Application of aqueous alkaline extraction to remove ash from algae harvested from an algal turf scrubber

Efforts to develop algae as a sustainable feedstock for the bioenergy economy have largely focused on cultivating microalgae for optimized lipid production. Because of this, filamentous algae are often overlooked in the algae biomass field because of their relatively low lipid content, although they do contain high levels convertible carbohydrates and proteins. Systems designed to produce filamentous biomass, such as algal turf scrubbers (ATS), have advantages over planktonic algae produced in raceway systems. Unlike raceway algae production, ATS systems do not require inoculum, nutrients, or CO2 inputs as these are readily obtained from the atmosphere or the natural waters feeding them. ATS systems are exceptional at removing nutrients from affected waterways. However, as a consequence of the growth environment (sand, silt and other suspended solids), and the composition of ATS biomass (diatoms), the periphytic biomass obtained from ATS can have very high ash, which represents non-convertible material that can complicate downstream conversion processes. We explored both physical and chemical approaches to remove ash with the goal of improving overall preprocessing costs and conversion yields while minimizing the loss of organic material. The simplest method for removing ash involved repeatedly rinsing the biomass with water at 25 °C, which removed 34.5 ± 3.4 wt% of the ash with no appreciable loss in biomass. When treated with 2.0 wt% NaOH at 80 °C, up to 87.8 ± 1.4 wt% of ash was removed. These severe conditions, however, also resulted in organic material losses of 29.9 ± 3.2 wt%. Ultimately, these results will inform future tests, both chemical and mechanical, and will provide input for models that identify energy bottlenecks and potential savings.