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Logistics systems vary based on technology, but these processes generally include harvesting algae from the cultivation system and then dewatering and concentrating the harvested biomass so that it's suitable for preprocessing into a product that can then be refined and upgraded into a biofuel.
Optimizing harvesting operations is critical to maximizing yields of algal biomass as well as ensuring sustainability. Recycling the remaining water back into the cultivation system reduces total water consumption. Algal biomass can be harvested from cultivation systems continuously or in daily or weekly batches; harvesting can be timed to take advantage of the algae's natural growth cycle.
Microalgae and cyanobacteria cultivated in water grow at dilute concentrations. Dewatering technology currently exists in wastewater treatment and mining industries to isolate solids from high-volume, low-concentration effluents, but these technologies may be too energy, capital, and chemical intensive to develop low-cost algal biofuels. Macroalgae and attached growth systems that cultivate multi-cellular algae do not require the same dewatering intensity.
Dewatered algal biomass may still be too dilute for effective preprocessing, and technologies are needed to boost these slurry concentrations. Concentration requirements are dictated by the preprocessing method that will be used. Centrifugation or membranes are typically used for concentrating the solids, but (as in dewatering) innovative adaptations and new technologies are needed to cut energy requirements and costs.
Algae preprocessing refers to the production of energy-dense intermediate products from harvested algal biomass. Preprocessing steps and requirements vary based on technology design and type of biofuel that will be produced; steps could include extracting lipids from algae cells or subjecting whole algae cells to high heat and pressure in a process, which is known as hydrothermal liquefaction. The end results of the preprocessing steps should be products that are suitable for refining into biofuels, such as diesel, jet fuel, gasoline, or ethanol. The Advanced Algal Systems Program works closely with the Conversion Program on intermediate production R&D.
To see an example of the potential algal biofuel intermediate production technologies, view Pacific Northwest National Laboratory's hydrothermal liquefaction process.
The algal components that will not be converted to biofuels can comprise 40%–75% of the biomass moving through the logistics system. Processing this remaining biomass can provide nutrients and power back to the production and logistics systems, or it can be converted to valuable co-products, such as animal feeds or commodity chemicals.