A new dimension in algal cultivation – 3D printed structures with a range of buoyancies

Abstract

The free-floating cultivation of macroalgae is a fundamental requirement for the efficient bioremediation of land-based waste streams as this cultivation mode fully utilises the water column available for production, translating to improved areal biomass productivities. To achieve the free-floating cultivation of species grown from propagules and dependent on the attachment to surface structures, we have used 3D printed structures made of polymers with negative, neutral and positive buoyancies to investigate the potential to manipulate the position in the water column for cultivation. Small wheel-shaped structures with internal protected spokes were designed, manufactured and seeded with zoids of Ulva tepida. Their settlement was quantified at different locations of the settlement structure and, although similar across polymer types, approximately 80% less zoids were on the outside of the wheel shaped structures made of polyethylene compared to protected locations 3 days post-seeding. Subsequently, all structures were maintained over 31 days as free-floating cultures under outdoor cultivation in aerated buckets and paddle wheel driven high rate algal ponds (HRAPs). Both negatively and positively buoyant polymers maintained their buoyancies with increasing algal biomass over time in the static test, while the neutrally buoyant polymer showed the highest variation and became exclusively positively buoyant after 14 days of cultivation. Productivities were generally higher and more variable in aerated buckets ranging up to 28.4 ± 1.7 g dw m−2 day−1, than in HRAPs ranging up to 18.9 ± 3.9 g dw m−2 day−1. There was no clear effect of polymer type, and consequently buoyancy, on algal productivity. Similarly, all tested polymers had maximum productivities between 12 and 22 days for cultivation in aerated buckets and 7–17 days in HRAPs. This study highlights the potential to use 3D printing to create settlement structures with a range of buoyancies for land-based cultivation.