Team:Calgary/PHB Fermentation

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PHB Fermentation

Overview

In this stage of the process, engineered E. coli is fermented in a VFA-rich stream obtained in the previous step to produce PHB. This continuous fermentation process occurs in a 5L stirred-tank bioreactor at 37 degrees Celsius and under anaerobic conditions. A continuous flow of the VFA-rich stream is generated by the pump. To achieve continuous fermentation, a mechanical self-cleaning filter with 0.2-micron filters is used to separate and recycle bacteria back to the bioreactor. The resulting bacteria-free harvest stream that contains PHB is then passed to PHB extraction and water recovery. The proposed process is shown in Figure 1.

Figure 1: Diagram of the proposed PHB fermentation process.

Other design options

Other considered design options include:

• External membrane bioreactor (EMB)
• Immersed membrane bioreactor (IMB): specifically the hollow fiber and the flat sheet designs
• Acoustic cell separation

The major problem associated with the membrane bioreactors is fouling. Fouling is a big counter-argument against a design on Mars because filter cleaning comes at a significant cost of crew-time input, which is extremely valuable. Fouling also comes at a cost of extra membrane material supply, which is expensive to ship to space.

The literature search on acoustic cell separation have proven to not be applicable to the scale of our process.

The stirred tank bioreactor, on the other side, comes with the advantages of continuous agitation and easy temperature control. The self-cleaning filter allows to eliminate the fouling issue and limited resources issue. The stirred tank bioreactor combined with the self-cleaning filter was chosen as the optimal design for the process.

Table 1: ESM analysis for the proposed PHB fermentation system.

	Self-cleaning filter	Stirred-tank bioreactor
Power (kW)	2	0.38
Weight (kg)	16	15
Volume (m^3)	0.028	0.1153
Spares & Consumables (kg/day)	0	0
Spares & Consumables (m^3)	0	0
ESM Estimation	196	73