Algae are being put to work performing a unique double duty: cleaning up sewage waste while simultaneously producing biofuel.
All algae feast on phosphates and nitrogen-containing compounds, converting them to lipids. Some of these oils can be converted to biofuel, but only a few algal species produce lipids of the right type and quantity to be easily converted to fuel. In theory, though, algae are a perfect renewable fuel source. The main obstacle is that brewing the right nutrient mix can be prohibitively expensive.
Now, in work for a master's thesis, Eric Lannan, a mechanical engineer at Rochester Institute of Technology (RIT) in New York and colleagues have identified three types of microalgae - Scenedesmus, Chlorella and Chlamydomonas - that efficiently convert nutrients to fuel on a diet of municipal waste water, while happily living in its harsh, salty environment. In a lab test, it took just three days for the algae to gobble up 99 per cent of the ammonia, 88 per cent of the nitrate and 99 per cent of the phosphates in a broth resembling that from a domestic sewage treatment plant, turning themselves into rich sources of fuel even as they purified the water.
"People had looked at algae to clean waste water, others to make biodiesel," Lannan says. "We're putting those ideas together."
In a few months, Lannan's group plans to install a 4000-litre pond at a waste-water treatment plant near Rochester. F. Drew Smith, who heads compliance for the area's treatment plants, believes it is an experiment worth trying, as algae could yield several times more biodiesel than crops grown on the same amount of land (see Illustration).
Jeff Lodge, a microbiologist at RIT, says the process should be economically feasible for treating up to 200,000 litres of waste water a day - the load of a typical rural US treatment plant. "We can treat the water and yield enough clean biodiesel for a rural area to run a fleet of trucks," he says.
The process has two phases. During the first three days, the algae produce lipids. Then, once the waste water is depleted of nitrogen and phosphates, the algae respond to starvation by turning their reserve nutrient stores into even more lipids.
After six days, the algae can be harvested. The team plans to use a mechanical pressing method to extract the oil, leaving behind biomass that could be composted, fed into an anaerobic digester to make methane, or sold as a feedstock to make ethanol.
There are limitations, though. In order to work year-round, the ponds must be heated in winter. For this, the team suggests using waste heat from the treatment plant. Too many sunny days in a row can also slow production, as algae divert resources into producing protection from the rays.
And large treatment plants need to process and discharge water quickly; accommodating a pond in which algae-infused waste water sits for six days could be a problem.
Smith says his biggest plant, which processes nearly 390 million litres a day, could accommodate a pond about the size of a small swimming pool to start with. "If the algae process turns out to be highly productive, we can certainly find enough land," he says.