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Home Stay Informed INFORM magazine Featured Articles
By Christine Herman
It sounds like the ideal scenario. Municipal wastewater—a rich concoction containing nitrogen and phosphorous—is fed to algae, which grow while simultaneously converting carbon dioxide into oxygen, treating the water, and creating biomass that can be harvested and converted to biofuel and other useful products.
Seem too good to be true? Some experts think so, while others continue to hold out hope that this approach will help keep wastewater from wreaking havoc on the environment while reducing our dependence on fossil fuels.
There may be a long road ahead for algal biofuels, but coupling algae growth with wastewater treatment could help pave the way. This feature presents examples of how researchers in both academic and industry settings are taking the lead in these efforts.
It is a matter of time before biofuels become essential for powering the planet. As the population grows and the fossil fuel supply continues to dwindle, we will need sustainable, bio-based fuels to take the lead in the energy market. But, biofuels, most notably ethanol, have received criticism because they often compete with food for land space, putting food stocks at risk.
For this reason and others, algae have come into the spotlight in recent decades as a source of biomass for fuel. The term algae refers to a diverse group of photosynthetic, eukaryotic organisms, which includes unicellular forms, such as microalgae and cyanobacteria, as well as multicellular macroalgae, more commonly known as seaweed. (For simplicity, we will refer to microalgae simply as "algae" in this report). Algal biofuels avoid the "food versus fuel" conundrum since they can be grown on nonarable land—typically in open ponds or enclosed photobioreactors. And, given their incredible diversity, algae have enormous potential for helping solve the world's energy crisis.
"There are conservatively 100,000 different species of algae, and up to closer to a million if you really dig into the literature," says Ryan Davis, senior member of the technical staff at Sandia National Laboratories (Livermore, California, USA). "Not all of those are going to grow really fast, and not all of them will be oleaginous like we'd like for biodiesel, but there's definitely some opportunity there."
Algal biofuels: the promise and the challenge
Algae have attracted the interest of biofuels researchers worldwide because of its high-oil content, which can be extracted and converted into biodiesel. Its other components—including proteins, carbohydrates, and metabolites—can be separately processed into valuable products using a variety of conversion technologies, including chemical, biochemical, and thermochemical processes. The process of extracting and isolating the individual components can be energy-intensive and difficult to optimize. So, others have developed methods to convert the entire algal biomass into biocrude, which can be further refined and blended in with petroleum as a "drop-in" fuel.
There are many strains of algae that naturally grow on marine water, and those that require freshwater can be grown on municipal wastewater, which contains nitrogen and phosphorus—nutrients necessary for growth. Biofuel production from algae has the potential to be a nearly "carbon-neutral" process, meaning the amount of carbon released to produce and process it is roughly equal to the amount of carbon that was captured in the process of growing it (see http://tinyurl.com/algae-carbon-neutral (pdf)). Also, on average, algae have a higher oil yield than traditional oilseed crops—anywhere from 30 to 100 times more oil per acre than corn and soybeans.
"We and others have demonstrated that you can grow algae to produce 70% of their dry weight as lipids that can be converted to biodiesel in a one-step process," Davis says.
So why has biofuel made from algae not taken the energy market by storm? A large part of the explanation comes down to the economics.
Peter Pfromm, chemical engineering professor at Kansas State University (Manhattan, USA), says there are two major cost considerations: the infrastructure for growing and processing the algae, and the ongoing costs for operating the facilities. The facilities can be quite expensive, he says, but "then you hopefully will make that money back over time and make some more money by selling biodiesel in the marketplace."
The problem is that algal biofuels are not cheap to make, and there is only so much you can do to make the process more efficient. "There is not unlimited room for improvement because ultimately you have so many photons coming from the sun per square meter and time," Pfromm says. "That is not something you can change . . . That's the physics that limits the process."
Pfromm and his coworkers conducted a 2014 research study on the economic feasibility of algal biodiesel, in which they concluded that the success of algal biodiesel will require "significant and continued support" from the government in the form of regulations and financial incentives. The analysis was performed with optimistic assumptions about algae productivity, such as an estimated yield of 50 grams of biomass per square meter per day. The academic study was performed without external financial sponsorship of any kind, Pfromm says, emphasizing that since he and his collaborators do not perform research on algal biofuels themselves, they did not have a bias going into the analysis to favor one outcome over another.
But, the economic challenges aren't stopping researchers from trying to make algal biofuels a success—with or without supportive governmental policies.
Bruce Rittmann, a professor of environmental engineering at Arizona State University (ASU; Phoenix, USA), is working on ways to maximize the output of algal biomass grown in photobioreactors with the goal of increasing its overall production rate. "The No. 1 holdup is we can't get high enough productivity to make it economically viable," Rittmann says. "If we had that now, everybody would be doing it. But, we're doing various types of research and development to get over this hump."
Photobioreactors, since they are closed systems, allow the algae to grow in a highly controlled environment, which results in higher yields. But, they are about 10 times more expensive than traditional open pond systems. "There's no question that you can get better performance in a photobioreactor," says Jim Flatt, president of the Genovia Bio Division of Synthetic Genomics (La Jolla, California, USA). "The challenge has been that that incremental performance benefit hasn't usually been justified by the added capital investment."
For this reason, those interested in producing algal biofuels tend to lean toward open pond systems, which come with their own challenges, namely contamination and water evaporation. Researchers in the field seem to agree that photobioreactors will likely be reserved for higher-value algae-based products, such as nutritional and food ingredients, and the more affordable open pond systems will be used for generating biofuels.
Still, it's not enough for algal biofuels to simply sell for greater than the cost of making them. They have to be produced cheaply enough to compete with other fuels on the market. "If you can get an energy return of greater than one, you're doing something," Sandia's Davis says. But at this point, petrochemicals are getting a much greater return on invested energy, so it's hard to compete.
Because of the tough technical challenges, Flatt says Synthetic Genomics is simultaneously pursuing algae for both intermediate- and high-value products in addition to a separate research effort with ExxonMobil (Irving, Texas, USA) on biofuels. In an effort to make algal biofuels economically feasible, the company is exploring various techniques, including genome editing and genetic modification, to improve photosynthetic efficiency and direct more of the carbon absorbed in the process to lipid production.
"Biofuels remain a strong interest for the company, but we also know they're very long-term," Flatt says. "The average selling price of fuel is still relatively low compared to many other commodity products. As such, it's really going to require the most mature technology available in order to be economical." In other words, in the near-term, the company plans to work its way up the learning curve with respect to growing, processing, and scaling up algal technologies while getting intermediate- and high-value algae-based products into the market. Then, when the economically competitive algal strains are ready for development into biofuels, the company will be have the know-how to cultivate and process them in an economical way, which will increase their chances of succeeding in the energy market.
Other companies, such as Muradel Pty Ltd (Whyalla, South Australia), are going full steam ahead toward using algae to create biofuels. Researchers at Muradel use a type of microalgal strain that is halophytic, meaning it can grow in shallow, open raceway ponds in water containing high salt concentrations—up to four times the level in seawater. "We knew from the beginning that to be sustainable we had to minimize the use of freshwater, which is a resource under enormous pressure in Australia," says Andrew Milligan, Muradel's business development manager. So, they have chosen an algal strain with demonstrated high productivity and high oil content that can grow in salinated water, such as seawater or the brine output of a desalination plant.
Since the company is based in Australia, which has an abundance of nonarable land, Milligan says that production of algal biofuels will not compete with food production. "We calculate that an area of 10,000 km2 dedicated to microalgae production could provide all of Australia's liquid transport fuels," Milligan says. "That equates to less than 0.2% of Australia's total land area." Although the company is still "precommercial" (meaning it cannot yet compete on price with fossil fuels), it hopes to create a full-scale commercial and profitable operation, comprised of 1,000 hectares of algal ponds, by 2019. It is also looking into ways to create valuable byproducts of fuel production, which could help boost the bottom line.
Coupling algae growth with wastewater treatment
Another potential avenue for making algal biofuels more profitable involves growing algae with the help of something that exists in great abundance all over the world: wastewater. Since wastewater is rich in nutrients that algae need to grow, some researchers think we need to start viewing it not as something to be discarded but as a resource to be exploited for the benefit of the environment.
Using algae to treat wastewater is nothing new. Microalgae are well-known for their ability to remove nutrients, organic contaminants, and heavy metals from the water they are cultivated in. If not removed from wastewater prior to its release into the environment, these components can wreak havoc, causing harmful algal blooms and areas known as "dead zones," where uncontrolled algae growth causes oxygen depletion in the water, killing off fish and creating major ecological catastrophes, Sandia's Davis says. So, wastewater treatment with algae is important—but even better if you can then harvest the algae to create useful products, such as biofuel.
"If we can start directly converting our waste . . . into fuels, then we're doing good on two ends," Davis says. "We're supporting our domestic independence from foreign fuel resources, we're diversifying our liquid fuels, and we're also solving the problem of uncontrolled nutrient release" into our water streams.
Jonathan Trent, adjunct professor at the University of California, Santa Cruz (USA) and a scientist with NASA (National Aeronautics and Space Administration; Ames Research Center, Moffett Field, California, USA), agrees. "There are huge amounts of useful nutrients in the waste streams from our cities and we're basically throwing it away," says Trent, who is working on a project known as OMEGA, which stands for offshore membrane enclosures for growing algae.
Currently, in San Francisco (California, USA), wastewater is processed by screening, settling, and biological nutrient removal to a level of cleanliness known as "secondary" treatment before it is released into the bay. Secondary-treated wastewater still has plenty of nutrients in it to grow algae. That is usually not a problem in San Francisco Bay, but it is a great resource for the algae grown in the OMEGA system, Trent says. By simultaneously growing algae and treating wastewater, companies can save money by not having to pay for other means of biological nutrient removal. Additional cost savings can be expected if the entire operation is moved off of land to an enclosed bay (a key component of the OMEGA vision for wastewater treatment and algae production). The water in the bay cools and mixes the algae with the wastewater to stimulate growth, and the offshore infrastructure can be coupled with aquaculture and energy-generating technologies, such as wind turbines and water-cooled solar panels. "As you start adding these value-laden additional activities, then the cost of the fuel, which is part of this overall system, comes down," Trent says. "The OMEGA project is about food, it's about water, and it's about energy from solar and from biofuels, but it's also about cleaning up the environment—all things we need to improve to sustain our lifestyle in a world of seven billion people and counting."
Trent and his colleagues have performed techno-economic analyses that suggest that if the OMEGA approach were scaled up to accommodate all 65 million gallons (or roughly 250 million liters) of wastewater produced by one of the treatment plants in San Francisco, the system would generate roughly 3 to 6 million gallons of biofuel a year. Instead of trying to grow a pure culture and go through the process of dewatering and extraction to create biodiesel, Trent's team uses a chemical process known as hydrothermal liquefaction to convert the partially dewatered but wet algal biomass into a biocrude that can be further refined and blended with petroleum fuels. They are also working on methods to use an underwater anaerobic digester to convert the biomass into natural gas. The products of the anaerobic digester—methane and carbon dioxide—can be used to produce either syngas (a mixture of carbon dioxide, carbon monoxide, and hydrogen) or burned to create heat to produce energy by turning turbines. The carbon dioxide produced by both the digester and by burning the methane is pumped back into the OMEGA system to feed the algae, creating a closed system.
The team has also developed an efficient method to recover the wastewater from the algae cultures as "super-clean water" ready to be used for industrial purposes or even drinking water. This wastewater-recovery technology combines algae pretreatment with forward and reverse osmosis (see sidebar). "The OMEGA system is about looking at wastes as resources and creating an 'ecology of technologies' by optimizing everything the system and the environment has to offer," Trent says.
Not everyone thinks coupling algal biofuels with wastewater treatment is a good idea. ASU's Rittmann, whose research directions include algal biofuels and wastewater treatment, although not a combination of the two, writes in an email, "I can describe much better ways to revolutionize wastewater treatment while not 'messing up' microalgae technology." In an interview, Rittmann explained that wastewater treatment with algae is "a very old technology . . . and it's generally proven to be not very reliable in terms of producing very good quality wastewater treatment." The efficiency of the process plummets on days when the sun is not out or the temperature is not just right. Add in the fact that wastewater is a highly variable stream and the whole approach is "not really commensurate with having a high production system to produce a lot of algae." In short, if you try to both grow algae and treat wastewater, Rittmann doesn't think you will be able to do either one particularly well.
Trent agrees that algae can be finicky. "And, if you're trying to grow a certain species that is a high oil producer, you may be frustrated by trying to do it in a wastewater environment," he says. "But, if you're open to just [creating] biomass, and your system is a community that is being enriched by these incredibly high concentrations of nutrients," there are ways you can manage the community to keep it working optimally.
This is precisely the approach Algae Systems (Daphne, Alabama, USA) takes to ensure that the algae they grow on wastewater are able to thrive. "We measure the incoming wastewater for the qualities we're looking for" and adjust the levels of nutrients and water accordingly, says Matthew Atwood, the CEO. The question the company considers is: "'Do we want to treat more wastewater or grow more algae?' and depending on what we're trying to do, we can change the recipe mix, so to speak."
Atwood says Algae Systems was inspired by the OMEGA project to create offshore membrane enclosures to treat wastewater, although the technology it has developed is different and separate from what NASA has done. The company's aim is to "develop technologies at the water-energy nexus and to try to reconceptualize the way that municipal infrastructure is developed." It is currently operating a demonstration plant and plan to take the next few years to gather data that will help it optimize the productivity of algae growth and wastewater treatment under various climate conditions. The plan is to take that information and learn how to scale up the technology and tailor it so that it can be applied in different geographic locations to meet the unique needs of the local environment and community. "We need to demonstrate that we can operate the system safely for a long period of time and find partners that are willing to work with us to develop these systems and scale it up," Atwood says.
Even if algae can grow on wastewater in useful quantities, and the wastewater can be adequately treated for safe release into the environment, Davis, sees another limitation to coupling the two processes, at least in the United States.
"The scale of liquid fuel consumption in the United States is much larger than the amount of nutrients we could ever recapture in any kind of realistic way," Davis says. "So, yes, we have this synergy between wastewater and algae, but there are limits to how far we can go."
That wastewater is a resource available in any industrialized locale may seem like a good thing for algae growers. The problem, says KSU's Pfromm, is that to exploit all available wastewater for cultivating algae, many small facilities will need to be built, as opposed to a few large facilities (the cost of transporting wastewater long distances would quickly cancel out the benefits of any resulting biofuels). "That makes it less efficient by definition," he says.
But, Davis is hopeful that the development of micromodules for processing algae grown on wastewater will help mitigate this problem. He and his coworkers are developing a technology for capturing, dewatering, and processing algae on a small scale. The scales will have to be adaptable, Davis says, and the techno-economics still have to be worked out to determine if the process is efficient enough to be viable. "It's a challenge, but I don't think it's insurmountable."
Even companies that are not trying to couple wastewater treatment with algae growth are looking into ways to exploit wastewater for biofuels. Using the same hydrothermal liquefaction technology that it uses to process algae, Australia's Muradel converts biosolids—one of the byproducts of wastewater treatment—into crude, and find that it has a high energy content that is suitable for mixing with petroleum fuels. "We are solving what was previously an intractable waste problem for wastewater treatment plants," Milligan says. "It's a win-win for both parties."
NASA's Trent thinks researchers and industries need to be persistent in working toward sustainable solutions if they are serious about putting a dent in fossil fuel consumption. "There are always going to be naysayers; these people who are arguing whether the cup is half empty or half full," he says. "But, the OMEGA project is saying no, no, no, you're using the wrong cup. We really need to move our thinking to how do we make an efficient system given that we need to make a [renewable] liquid fuel for the forseeable future . . . The right attitude is not to ask: 'What's wrong with the system?,' but 'How can we make this work and be sustainable?'"
Now, Trent says his goal is to find people who embrace the OMEGA vision and want to make it work in their location. He hopes to serve as a resource in those efforts. Since the first three years spent on the project were full of false starts and mistakes, Trent says, "I can tell people what not to do." As he explained in a 2012 TED talk he gave on the OMEGA project, like Thomas Edison, he has learned many ways the system does not work. And now he says, "I'm willing to go anywhere in the world to make OMEGA work for the sake of future generations.
"It's really about creating a sustainable system for society," Trent says, "and failure is not an option." (Although Trent works for NASA, he was not interviewed as a NASA civil servant, and his views are not necessarily representative of NASA's opinion or that of the US government).
Algae Systems's Atwood understands there are challenges at the water-energy nexus but says he's determined to find a way forward. "We're as much a wastewater treatment company as we are a fuel producing company," he says. "We believe fundamentally that the future requires a different relationship to waste, and . . . we cannot continue to develop a municipal infrastructure or [wastewater treatment] plants that externalize the costs of releasing large amounts of nutrients into the ocean and losing them.
"We have to close those cycles, and algae [are] nature's way of doing that," he adds. "We can grow the algae in a closed system and gain the benefits of it by being smart about how we deal with it downstream."
Algal biofuel technology helps convert wastewater to drinking water
Reverse osmosis of seawater is a technique commonly used to produce clean water and brine. The water is kept and the brine is considered waste, although it took as much energy to make the brine as it did to make the clean water. In a project known as OMEGA (offshore membrane enclosures for growing algae), NASA scientist Jonathan Trent and his colleagues use the waste brine from reverse osmosis to drive forward osmosis of algae-treated wastewater.
"The brine pulls algae-wastewater through forward-osmosis membranes, which cleans the water and dilutes the brine to about 1% salt," Trent says. "This is less than 1/3 the salt concentration in seawater, which is 3.5% salt. The forward-osmosis treated wastewater-brine is then processed by reverse osmosis to produce clean water and to recover the brine, which is used again."
The OMEGA wastewater, treated by algae, forward osmosis, and reverse osmosis, is recovered as clean water, and the process utilizes and detoxifies the brine, which is currently a problematic waste-product of reverse osmosis. In this way, the process of producing algae biomass with the OMEGA approach turns wastewater into drinking water, Trent says.
Christine Herman is a science writer for AOCS.