Macroscale View of the Phases in an SSF Bioreactor

From a macroscale perspective, the bioreactor contains three phases (Fig. 2.4(a)):

• the bioreactor wall;

• a headspace full of gas, the extent of which depends on the bioreactor type;

• a substrate bed, composed of particles and air within the inter-particle spaces.

The bioreactor wall is important as a barrier. It should be a complete barrier to mass transfer. Matter can only enter the bioreactor through holes in this wall (addition ports, sampling ports, loading/unloading ports). It is a partial barrier to energy transfer. Energy can cross this boundary by conduction, at a rate that depends on the thermal properties of the material from which the bioreactor was constructed. Note that energy can also be stored in this wall, this storage being manifested as an increase in the temperature of the wall. The headspace has functions such as:

• allowing the air that leaves the bed to reach the air outlet of the bioreactor. Foaming will typically not be a problem in SSF bioreactors, so the headspace does not play the role in foam control that it does in SLF bioreactors. In some SSF bioreactors, in which the solid particles within the bed are suspended in an air stream (gas-solid fluidized-beds), the headspace allows room for bed expansion and for disengagement of particles and air;

The organism needs optimum values of

• temperature

• water activity

But the organism is

• consuming nutrients

• altering the local pH

• releasing waste metabolic heat

and there is likely to be evaporation of water to the air

Therefore, the conditions will tend to deviate from the optimum during the process

The operating and design variables that we can manipulate in an effort to maintain optimal conditions are:

Bioreactor type and geometry

Design Liquid Phase Bioreactor

Aeration' system

Temperature, humidity, and flow rate of the air

Aeration' system

Temperature, humidity, and flow rate of the air

Additions to the bioreactor (water, nutrient solutions, pH-correcting solutions)

Incorporation of a water jacket or heat transfer plates, the flow rate and temperature of the water in the jacket/plates and the location of the plates

Agitation system

Frequency, intensity, and duration of agitation events

We want to control the conditions in the interior of the substrate bed

We are limited to manipulation of variables external to the bioreactor, such as the conditions of the inlet air

We want to control the conditions in the interior of the substrate bed

Fig. 2.3. The difficulty of controlling the conditions within an SSF bioreactor. (a) The organism changes the values away from the optimum values for growth and product formation, and we have only a limited number of operating variables, involving manipulations external to the substrate bed, with which we can attempt to bring the conditions back to the optimal values. (b) Transport phenomena within the bed determine the effectiveness with which any manipulation of the operating variables can control the conditions in the interior of the bed

Transport phenomena determine the efficiency with which our manipulations of the operating variables influence the conditions within the substrate bed

Fig. 2.3. The difficulty of controlling the conditions within an SSF bioreactor. (a) The organism changes the values away from the optimum values for growth and product formation, and we have only a limited number of operating variables, involving manipulations external to the substrate bed, with which we can attempt to bring the conditions back to the optimal values. (b) Transport phenomena within the bed determine the effectiveness with which any manipulation of the operating variables can control the conditions in the interior of the bed

Picture Ssf Bioreactors

headspace substrate bed (particles, biomass, and void spaces)

bioreactor wall void spaces may contain droplets of water, but water does not form a continuous phase headspace substrate bed (particles, biomass, and void spaces)

bioreactor wall void spaces may contain droplets of water, but water does not form a continuous phase void spaces partially filled with microbial biomass void spaces partially filled with microbial biomass

Pictures Ssf Bioreactor

aerial hyphae biofilm aerial hyphae biofilm

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