Innovative membrane photobioreactor for sustainable CO2 capture and utilization

The rising of greenhouse-gas emissions (GHG), during the last 200 years, is associated to the well known global warming phenomena. One of the main sources of CO2-equivalent GHGs emissions are the environmental protection plants accounting for 1.57% of the global emissions and thus sustainable and effective technologies for their mitigation are strongly needed. The current paper presents and discusses the assessment of an innovative membrane photo-bioreactor (MPBR) whose aim was the promotion of CO2 capture from conveyed flows, such as those from wastewater treatment plants (WWTPs), landfill and composting plants, for production and energy valorisation of algal biomass. Chlorella vulgaris microalgae strain was selected as photosynthetic platform for the abovementioned purposes. The influence of various operating parameters has been explored, including the photosynthetic photon flux densities , liquid/gas ratio and CO2 concentration in order to investigated their effects on carbon capture effectiveness and biomass production.

Figure 1 — Schematic diagram of the experimental set-up.

The apparatus used for capture and conversion of CO2 in algae biomass is depicted in Figure 1 and consisted of: i) a cylindrical reactor made of poly(methyl methacrylate) (PMMA), with internal diameter, height and wall thickness respectively of 29, 59 and 0.5 cm, corresponding to a volume of processable liquid of 40 L; ii) a self-forming dynamic membrane (SFDM) with filtering area (A) of 0.02 m2 composed of a PMMA rectangular frame holding on both sides two meshes and iii) an absorption column (AC) made of PMMA, with internal diameter and height respectively of 6 and 290 cm, and a resulting working volume of 8 L.

Long-term performances of a novel MPBR configuration for CO2 capture and biomass production have been evaluated. The experimental set-up investigated showed an efficient removal of CO2 which during the last stage (Stage IV) averaged a RE value of 78.72 ± 1.67% for an IL of 54.26 ± 2.85 g m-3 h-1 and a corresponding average value of algal biomass concentration of 928.98 ± 77.28 mg L-1. Moreover, the innovative membrane set-up used for the harvesting phase not only showed an efficient harvesting rate (41 g m-2 h-1), but also demonstrated the capacity of: i) controlling the oscillation of the pH of the permeate, ii) decreasing the SatO2 in permeate, iii) reducing the nitrate content in permeate; iv) reducing the turbidity in the permeate. Moreover, all those abovementioned chemo-physical parameters, if not controlled, could negatively affect the biological activity in aquatic environments receiving the process effluents.

Overall, this study confirmed the potential of closed MPBR for CO2 capture and biomass production as well as the synergisms of using SFDM as convenient and effective tools for the simultaneous harvesting of the biomass. Indeed, the use of SFDM filtration, cheaper alternatives to microfiltration and ultrafiltration techniques, and the resulting production of highly valuable algal biomass, may promote the MPBR investigated as a cost-effective and sustainable technology for GHGs emissions control.

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