Aeration Rate and Power Requirements for CO2-replete Cultivation of the Red Seaweed Devaleraea Mollis (pacific Dulse) in a Tumble Tank

The red seaweed Devaleraea mollis, commonly known as Pacific dulse, is a sustainable source of healthy food, food ingredients, and plant-based proteins. Land-based aquaculture of red seaweed typically employ tumble culture systems, where compressed air is delivered to the bottom of the tank to provide gas exchange and promote mixing. However, aeration is a potentially costly process because compressed air must be bubbled into the liquid, and power is required to drive the air compressor. In this study, fundamental relationships between aeration rate, biomass productivity, and aeration power consumption were developed for tumble tank cultivation of the red seaweed Pacific dulse under CO2-replete growth conditions. The biomass productivity (g biomass L-1 d-1) of D. mollis in tumble culture was measured over seven weekly harvest cycles over a range of aeration rates (0.086-0.684 L air L-1 liquid min-1) for fine-bubble (1-5 mm) and coarse-bubble (2-3 cm) aerators, and then correlated to the CO2 transfer rate (CO2-TR, mmol CO2 L-1 h-1). The CO2-TR consolidated aeration rate, CO2 partial pressure, boundary layer diffusion, and gas sparger characteristics (bubble size, interfacial area) into a single parameter. This analysis required measurements of the gas-to-liquid mass transfer coefficient (kLa) for estimation of CO2-TR, and pressure drop across the gas diffuser for aeration power consumption. CO2-replete biomass productivity required a CO2-TR that was three times the CO2 demand. For fine-bubble aeration, both CO2-TR and aeration power consumption were higher than for coarse-bubble aeration over the range of aeration rates tested. However, when the aeration rate needed to achieve CO2-replete biomass productivity was approached, fine-bubble aeration required less energy consumption due to lower aeration requirement. The optimum was achieved at an aeration rate of 0.26 L air L-1 culture min-1 using fine-bubble aeration, which provided an areal biomass productivity of 490 g FW m- 2 d-1 (fresh mass) or 32 g AFDW m- 2 d-1 (ash-free dry mass), at an aeration energy power requirement of 1.5 kWh/kg FW (22 kWh/kg AFDW). Overall, this study showed that aeration systems can easily limit biomass productivity and result in high aeration energy consumption per kg of biomass produced if the aeration processes are not thoughtfully considered. The key tradeoff was that aeration energy consumption increased significantly as CO2-replete biomass productivity was approached. Therefore, optimization would require the use of an aerator system that provides the best possible kLa and lowest pressure drop at the lowest possible aeration rate.