The motivation to overcome the challenges with respect to environmental and economic sustainability of microalgal culture for biodiesel production and other renewable products stems from the significant advantages of the microalgal sys­tem as a biomass source. These include the potential to use nonarable land for microalgal cultivation, the homogeneity of the biomass formed and the ability to process all components, the much-improved oil production per unit area (15 to 300 times greater), the higher growth rate, and the photosynthetic efficiency of microalgae compared with terrestrial plants (up to tenfold increase) (Chisti, 2007; Schenk, 2008; Rodolfi et al., 2009). Freshwater, seawater, brines, and wastewater are all potential water sources for algal growth (Vasudevan and Briggs, 2008). CO2 uptake by the autotrophic algae enables CO2 cycling through uptake for bio­mass generation and release on fuel combustion. Furthermore, the multiple energy forms attainable from microalgae span liquid fuels, and heat and electricity genera­tion, enabling ongoing support of existing technologies while developing a reduced carbon economy. On attaining an energy economy in which dependence on carbon combustion is reduced, the technology lessons learned through achieving environ­mental and economically sustainable algal biomass will readily be transferred to the production of carbon-based commodities with simultaneous carbon sequestra­tion or cycling.

The NER and LCA studies conducted to date have highlighted the great sensi­tivity of the GWP, fossil fuel requirements, and NER on the productivity of algal biomass and of algal oil attainable in the algal cultivation process. While maxi­mum specific growth rates and lipid content are partly defined through the algal species selected, culture conditions may be used to enhance these through improved light supply, mass transfer, and mixing. The energy requirement of the bioreactor to achieve mixing and mass transfer is a major contributor to the energy requirement of the integrated algal process, as is the CO2 provision to the reactor. Based on the volumetric concentrations attainable, pumping energies can be defined.

Supply of nutrients, yield of products from these nutrients, and recycle of unused nutrients impact GWP and fossil fuel requirements significantly. Opportunities exist in selecting algal species able to scavenge low nutrient concentrations, having reduced nitrogen content, as well as to utilize nitrogen and phosphorous resources efficiently, either by their provision from wastewater or through their recycle.

The typically dilute biomass concentrations required to minimize light limitation result in the need to process large culture volumes; hence, natural flocculation to facilitate settling is beneficial. In downstream processing, the most important factors pertain to the ability to process wet biomass, thereby eliminating the drying process, as well as the ability to recover product from the algal cell readily, thus minimizing the requirement for conventional, energy-intensive cell disruption.

As reported by Harding (2009), the production phase typically has the greatest impact on the overall LCA; hence, its optimization is required in the first instance.

Opportunities to improve the economics correlate well with the environmental analysis with respect to operating costs. These highlight mass transfer and mixing, provision of CO2, provision of nutrients, biomass recovery, and the avoidance of rigorous drying. More importantly, the economic studies suggest that algal biofuel technology requires further enhancement prior to its economic feasibility as a stand­alone technology. However, opportunity exists to establish a cost-effective algal biorefinery delivering a combination of products such as biodiesel, biogas, animal feed, and protein extracts. To this, high-value products may be added. Further, the predicted cost of algal biomass positions it attractively as a raw material source for bulk products, including fuels, chemicals, materials feeds, and food supplements.

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