constant volume and steady state conditions, meaning that dV/dt=0, d(VSd/ dt=0, d VX1)/dt=0. 

A continuous fermenter with sterile feed is referred to as a chemostat. For constant volume operation, the inlet volumetric flow rate is equal to that at the output. With this model chemostat start-up, resultant steady state behaviour and cell washout phenomena are easily investigated by simulation. 

What is the effect of operating the chemostat at a dilution rate of D>(um ... 

Suspension systems can be operated in different modes batch, fed-batch, chemostat, and perfusion (Fig. 1). These operation modes differ basically in the way nutrient supply and metabolite removal are accomplished, which in turn determines cell concentration, product titer and volumetric productivity that can be achieved [8]. The intrinsic limitation of batch processes, where cells are exposed to a constantly changing environment, limits full expression of growth and metabolic potentials. This aspect is partially overcome in fed-batch cultures, where a special feeding strategy prolonges the culture and allows an increase in cell concentration to be achieved. In perfusion and chemostat processes nutrients are continuously fed to the bioreactor, while the same amount of spent medium is withdrawn. However, in perfusion cultures the cells are retained within the bioreactor, as opposed to continuous-flow culture (chemostat), which washes cells out with the withdrawn medium [9]. 

Fig. 1. Schematic view of the different operation modes of suspended-cell processes batch, fed-batch, chemostat and perfusion...

There are two different ways of operating continuous stirred-tank fermentors (CSTFs), namely chemostat and turbidostal. In the chemostat, the flow rate of the... 

The characteristics of the CSTF, when operated as a chemostat, are discussed in Chapter 12. 

In the chemostat, the dilution rate is set at a fixed value, and the rate of cell growth then adjusts itself to the set dilution rate. This type of operation is relatively easy to carry out, but becomes unstable in the region near the washout point. 

In the turbidostat, P and F are kept equal but the dilution rate D is automatically adjusted to a preset cell concentration in the product by continuously measuring its turbidity. Compared to chemostat, turbidostat operation can be more stable in the region near the washout point, but requires more expensive instruments and automatic control systems. 

There are certain circumstances when it may be desirable to operate a series of stirred-tanks in cascade, with the effluent of one forming the feed to the next in the chain. In the case of two such fermenters the first will behave in the manner of a simple chemostat and the performance equations of the chemostat will apply. The second tank, however, will not have a sterile feed so that some of the simplifications which led to those relationships will not be valid. 

However, inspection of Fig. 5.57 suggests that the operation of a chemostat at dilution rates other than those approaching washout conditions results in growth at low values of substrate concentration S. For this condition the endogenous respiration rate becomes significant and Ya Yx/S, as indicated in equation 5.54. It therefore becomes more realistic to express the specific growth rate p as ... 

The chemostat is a biological CSTR where the substrate concentration in the tank is maintained constant. The tur-bidostat is similar to the chemostat except that the cell mass in the reactor is kept constant. The primary distinction between the two reactors is the control mechanism used to maintain continuous operation. A unique feature of a biological CSTR is the washout point. When the flow rate is increased so that the microbes can no longer reproduce fast enough to maintain a population, the microbes wash out of the tank, and the reaction ceases. This washout point represents the limits of maximum flow rate for operation. 

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