The ICARUS Floating Membrane Photobioreactor for Microalgae Cultivation in Wastewater: Advancing Technology from Lab to Field Prototype

The ICARUS membrane photobioreactor, conceptualized and preliminarily tested at the University of South Florida, provides a viable platform for algae cultivation using wastewater as a growth medium. Progression of the technology is tracked using NASA’s Technology Readiness Level scale, and is herein described. Through the work described in this manuscript, the technology has evolved from TRL 3 to TRL 5. Completing the suggested future work will readily move the technology to TRL 6 and beyond. The first study investigated the co-location of algal cultivation with wastewater treatment. This coupling provides a steady source of water and nutrients for algal growth. However, wastewater microbiota has the potential to positively (through symbiosis) or negatively (through grazing or competition) affect algal growth. A wastewater sample was collected and filtered with two membranes; the four series were: unfiltered control, microfiltration (MF- 0.1μm pore size), ultrafiltration (UF- 40kDa MW cutoff), and growth media as reference. Cultivation was conducted with diurnal lighting and temperatures simulating Exeter summer (i.e., 16-hour days with13-20°C temperature extremes). Nutrient removal and growth rates were comparable for all growth mediums. However, high-resolution growth curves show differences in diurnal growth fluctuations, with MF series having the largest daily changes in growth curve amplitude. The OD fluctuations suggest diurnal changes in cell size, composition, morphology or agglomeration. While the exact causes for these diurnal density fluctuations are not currently known, there are several finding implications. First, single daily growth measurements miss significant oscillations in growth and may lead to misinterpretations of crop density. Second, diurnal fluctuations in algal density may suggest optimal times of day when harvesting is most efficient. Successful results from TRL 3, described in detail in a previous doctoral manuscript, led to more aggressive experimentation and assessment of ICARUS membrane performance. Mass transfer experiments were conducted to assess the relative effect of membrane diffusion, mixing, and neither (i.e., suspended control) on algal growth at this larger reactor size. Membrane separation has proven successful at maintaining physical separation between the algae crop and its predator, in this case, rotifers. Further testing of the system will include emphasis on enhanced algal growth for better biomass production. Results from mass transfer assessment yield insight into the need for backwash or mixing, and establishing a time to harvest parameter. Crop protection was tested against known algal predators. Further lab testing investigated crop protection effectiveness and mass transfer through the membrane over time. Crop protection capabilities were tested in the lab using two membrane types and pore sizes (50kDa PVDF and 7μm Nylon) against a common algal grazer, the rotifer. Membrane separation has proven successful at maintaining physical separation between the algae crop and its predator, in this case, rotifers, and further testing will aim to enhance algae crop yields within the system. Preliminary membrane foulant analysis was done using confocal microscopy. This testing revealed that cells were settling at the membrane surface; limited-conditions are likely contributing to cell death closest to the membrane surface suggesting the need for better mixing within the reactors. The next phase of this testing required reactor redesign and scale-up in order to move the technology to TRL 5 and beyond. A new prototype reactor was fabricated in-house using a vacuum-forming process. This new prototype incorporates a tapered and two-panel membrane bottom in an attempt to enhance algae harvesting and minimize membrane fouling. The new functional volume is 2.5 L compared to the 80 mL proof-of-concept jar used in TRL 1-3 testing. These new prototypes are tested and compared to the previous reactor iteration in the lab and outdoor tank setting with simulated wastewater, and then onsite in a municipal wastewater treatment plant clarifier. In the lab setting, the ICARUS reactors tested, both jars and prototype, outperformed the suspended control when comparing both culture density (jar, 0.328 g L-1; prototype, 0.307 g L-1; control, 0.208 g L-1) and day 0-5 specific growth rates (calculated based on OD taken at 680nm) (jar, 0.367 day-1; prototype, 0.476 day-1; control, 0.342 day-1). Similarly in the outdoor tanks, the ICARUS reactors tested, both jars and prototype, outperformed the suspended control when comparing both culture density (jar, 0.682 g L-1; prototype, 0.320 g L-1; control, 0.108 g L-1) and day 0-5 specific growth rates (jar, 0.343 day-1; prototype, 0.230 day-1; control, 0.207 day-1). WWTP cultures achieve nearly a 6x higher density and 10x higher percent solids. Both the jars and prototypes achieve comparable performance in the lab and outdoor setting, but enhanced performance is documented when grown in the wastewater. This enhancement is much more exaggerated in the prototypes likely due to the larger membrane surface area allowing for greater influence of WW water quality characteristics beneficial for algal growth. This body of research describes a functional algae cultivation platform successful at the TRL 5 level. With further work and testing, it could sustainably support algal cultivation, not only in the wastewater treatment plant setting, but also in any water body with suitable water quality to support algal growth