Dissolved Oxygen control in biomass production


Thread Starter



I'm developing a bioreactor (fermentor) for biomass production in our lab. We chiefly need to control three parameters - temperature, pH and
DO (Dissolved Oxygen).

Temperature control is pretty straightforward (at least I think so) - We plan to use a PID controller in the market which regulates a steam
jacket covering the bioreactor.

For controlling the pH we plan to incorporate a controller which is connected to a dosing-pump that switches between pumping an alkaline
liquid (KOH/NaOH) or an acidic one (diluted HCl) depending up on the setpoint of the pH.

But I'm having problems designing a DO control system. For pumping oxygen (air) into the bioreactor, we plan to use a diaphragm pump
connected to a sparger immersed in the biomass inside the bioreactor. Just above the sparger is a stirrer (RPM 200 - 1000) which will
'break' the oxygen bubbles (as they come out of the sparger) into smaller ones so that the oxygen has a better chance to get absorbed in the
biomass. Further, DO takes a long time to get registered in the biomass. So if we start pumping in oxygen now, it may be a while before it gets absorbed in quantities enough to be effectively detected by the sensor. To sum-up the DO problem:
1. We have to control the oxygen level in the biomass.
2. The oxygen takes a while to get dissolved.
3. We can control the oxygen dissolution by two factors -
a. By increasing/decreasing the oxygen flow through the sparger, i.e. pump more/less.
b. By increasing/decreasing the RPM of the stirrer.

Can anybody help me how to design a system for controlling the DO to regulate the oxygen pump and/or the stirrer speed?

Thanks in advance,
Mohit Mahajan,
Bioprocess engineering,
DEI, Agra,

Jaime Renovell

Hi, I'm an absolute beginner but this is my opinion:
The control of DO is the bigger problem you will find in a bioreactor and I think that there isn´t an optimal solution. I think that modulate the stirrer speed will be more effective because usualy the problem is that the mass transfer coefficent is not enough to reach the equilibrium even if your are putting more O2 than the water can absorbe. The problem is that is an expensive solution. You can think on
changing the reactor design (for example with a plate with a lot of small holes in the bottom of the reactor) in order to get a good
distribution and smaller bubble size.
I would use compressed air. If you have the money you'll want to get a mass flow controller, otherwise try a solenoid with a time based strategy at constant supply pressure. You can make your own sterilizable galvanic D.O probes, but they must be re-built now and then, but if you can go polargraphic (i.e. ingold). Make sure the mass flow control speaks 4-20ma and get an analog devices or NI 6B backplane to tie it all together with the appropriate modules. Serial to the PC is slow as bus, but it is simple, far away from the bioreactor, and scalable. Otherwise set your data acquisition PC next to the firm and purchase a PCI data card.

Agitation could also be controlled via a 4-20ma into a drive controller.

Regards, Brad
Add a recirculating line through a pump by taking a slip stream from the clear water in the bioreactor. Measure the DO in the bioreactor in a location with the least amount of turbulence, hopefully in an area of laminar flow. ADD air to the suction side of the pump through a simple venturi at the suction side of the recirculating pump. You should be able to maintain contollable levels of DO by using a needle valve on the venturi tube feeding to the suction of the pump.

e-mail me if you need more clarity.