February 17, 2009 | General

Cultivating Algae In Wastewater For Biofuel

BioCycle February 2009, Vol. 50, No. 2, p. 36
New technologies are being designed to grow algae for their potential to produce more oil per acre than corn or soybeans.
Diane Greer

ENORMOUS clear plastic bags filled with churning green water lay on a hill at East Austin’s Hornsby Bend Biosolids Management Plant in Austin, Texas. The bags, which look like waterbeds, are essentially miniature greenhouses growing algae fed with wastewater and carbon dioxide (CO2) emissions from the plant. If all goes as planned, Houston-based Sunrise Ridge Algae will expand this small pilot facility to hundreds of acres, cultivating commercial scale quantities of algae to make biodiesel and animal feed.
Sunrise Ridge is among a number of start-ups and universities designing systems to grow algae in wastewater from municipal and industrial treatment facilities. The technology promises to produce biofuels from nonfood sources while cleaning wastewater and reducing greenhouse gas emissions. But the devil is in the details. “Growing algae sounds very simple,” says Sunrise Ridge’s CEO Norm Whitton. “It grows everywhere. But it turns out to be really a complex process.”
Algae are touted as a superior crop for biofuel production. They grow in all types of water and can be cultivated on land unsuitable for food production. The plant-like organisms employ photosynthesis to convert sunlight and CO2 into energy so efficiently that they can double their weight several times a day and have the potential to produce significantly more oil per acre than crops like corn or soybeans.
Algae thrive on a diet of wastewater and carbon dioxide, a greenhouse gas. “Waste-water is attractive because it has the nutrients algae need to eat, especially phosphates and nitrates,” Whitton notes. But wastewater alone does not provide a balanced diet. “Most wastewater has too much nitrogen and phosphorus and not enough carbon,” explains Tryg Lundquist, professor of Civil and Environmental Engineering at California Polytechnic State University. “The algae can’t assimilate all the nitrogen and phosphorus.” The solution is to add CO2.
CO2 is created at many wastewater treatment plants when sludge is incinerated or processed in an anaerobic digester to produce biogas (a mixture of methane and CO2). Hornsby Bend produces biogas in its digesters, which is used to generate electricity. Sunrise Ridge takes CO2 from the plant’s flue stacks and bubbles it through the plastic greenhouses. The enclosed greenhouses maintain desired CO2 concentrations by preventing the gas from bubbling out into the atmosphere. In initial testing, algae reduced nitrate levels in the wastewater to as low as two part per million.
Opinions differ on the best technologies to grow algae in wastewater. Open ponds offer the simplest and lowest cost solution, but are more susceptible to contamination and variations in temperature and light. At Old Dominion University in Virginia, geochemist Dr. Patrick Hatcher is leading a team working on efficient ways to grow algae at wastewater facilities in open ponds.
Because algae grown in open pond systems are susceptible to invading species, Hatcher is working an a system to grow desired algae species in closed tanks. Periodically the algae from the tanks will be infused into the bigger ponds. “That is our way of insuring that the guys in the big ponds are overwhelmed by the guys that we grew in tanks,” Hatcher says. “We give them a bit of an advantage.”
Nitrogen removal is a primary focus of the research, since local treatment facilities must reduce nitrogen in treated effluent flowing into the Chesapeake Bay by 2010, Hatcher explains. Current biological nitrogen removal techniques using bacteria are very expensive. “It costs three to four times more to remove nitrogen than our projected costs for algae systems,” he says.
Cost calculations for an algae system at a combined industrial and municipal wastewater plant in Hopewell, Virginia, which included the land, algae ponds and harvesting and biodiesel conversion technologies, were about $25 million. Costs for a conventional biological nitrogen removal system were quoted at $100 million. But despite lower costs, wastewater treatment plants are not sold on the technology. “They are skeptical about algae because no one has done this on a large scale,” Hatcher says.
Lundquist is looking at adding CO2 to high-rate algae ponds used at wastewater treatment facilities to achieve complete tertiary nutrient removal. “The trick is to eliminate the CO2 limitations of municipal wastewater,” he says. High-rate ponds are shallower than conventional ponds and designed with continuous raceways. A paddlewheel mixes the water, pushing it around the raceway. “It is simulating a shallow river, an environment that favors rapid algal growth,” says Lundquist. Higher algae levels produce more oxygen, which promotes the growth of bacteria that decomposes organic wastes. “Producing oxygen at a faster rate means you can treat the water with a lower residence time.”
Since the paddlewheel moves slowly, at two to three revolutions per minute, the system uses substantially less energy than conventional mechanical aeration systems. “High-rate ponds with settleable algal flocks and supplemental aeration in winter can save 40 to 70 percent of the power used in secondary activated sludge plants, on an annual average basis,” Lundquist explains. “During the high insolation of summer, the savings should be 80 to 90 percent.” Lab studies adding CO2 to wastewater have removed soluble nitrogen and phosphorus to less than 1 mg/liter with algae over a 3-day residence time. The next step is to add CO2 to a pilot-scale facility.
Sunrise Ridge’s plastic greenhouses, known as photobioreactors, permit more control of the growing environment, compared to ponds. However, the systems face high infrastructure costs, and an indoor system may require artificial lighting, which increases energy costs.
A key issue at Sunrise Ridge is lowering the costs of the greenhouses while maintaining a high production of algae, insuring a return on capital invested in the system, Whitton explains. Maintaining a high production rate is dependent on selecting algae strains with high oil content that grow well in the greenhouse environment. Oil content can vary from 4 to 50 percent by weight, depending on the species and growing conditions.
Whitton is testing local species and working with the University of Texas to select algae from its extensive collection. He is finding that algae that work well in the lab do not always perform well under field conditions, because the sunlight levels are higher and temperature, CO2 quality and nutrient levels vary more. “You pick a guy that grows really well under optimum conditions and you take him to a place that is not optimum, and you get different results,” Whitton says.
Pilot testing to better understand algae growth dynamics, to optimize the system and refine the oil separation systems will take another 18 months. Whitton expects to scale to a demonstration system in the next year or two. Scaling will require additional land and investment. Expansion plans are contingent on the economy and the price of oil. “If the price of oil is still around $40/barrel, I suspect we will be delaying that.”
In St. Paul, Minnesota, a pilot plant with two photobioreactors is growing algae in the basement of the solids management building of the Metropolitan Council’s Wastewater Division. The Council operates seven wastewater treatment plants; the largest processes about 185 million gallons a day. Solids produced during treatment are dewatered in centrifuges and burned. The centrifuges produce about one million gallon per day of liquid centrate with relatively high phosphorus content, which is recycled back into the plants.
The Council is collaborating with the University of Minnesota on the pilot bioreactor system intended to lower the phosphate content of the centrate, explains Bob Polta, R&D Manager for the Council’s Environmental Services. The University is researching algal species to use in the process and will take the cell matter and extract the oil to produce biodiesel. Initially, cylinder CO2 will be used to control pH and provide carbon for algae growth in the bioreactors. Eventually, the system could use CO2 produced when sludge is burned.
Polta sees the system as a potential low cost alternative for removing phosphorus and some of the nitrogen. “Our treatment plants now have a permit limit of 1 mg/liter for effluent phosphorus,” he says. “The regulatory agency has already informally suggested that we are going to have to go down to 0.3 mg/liter.”
Removing 100 lbs of phosphorous will result in a reduction of 700 lbs of nitrogen. “So if we treat 1 million gallons a day of centrate, we can remove most of the phosphorous and take a big chunk of the nitrogen out as well,” says Polta. The pilot plant will treat about 1,000 gallons a day of centrate. The reactors currently operate under artificial light. Expanding the system will probably require building a greenhouse to utilize natural light, according to Polta.
Long term, the goal is to produce biodiesel to power the Council’s fleet of buses. Polta questions if the project will produce sufficient biodiesel for the entire fleet, but believes the volume could be substantial. “It depends on what you assume for the oil content of the algae.”
Indianapolis, Indiana-based Algaewheel’s system was originally designed to reclaim fish waste to grow algae, which was then fed back to the fish. “We ended up in the energy business by accident,” says Chris Limcaco, CEO of Algaewheel.
About eight years ago, Limcaco started testing the system on different types of wastewater. “The goal was not to produce algae for energy. The goal was to provide an energy efficient wastewater treatment process.” Typically, wastewater treatment plants employ bacteria to breakdown organic matter. Because bacteria require oxygen, large blowers are used to aerate the wastewater. Aeration is an energy intensive process, accounting for 45 to 75 percent of a plant’s total energy costs.
Algaewheel employs algae in conjunction with bacteria. Algae consume CO2 and give off oxygen, which is utilized by the bacteria. The bacteria give off CO2 that is used by the algae. In essence, Limcaco’s system employs algae as a solar powered air blower.
The system is comprised of large wheels placed in wastewater tanks that promote algae and bacteria growth. Fins on the exterior of the wheel provide a surface area for the algae to grow. Algae attach to the fins via their root system. Bacteria congregate in the wheel’s interior. Low power blowers rotate the wheel, which is partially submerged in the treatment tank. As the wheel rotates, the algae is alternately submersed in the wastewater and then exposed to the sunlight. The bacteria remain submerged at all times.
As the algae grow, the rotation of the wheel sheers the algae. “It is like mowing grass,” Limcaco says. “Constantly harvesting the algae keeps it in an aggressive growth mode.” Algae sheered from the wheel are collected off the bottom of a downstream clarifier. The three-dimensional wheel maximizes surface area for algae growth while minimizing the system’s footprint. “With a pond or raceway you are limited by the square footage of the pond for growing area,” he adds. “With the 3-D design, for every one square foot of footprint we get 10 square feet of algae growing surface.”
Until a viable commercial oil extraction system is available, Limcaco plans on dewatering the algae/bacteria mixture in a centrifuge to 70 percent solids. Tests to gasify the solids yield a BTU content equivalent to burning natural gas and should produce enough electricity to power a plant, according to Limcaco. Flue gas from the gasification system will supply additional CO2 to the tanks. “All we have to do is direct the exhaust from our thermal system to the blowers used to rotate the Algaewheels,” Limcaco explains. The system can be retrofitted into existing wastewater treatment plants. In colder climates the tanks housing the Algaewheels will need to be housed in a greenhouse. Currently the company has five small-scale systems, ranging from 3,000 to 7,000 gallons/day.
While using wastewater to produce algae looks good on paper, there are still a lot of unknowns, says Polta of the Metropolitan Council. “I get a little concerned that things are hyped too much and people do not realize that there is an awful lot of work that needs to be done to get this to work.”
Whitton of Sunrise Ridge points to a recent DOE Algae Roadmap Conference that identified six major areas requiring more research. “We are not quite there yet. It is going to take a bit more time and a bit more money.”
p. 38 Sidebar
SEATTLE-based Blue Marble Energy’s system to grow algae is based on studying natural algae blooms. The company determined the special cocktail of nutrients and conditions causing the blooms, explains Kelly Ogilvie, Blue Marble’s President. Its process mimics nature’s methods in treatment facilities by supplementing the wastewater with, “a combination of metals, nutrients and a couple of other things that are in our secret sauce.”
The Blue Marble equipment used to grow algae can be placed in existing clarifying tanks and open lagoon systems. “It looks a bit like a coffee filter that latches onto the clarifier,” Ogilvie says. Algae are harvested by raising the cone-shaped device out of the water. “The algae gets forced into the bottom and sucked out into a holding tank.” The system significantly lowers the BOD (biological oxygen demand) and nutrients in the wastewater.
The company has tested 10-foot and 20-foot diameter models that “work like a charm.” However, says Ogilvie, “going from 20 feet to 140 feet in diameter, therein lies the technology and engineering challenges.” Algae in the systems are comprised of wild species. “You have a variety of organisms that lead to overall higher densities,” he adds.
The process is not targeting algae for the production of oils for biodiesel. Instead, the algae biomass is fed to other bacteria. “So you have bug eat bug,” Ogilvie says. Different strains of bacteria consume different parts of the algae and in turn produce a variety of products such as anhydrous ammonia and organic acids. A by-product of the process is biogas.
Organic acids are reacted into bioesters, chemical building blocks used to make a variety of biochemicals, which represent an underaddressed market worth significantly more money than biodiesel, according to Ogilvie. A demonstration plant is under construction that will house three 10,000 gallon tanks. The goal is to produce 10 dry tons/month, or about 100 wet tons. The demonstration will use both algae from wastewater treatment plants and algae harvested from the ocean.

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