## Chemostat

The simple continuous process takes place in a CSTR (completely stirred tank reactor) as a steady state process with a bacteria-free influent and with no recycling of settled sludge. This reactor is called a chemostat. The growth of bacteria was initiated by inoculation. After a non-steady period, a constant concentration of bac teria and substrate are achieved for mean retention times which are long enough to sustain bacterial growth. Different steady states can be obtained by changing the flow rate, substrate influent concentration or dissolved oxygen concentration at different aeration rates. A very simple solution of the problem follows from a bacterial balance:

flow rate out growth rate of the reactor

With Eq. (6.1) and the mean retention time: tR = — (6.20)

or the dilution rate: Q0 1

pmax tR 1

With Eq. (6.15), the corresponding equation for X is: X = YX/S (So--) (6.24)

with the mean retention time tR used as a parameter (Fig. 6.1).

It is typical for all bioreactors with suspended microorganisms in continuous operation that the organisms can be washed out at a critical mean retention time (tRC) which follows from Eq. (6.24) for X = 0 as:

Pmax S0

and for S0P KS as: 1

-max

Using Eq. (6.21), a well known solution is obtained: Dc = —max (6.27)

All suspended organisms are washed out if the dilution rate is increased beyond a critical value, which corresponds to the maximum specific growth rate, -max. From a theoretical point of view, a suspended culture of two organisms (1 and 2) with different —max can be separated by increasing D up to a value greater than DC1 and less than DC2.

Fig. 6.1 Effluent COD S and bacterial concentration X of a chemostat versus mean retention time tR. tRC = critical mean retention time, calculated with ^max = 7.2 d-1, KS = 100 mg L-1 COD.

Fig. 6.1 Effluent COD S and bacterial concentration X of a chemostat versus mean retention time tR. tRC = critical mean retention time, calculated with ^max = 7.2 d-1, KS = 100 mg L-1 COD.

The growth rate follows directly from Eqs. (6.2) and (6.24):

it increases with increasing tR>tRC, get a maximal value rx max, which can be obtained from: drx/dtx = 0 and decreases.

Because the chemostat maintains only a low concentration of microorganisms, it has no practical significance, and as a consequence, it exhibits only a low substrate consumption rate. But experiments can be used to study the reaction kinetics and to determine the coefficients. It can be used to determine S for different tR values while Eq. (6.22) is linearized as follows:

1 Vmax 1

S ks kS

A plot of 1/S versus tR must give a straight line with a negative ordinate intercept -kS-1.

As discussed in Section 6.2.1.1, oxygen limitation must be avoided during these experiments and the dissolved oxygen concentration must be c'>1.8 mg L-1 (T = 20°C, municipal wastewater). The region of oxygen limitation can be studied if an oxygen balance is used:

VmaxX c'

Yx/02 K + c and if S is high enough, whereby S/(KS + S) ; 1 during aeration.

With:

it follows that:

Equation (6.32) can be linearized using the Langmuir plot:

Figure 6.2 presents results from measurements at high bacterial concentration X0 and high substrate concentration S0 for 17 °C and 24 °C. X0 was added to the influent wastewater (not chemostat operation).

A good linearization was obtained with K' = 0.19 mg L-1 (17 °C) and K' = 0.06 mg L-1 (24°C; Putnaerglis 1987).

c r max max

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