UC San Diego/Scripps use unique DAF process to harvest algae

By Jim McMahon | March 27, 2013

Since 2008, the San Diego Center for Algal Biotechnology (SD-CAB) has been conducting research into algal applications for biofuel. This has been a joint collaboration between several departments of the University of California/San Diego faculty and students, the University’s Scripps Institution of Oceanography, the Scripps Research Institute, as well as nonacademic industrial corporate participants. In 2011, the center moved beyond its campus-oriented algae research facility in San Diego and utilized a field laboratory in the Imperial Valley of California, about 100 miles east in the Sonoran Desert. One-acre ponds were made available by Carbon Capture Corp., which also outfitted some of the on-site lab necessities.

The field location, which SD-CAB used for two months, gave the center the opportunity to considerably expand its algae growing volume from in-laboratory vats to one-acre ponds. During this two-month effort, the ponds used by SD-CAB were the largest field laboratory of any academic institution in North America for the growing and research of algae for use as a biofuel.

Algae thrive in fresh water and seawater, and all types of water up to a salinity of about 0.5 percent (50 percent higher than seawater). It can grow in desert ponds, using high-saline water from aquifers that cannot otherwise be used. Algae growth is not dependent on a particular season, and can proliferate wherever there are nutrients and light.

Having performed much prior in-lab research with the processing of smaller quantities of algae, SD-CAB was well versed and outfitted with a) growing algae; b) handling the drying the biomass on racks to remove 90 percent of the water; c) extracting the oil with a screw press, then purifying it in a centrifuge; and d) converting the oil to biodiesel using a traditional open-processing reaction method. The algae would be tested first to determine the levels of free fatty acids and moisture content to determine what mixture of chemistry was needed, balancing the methanol and sodium hydroxide to effect the desired reaction. But the center had little experience with how to harvest algae from one-acre ponds, which posed a considerable obstacle for the group.

“We knew, starting off, that the biggest challenge would be the harvest,” said Kristian Gustavson, part of the UCSD student-led, algae-based biodiesel research study. “We could grow the algae all day long, and once we had the dry biomass we could turn it into fuel, but it was the matter of harvesting. We experimented with several methods that were not very successful.”

Gustavson attracted the interest of World Water Works, which had developed a unique dissolved air flotation system specifically equipped for the harvesting of algae from water. After careful review, SD-CAB selected World Water Works’ Algae Harvesting Technology Optimized (AHTO) dissolved air flotation system (DAF), which provided a highly efficient and compact treatment method.

AHTO is specifically tailored to algae harvesting. Pond water is pumped into the DAF where suspended solids in the form of algae are separated from the water by a process of dissolving air into the water under pressure, with the addition of a polyacrylamide flocculent. Upon release of that pressure, microbubbles form. These microbubbles interact with the algae particles, attaching to the biomass surface and affecting the particle density, causing them to float to the surface of the DAF. They are then skimmed with a chain and flight mechanism to a sieved Thickening Beach (trademarked). The Thickening Beach allows free water to be drained and thickens the algae particles, achieving an efficient liquids and biomass separation.

Patented Nikuni air dissolving technology is used to create the robust whitewater in the AHTO system, which saturates the effluent pond water entering the DAF with atmospheric air.

Heavy sand and grit particles settle to the bottom, where a timer function controls the removal. The clean water is continuously removed from the DAF and piped back into the ponds, allowing new pond water laden with algae to enter for separation.

“Within a few hours of receipt of the AHTO dissolved air flotation unit from World Water Works we had it up and running,” said B. Greg Mitchell, research biologist and senior lecturer at Scripps Institution of Oceanography, UC San Diego, and associate director of SD-CAB. “We harvested a good amount of algae, which was pretty easy to dry. It was high-quality biomass.”

“Our collaborators at the Biodiesel Awareness Action Network, at UCSD’s chemistry department, were ready to convert the biomass into fuel,” Mitchell continued. “We got the chemistry set and were able to harvest continually from the ponds. We added fertilizer to the ponds as it was being harvested to keep it at a steady state where it was still growing in a nutrient-rich environment, to help maximize the lipid content.”

The AHTO process is a highly efficient system for separating algae from liquids. The system can achieve biomass removal efficiencies exceeding traditional DAF performance. Up to 9,000 gallons of algae-laden water can be processed per minute, at a 95 percent capture rate, yielding up to 20 percent algae concentrations.

“The DAF that World Water Works provided greatly accelerated our ability to harvest algae in our Imperial Valley facility,” added Mitchell. “We harvested a lot more algae, and more efficiently than with anything else we have used. It is the kind of system that many in this industry should be looking at using. The AHTO dissolved air flotation unit certainly appears to be the most effective way to harvest algae for biodiesel processing.”

SD-CAB scientists plan to make sustainable algae-based fuel production a reality within five to 10 years. Its goal is to create a facility that provides a national and global model for the commercialization of algae fuel.

As the popularity of alternative fuels gains momentum, biodiesel, and specifically algal-based biofuel, continues to strengthen its position as an attractive option to offset petroleum-based diesel usage. The introduction of noncommodity feedstocks like algae, along with attractive U.S. federal and state subsidies both for biodiesel production and consumption, are inciting equipment manufacturers to develop better processing systems that are faster, safer and more efficient, such as the AHTO DAF system used by SD-CAB.

Author: Jim McMahon



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