2 over time may be determined.

It is understood that the measuring unit 6 measures the microorganism content X generally from a sample of liquid 2 taken from the container 3.

Returning to the device 1 of FIG. 1, a control unit 7 is connected to the measuring unit 6 so that it has access to the microorganism content X measured by the measuring unit 6 in each case. The control unit 6 has, for example, a proportional integral (PI), a proportional integral differential (PID) controller or the like for controlling the microorganism content X. For this purpose, the control unit 7 determines a dosage D of a biocide B which is to be added to the liquid 2 in order to achieve the predefined microorganism content X*. The predefined microorganism content X* may be predefined here to be constant on the one hand or to vary over time on the other hand. The temporal variation of the predefined microorganism content X* is based, for example, on an adaptation to a previously planned course and/or a measured state of the industrial process. For example, in the context of a so-called “fed-batch” process successively supplying liquid 2—for example in a fermentation process—not only the amount of liquid 2 depends on the process progression, i.e. varies over time, but usually also the predefined microorganism content X*; in particular, the quantity of liquid may be small and a, for example, high predefined microorganism content X* may be desired in an initial stage of the process, whereas in a later stage of the process the quantity of liquid is larger and the predefined microorganism content X* is to be reduced.

The device 1 further comprises a supply unit 8 with one or more reservoirs from which it supplies biocide B to the liquid 2, for example by means of a controllable pump or a control valve. The supply unit 8 is connected to the control unit 7 to supply the biocide B to the liquid 2 in the dosage D determined by the control unit 7.

In order to determine the dosage D, the control unit 7 uses a model with at least one parameter k. The model represents a relationship between an amount P of biocide B supplied in a time interval Δt and the change in the microorganism content X caused thereby in this time interval Δt, i.e. it is a kinetic model of the reaction of the liquid 2 and thus of its microorganism content X to the biocide B supplied. It is understood that the amount P of biocide B supplied in the time interval Δt coincides with the dosage D of biocide B to be supplied to the liquid 2 if this has been determined for the same time interval Δt. The control unit 7 uses the model to adapt one or more time constants and/or gains of its controller to the reaction of the liquid 2, i.e. the change in its microorganism content X, to biocide supply.

The model may be of various types, in particular a model that assumes a linear, quadratic or exponential decrease in the microorganism content X with biocide addition. In one embodiment, the model is given by the following equation 1:

dX
dt

=

-

kB
n

⁢
X

(
1
)

with:

dX
dt

- change in microorganism content X in a time interval Δt (here: the “infinitesimal” time interval dt),
- k parameter of the model concerning a deactivation of the microorganisms M,
- B amount P of biocide B supplied in this time interval Δt,
- n dilution coefficient of the biocide B and
- X measured microorganism content X.

In an alternative embodiment, the model is given by the following equation 2:

dX
dt

=

-

k
1

⁢

B
0
n

⁢

e

-

k
2

⁢
nt

⁢
X

(
2
)

with:

dX
dt

- change in microorganism content X in a time interval Δt (here: the “infinitesimal” time interval dt),
- k₁parameter of the model concerning a deactivation of the microorganisms M,
- k₂parameter of the model concerning an inactivation of biocide B after its supply to the liquid 2,
- B₀amount P of biocide B supplied in this time interval Δt at the time of its supply,
- n dilution coefficient of the biocide B and
- X measured microorganism content X.

The extended model of equation (2) makes it possible, through the parameter k₂, to take into account an inactivation of the biocide B after its supply to the liquid 2.

In both of the above-mentioned models, the dilution coefficient n of the biocide B is either known in advance if the biocide B used and the liquid 2 and the microorganisms M contained therein are known; otherwise, the dilution coefficient n is another parameter of the model, so that the model given by equation (1) has two parameters k and n and the model given by equation (2) has three parameters k₁, k₂and n.

If the model is also to represent an influence of any side reactions of the industrial process on the relationship between the amount P of biocide B supplied and the change in the microorganism content X caused thereby, the model may optionally be given, for example, by a system of coupled differential equations, which in each case describe the influence of the industrial process itself or that of its side reactions.

In order to adapt the model or its at least one parameter k, k₁, k