(25d) Development of a Microalgae Biocomposite-Integrated Spinning Disc Bioreactor (SDBR) for Bioprocess Intensification of Light-Driven CO2 Absorption

Development of a microalgae
biocomposite-integrated Spinning Disc Bioreactor (SDBR) for bioprocess
intensification of light-driven CO₂ absorption

K.Boodhoo^1*, T. Ekins-Coward¹, S.
Velasquez-Orta¹ and M. C. Flickinger²

 ¹ School of Engineering, Merz Court,
Newcastle University, Newcastle Upon Tyne, NE1 7RU, UK

² Chemical and Biomolecular Engineering,
Engineering Building 1, North Carolina State University, Raleigh, North
Carolina, 27695,USA

*Corresponding/presenting author (kamelia.boodhoo@newcastle.ac.uk)

We report the development of an
entirely new, scalable, solar energy driven microbial gas absorber-converter
technology by a novel combination of advanced photoreactive biocomposite
materials with a continuous thin film flow absorber design based on a spinning
disc concept. A methodology was developed to integrate Chlorella vulgaris
microalgae cells into a porous paper biocomposite for the first time with the
addition of the common natural biopolymer, chitosan which acts a biopolymer
bridging structure and charge neutralizer to significantly improve the
integration of the C. vulgaris cells within the paper matrix. A comprehensive
study was undertaken to assess and understand the effects of the main constituent
components of “fresh” C. vulgaris biocomposite paper highlighted that increases
in chitosan dosage, MFC content, and the total cell loading had the greatest
positive impact on the photoactivity of the immobilised microalgal cells
measured via a fluorescence technique. Interactions between chitosan dosage and
basis weight and between cell loading and MFC content were also deemed
significant. For instance, highest Fv/Fm values of the order of 0.6 were achieved
with the highest total cell loading of 2.4x10¹³/m² of
biocomposite surface and low MFC content (<20%) or with high chitosan levels
(75 mg) and low basis weights (50 g/m²). The microalgal cells in the
biocomposite have also been shown to thrive after exposure to long periods of
desiccation and shear stress where higher recovery of photosynthetic reactivity
was achieved at high chitosan levels and high total cell loading.

The proof of concept of employing an immobilized C.
vulgaris biocomposite paper on a spinning disc for CO₂ capture
has been demonstrated over a period of 15 hours. Using a biocomposite paper
containing 75 mg chitosan, 10% MFC, 8.8x10¹² cell loading/m²
of biocomposite surface and a basis weight of 95 g/m², high
photoactivity was maintained at a spin speed of 300 rpm throughout the duration
of operation, more so in the presence of bicarbonate in the liquid medium and
5% CO₂ in the gas environment (Experiment 1 in Table 1). The
immobilised microalgae in the biocomposite preferentially consumed the
dissolved CO₂ over HCO₃^-, as the dissolved CO₂
was consistently significantly lower during the SDBR runs with the C. vulgaris
biocomposite when compared to the control blank for Experiment 1 in Figure 1. On
average the liquid medium contained 54.9±0.9 % less dissolved CO₂
when compared to the blank SDBR runs. The CO₂ biofixation rate was
estimated to be 144.05 mg hr^-1 when calculated from the average
difference in dissolved CO₂ per hour between the biocomposite and
blank. Nayak et al. [1]
reported a corresponding value of 41.6 mg L^-1 hr^-1 (996.4
mg L⁻¹ d⁻¹) or 83.2 mg hr^-1 for
their 2 L reactor volume when simultaneously supplying supplementing 1% (v/v)
CO₂ and NaHCO₃ . Given that the result reported by Nayak
et al. was from a 2 L flat-panel photobioreactor compared to the C. vulgaris
biocomposite surface area on the disc of 62.2 cm², a considerable
enhancement of CO₂ biofixation is evident in the present study. Interestingly,
with the biocomposite paper securely fixed to the disc surface using a pH
neutral spray adhesive, practically all C. vulgaris cells in the
biocomposite successfully remained attached to the disc throughout the 15 hour
rotational period at 300 rpm (equivalent to 5× g at disc edge). This is a
significant achievement in the context of rotating biofilm technologies which
generally must operate under gentle rotations below 20 rpm typically to prevent
the microbial cells employed in wastewater treatment from detaching from the
surface of biofilm structure. Overall, the increased biofixation for a much
reduced spinning surface area and the high cell retention in the spinning
biocomposite highlights the process intensification potential of the SDBR.

The scaling up of the biocomposite-integrated SDBR
technology offers the potential of a high performance air filter, enabling many
different types of microbial uptake of waste carbon gases at industrial scales such
as CO₂ but also syngas (CO), biogas (CH₄), and volatile
organic carbon (VOC) pollutants. With carefully selected photosynthetic
micro-organisms, economically viable biotransformations of waste carbon gases into
useful products such as platform chemicals and liquid fuels may also be envisaged. 

Keywords:
bioprocess intensification; spinning disc bioreactor; biocomposite; microalgae;
C. vulgaris; CO₂ biofixation

Table 1.
Effect of carbon source on photosynthetic activity of C. vulgaris
biocomposite in SDBR

Figure 1.
Results for Expts 1,2 and 3 in a SDBR with control blank disk (CD) and the C.
vulgaris biocomposite (BC) A)pH B) Bicarbonate (HCO₃^-) C)
Carbonate (CO₃^2-) D) Dissolved carbon dioxide (CO₂)