Algal Bioflocculation for Solid-Liquid Separation

Algae production in conjunction with wastewater treatment is a low-cost path to algae biofuels. Whole biomass is available for conversion to biofuel because algae biomass co-product sales are not required for good economics. Instead, added revenue is derived from wastewater treatment. Existing wastewater treatment pond facilities covering hundreds of acres are the only producers of microalgae approximating the huge scale envisioned for future algae biofuels systems. A key requirement for algae biofuels is a lower cost harvesting process than the current commercial process of chemical coagulation and separation by dissolved air flotation or sedimentation. The only option of sufficiently low cost for algal biofuels production is sedimentation of bioflocculated algae. In bioflocculation, algal cells aggregate and grow into large colonies and flocs. After flowing out of paddle wheel mixed cultivation ponds, flocs readily settle to the floor of simple settling basins. From the settling basin, biomass can be pumped as a slurry to thickening equipment such as centrifuges, and the output algal paste can be processed to biofuels. However, a lack of highly reliable >90% harvest efficiency) bioflocculation has limited this technology. Developing methods to manage bioflocculation and thereby improve its reliability is the goal of the proposed research. Management of bioflocculation requires cultivation of suitable algal strains under specific operating conditions. Strains with large cell size >20 micrometers) or that grow in colonies facilitate the process. It is possible to maintain cultures of microalgae that exhibit such properties through a selective process of enrichment cultivationalgae that bioflocculate are harvested and used to inoculate the next culture. Cultures are operated to promote bioflocculation, allowing for efficient and stable harvesting. Enrichment cultures are used in the popular activated sludge process in which bioflocculating bacteria are enriched in conventional aerobic wastewater treatment. This process will be adapted to the needs of microalgae harvesting in this project.

The research will be done in outdoor raceway ponds, the only scalable technology for algal biofuels production. Several operating conditions will be tested over six months to test both algal strains and the conditions needed to achieve the >90% settling efficiency required for affordable biofuels. The growth medium will be treated wastewater, a low-cost source of water and nutrients. Energy return-on- investment and cost competitiveness will be compared with other biofuel and wastewater treatment technologies. In Phase 2, the developed strains/process will be scaled up at full-scale raceway systems already operating in California for wastewater treatment, providing sufficient biomass for pilot-scale algae-to-fuel conversion. Successful commercialization would reduce costs of wastewater treatment and open doors for microalgal biomass use in biopharmaceutical, nutraceutical, pigment and biomaterial industries.