Modelling and Performance of a Hydrogel-Based Photobioreactor

Abstract

This work is motivated by the need for in situ food production with respect to future space activities due to the technical and economic in-feasibility of long-term earth-based resupply. The unique size constraints of space have prevented conventional food systems from demonstrating feasibility. Owing to their high growth rates and phototropic activity, microalgae are a promising candidate to meet the caloric and respiratory needs of astronauts as part of a biological life support systems (BLSS). However, the gravity dependence and size of transitional photobioreactors poses a challenged to their utilization in space. As such, a solid-state hydrogel-based photobioreactor (hPBR) is proposed to achieve inherent phase separation allowing for extra-terrestial use. Initially proposed for the Canadian Space Agency (CSA) Deep Space Food Challenge (DSFC) (Design A), this design was further iterated to improve productivity and reactor performance (Design B). Using Chlorella vulgaris, Design B achieved a biomass productivity of 2.4 and 3.2 g m−2d −1 when using physically (pPVA) and chemically (cPVA) crosslinked poly(vinyl) alcohol (PVA) respectively with a water demand of 0.44 kg g−1 biomass. Over 23 days of growth, the lipid content increased from 18.9% to 56.6% and 13.8% to 43.2% for pPVA and cPVA respectively, and the chlorophyll content also decreased. However, cell viability remained high at over 97% and surface coverage analysis showed good coverage within a few days. Observations made with the prototype suggested mass transport limitations were impacting growth, and that poor humidity control led to the hydrogels drying out. To this end, a continuum model of the hydrogel was proposed to better understand mass transfer and to inform future design iterations. Hydrogels are two phase systems where the polymer is fixed due to crosslinking leading to a moving boundary with changes in water content. The proposed model did not require any parameter fitting as values were determined with independent experiments. The model enabled the prediction of the transient material response to changing relative humidity. This helped to explain why humidity control was critical in maintaining cell viability. Humidity impacted the water content of the gel’s surface which needed to be high enough to support algae growth. Using the steady-state solution to the model, the solute transport through the system was also modelled. The solute profile suggested that nutrient concentrations throughout the hydrogel were similar to that in the media tank. This suggests nutrient supply was not the cause of the diminishing biomass quality and that other factors such as photo-inhibition, and mechanical stresses from solid-state cultivation may be issues to address in future work.