Open thin-layer cascade reactors for saline microalgae production evaluated in a physically simulated Mediterranean summer climate

While microalgae hold the promise for conversion of sunlight and CO₂ to a wide variety of products, the economics of algae processes are still debatable. We have designed an open thin-layer cascade photobioreactor for high-cell density cultivation of saline microalgae to advance economic microalgae mass production. Pilot-scale reactors with a surface area of up to 8 m² (cultivation volume 50–140 L) were constructed and evaluated using a dynamic climate simulation technology (light, air temperature and humidity) integrating natural sunlight and multi-color LED arrays for a highly realistic reproduction of the sunlight spectrum. Batch processes with Nannochloropsis salina were performed in these reactors in the physically simulated Mediterranean summer climate of Almería, Spain – an ideal location for outdoor microalgae cultivation. Two reactor variants were examined: one with a smooth but expensive rigid channel made of polyethylene sheets, and one with a more uneven but significantly less expensive channel made of pond liner. Maximal intra-day growth rates of 1.9 d⁻¹ were observed at a cell density of 1–3 g L⁻¹. The maximal cell density of 50 g L⁻¹ was obtained within 25 days. These high growth rates and cell densities markedly exceed literature data. No difference in growth between the channel variants was observed. This suggests that cost-efficient large-scale thin-layer cascade reactors with inexpensive pond liner channels are feasible. The high cell density allows a reduction of harvesting cost. Optimal process conditions were identified by analyzing the batch and daily economic bioprocess metrics: At a cell density of 17 g L⁻¹, an areal biomass productivity of 25 g m⁻² d⁻¹ (volumetric productivity 4 g L⁻¹ d⁻¹) and a photosynthetic conversion efficiency of 4.6% were observed. The reactor design is discussed in detail to encourage further advancement of thin-layer algal cultivation technology.