Transcript Continuous cultivation

Continuous cultivation
by
E. Börje Lindström
Theory
• Two types of apparatus are usually used; the chemostat and the turbidistat.
aeration
Pump
Medium tank
(S0)
Reactor
(S)
Recovery tank
• In both types there is a flow of fresh medium into the growth vessel (reactor)and the
same amount of volume is leaving the growth vessel.
• The cell concentration is regulated in different ways:
Stämmer verkligen detta?
Något ämne i mediet är väl
- In a chemostat the level of the bacterial population is
alltid begränsande förr eller
dependent on a limiting factor (S0) in the medium tank and senare?
the flow rate of the medium to the reactor is kept constant.
- In a turbidistat the cell concentration in the reactor is registered continuously and
regulated by changing the flow rate of medium into the reactor with no limiting factor
in the medium.
Chemostat
• Cultivating an aerobic bacterium in a continuous way demands
effective aeration and stirring. These prerequisites can be
provided in stirred tank reactors.
• With a good stirring the composition of the medium and the
air supply in the reactor will be homogeneous throughout the
reactor and the bacteria will grow optimally.
• However, the process is always started batch-wise with
no addition of new medium. The inoculated bacteria will
then grow as fast as possible (mmax) until the concentration
of the limiting substrate, (S), will decrease and hence the
growth will slow down.
Chemostat, cont.
• Then the pump is started and new amount of nutrients
will be available for the bacteria. The added limiting
substrate, S, will now be consumed rapidly by the
bacterial population in the vessel
Chemostat, cont.
• At each fixed flow rate of medium a ”steady-state” will be
obtained which means that:
• The cell population will be constant and kept at this level until the flow
rate is changed.
• At this steady state the growth of bacteria is equal to the removal of
bacteria to the recovery vessel.
• The growth of the bacteria in a continuous process is therefore dependent
on the flow rate of new medium into the reactor and the limiting substrate,
S0, in the medium.
Calculations
• For bacteria in a continuous one-reactor process using nutrients which all are
in solution the following exponential equation is used:
Nt = N0 x emt (1)
Where:
• m = the specific growth rate constant
• Nt = number of cells at time t
• N0 = the number of cells at time = 0.
• According to Monod et al. (ref.??) the following equation is
valid:
Calculations, cont.
m = mmax x (S)/Ks + (S)
Where:
(2)
• mmax is maximal m for the bacterium
and the medium used
• (S) is the concentration of the limiting factor in the
reactor
• Ks is a concentration constant for the limiting factor which
gives m = 0.5 mmax.
m
mmax
mmax/2
Ks
(S)
Calculations, cont.
Using the following nomenclature:
• V = the reactor volume (liter, L)
• F = the flow rate of the medium into the reactor (L/hr)
• D = dilution = F/V (tim-1)
• (X) = the cell concentration (bacteria/L)
• Y = D(X)/D(S) = the recovery constant of cells per unit
limiting factor
• The following can be obtained:
Changes of cell concentration
At steady state we have:
• Growth of bacteria - outflow of bacteria to the recovery vessel =
V x (X) x m - F x (X) = 0
This gives:
m = F x (X)/ V x (X) = F/V = D
• This means that increasing the dilution can be done until the
growth rate constant reaches mmax
Conclusion:
- At a dilution higher than mmax the culture will be washed out because the
growth of bacteria can’t compensate for the loss of bacteria to the
recovery vessel.
Changes in the limiting factor
At steady state we have:
• Amount of factor entering the reactor – amount of factor leaving the reactor consumption of the factor by the growing bacteria =
F x (S0) – F x (S) –V x m x (X)/ Y = 0
This gives:
(X) = Y((S0) –(S))
Conclusion:
- This means that knowing Y, (S0) and (S), the amount of bacteria in the reactor can
be estimated.