Continuous Cultivation - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 608

growth, we have learned how to analyze batch growth. We can infer that there is no control
being exerted in batch growth other than the initial substrate addition and maintaining the
reactor temperature. In this chapter, we shall focus more on the controlled growth where
control can be exerted on the growth more than the simple substrate/inoculums addition.

12.1. CONTINUOUS CULTURE

The medium conditions change continually in a batch culture due to cell metabolism and
lack of control. Growth, product formation, and substrate utilization terminate after a certain
time interval, whereas, in a continuous culture as it implies, fresh nutrient medium is contin-
ually supplied to a well-mixed (or more precisely uniform) culture, and products and cells
are simultaneously withdrawn. Growth and product formation can be maintained for pro-
longed periods in a continuous culture. After a certain period of time, the system reaches
a steady state where cell, product, and substrate concentrations remain constant. Continuous
culture provides constant culture environmental conditions for growth and product forma-
tion and supplies uniform quality product. Therefore, continuous culture is a controlled
culture. Continuous culture is an important tool to determine the response of microorgan-
isms to their environment and to produce the desired products under optimal environmental
conditions.

12.1.1. Chemostat Devices for Continuous Culture
The primary type of continuous cultivation devices is the chemostat (with many variations
pending on the feedback control mechanism: cytostat , turbidostat , and perfusion reactor are
some of the classic chemostats equipped with different external peripherals or recycle capa-
bilities) although plug flow reactors (PFRs) are also used. In some cases, these units are modi-
fied by recycle of cells (perfusion reactor). Figure 12.1 is a schematic of a continuous culture

Acid

Exhaust

Filter / sterilizer PH

Feed
Effluent

Filter

Pumps

Rotameter

Compressed air

FIGURE 12.1 A continuous-culture laboratory setup (chemostat).