Sequestering floating biomass in the deep ocean : "Sargassum ocean sequestration of carbon" (SOS Carbon)

Abstract

Sargassum seaweed has been growing ever faster due to climate change and agricultural runoff and has become a terrible problem for the Caribbean. The floating algae has been inundating Caribbean coasts in unprecedented and increasing quantities for the past decade and there are no indication of abating. When the seaweed makes landfall, it dies, rots, dyes water dark brown, creates an eyesore, and emits a terrible smell ("rotten egg" smell of hydrogen sulfide). Moreover, sargassum negatively affects tourism (decreased by as much as 25%-50% in some locations), fishing (decreased harvests & damaged equipment), industry (clogs in power plant cooling intake & fumes/particulate corrosive to machinery), human health (hydrogen sulfide & heavy metals), standard-of-living (civilian coastline with no resources to address), coastal ecology (eutrophication), and the global climate (methanogenesis in landfill & anaerobic coastal water).

Responses thus far have been bootstrapped (manual cleanup predominant) and technologies recycled from other industries --

containment barriers, large conveyor systems, excavators, bulldozers. These measures haven been proven to be costly, insufficient, and unsustainable. This thesis presents the strategy of "sargassum ocean sequestration of carbon" (SOS Carbon) based on the discovery that sargassum pumped to a critical depth in the ocean ("pumping-to-depth"; identified as ~150-200m) is rendered negatively buoyant by the ambient hydrostatic pressure (air bladders that make the algae buoyant are compressed) and continues sinking to the ocean floor. Pumping-to-depth is a simple and energy-efficient (<<<10MJ/m3) process that could generate carbon offsets (if sargassum deposited sufficiently deep; estimated to be >0.25 tCO2/tonne wet sargassum including process emissions) and enabled the design of ideal systems for low-cost, effective, long-term sargassum management. Presented herein are designs for two physical sargassum collection/disposal systems, "in-situ" and "ex-situ".

This thesis documents initial investigation, early experimentation, systems analysis, concept selection, proof-of-concept tests, and the design/construction/testing of a full-scale "SOS Carbon Pilot" vessel.