11. Understanding Critical Dilution Rate | Bioprocess Technology | Notes and MCQ's in description.

MCQ's
Q1. At Critical dilution rate of a continuous stirred tank bioreactor is characterized by [GATE 2011]
(S = substrate concentration inside bioreact
S(in) = Substrate concentration inside feed
P = Product concentration inside bioreactor
X = Biomass concentration inside bioreactoror)

a. X =0, P=0, S=0
b. X =0, P=0, S=S(in)
c. X= 0, P is less than zero, S=0
d. X more than zero, P=0, S=0

Q2. Match the microbial growth characteristics in group 1 with the corresponding features in group 2 [GATE 2011]
Group 1
P. Growth associated product formation
Q. Non growth associated product formation
R. Product inhibition
S. Substrate inhibition
Group 2
1. Specific growth rate decreases with increasing product concentration
2. Specific product formation rate is constant
3. Specific product formation rate is proportional to specific growth rate
4. Specific growth rate decreases with increasing substrate concentration

a. P(1), Q(2), R(4), S(3)
b. P(3), Q(2), R(1), S(4)
c. P(2), Q(1), R(3), S(4)
d. P(2), Q(3), R(4), S(1)

Q3. In a chemostat operating under steady state a bacterial culture can be grown at dilution rate higher than the maximum specific growth rate by [GATE 2010]
a. Partial cell recycling
b. Using sub-optimal Temperature
c. PH cycling
d. Substrate feed rate cycling

Q4. Culture of rhizobium is grown in a chemostat (100m3 volume). The feed contains 10g/l sucrose, Ks for the organism is 0.2g/l and umax = 0.3 per hr.
Flow rate required to result in steady state concentration of sucrose as 1.5g/l in the bioreactor will be [GATE 2010]
a. 15 m3/Hr
b. 26 m3/Hr
c. 2.6 m3/Hr
d. 150 m3/Hr

Answers -
1. b
2. b
3. a
4. b

NOTES-
What is dilution rate?
The dilution rate is the feed flow rate per unit volume.

What is critical dilution rate?
The critical dilution rate is the value of dilution rate at which the steady-state Biomass concentration just becomes zero.

For understanding the concept of critical dilution rate we first need to understand the relationship between dilution rate and steady-state Biomass concentration.

Change in Biomass concentration over time can be denoted by biomass produced minus biomass removed.

dx/dt = ux - Dx

Now we know that under study state conditions there is no change in the biomass concentration, hence, dx/dt is equal to zero.
so u = D

Now, what if we decrease the dilution rate.
so, if we decrease the dilution rate the substrate flowing inside the bioreactor decreases leading to decrease in u and hence decrease in x.
but after some time a stage is reaching when u and D again become equal and a strady state is achieved with lower biomass concentration.

what if we increase the dilution rate.
so, if we increase the dilution rate the substrate flowing inside the bioreactor increases leading to increase in u and hence and increase in x. But u it can only increase up to a certain level that is umax and if we still further the increase the dilution rate beyond umax washout will occur which means that the biomass inside the bioreactor will be washed out.
we can understand from the above equation that specific growth rate remains at umax and dilution rate increases which causes an increase in the rate of removal of biomass and finally a state will be reached when there will be no Biomass left inside the bioreactor. And the dilution rate at which the biomass inside the bioreactor just becomes zero is known as critical dilution rate.
The formula for dilution rate is given by
D = umax S/(Ks+S)
Here, S = substrate concentration in a bioreactor under steady-state conditions.
As there is no biomass present inside the bioreactor at D critical, there is no consumption of substrate and hence, steady state substrate concentration in the above equation is replaced by substrate concentration in the feed.
so now the formula for critical dilution rate is given by
D = umax Si/(Ks+Si)
here, Si = substrate concentration in the feed.

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