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.

A FILTER made from natural crystals may help dirty, carbon-emitting fossil fuels green up.

Zeolite crystals are a popular industrial “sieve” because their tiny pore spaces allow only certain molecules to slip through. Now a group of researchers led by Steven Kuznicki at the University of Alberta in Edmonton, Canada, and Anthony Ku at General Electric think they can be used to screen out the carbon dioxide produced when processing or burning fossil fuels.

The vast oil-sand deposits of Alberta are estimated to hold up to 170 billion barrels of recoverable oil, second only to Saudi Arabia. But extracting the oil accounts for millions of tonnes of carbon emissions each year, and the industry is growing rapidly. Much of the emissions are generated when superheated steam is pumped into the deposits. The steam pushes oil to the surface, but also picks up carbon from the oil and surrounding bedrock, which is then released into the atmosphere.

The team found that passing such dirty steam through a zeolite crystal, such as the mineral clinoptilolite, traps everything but water, hydrogen, helium and ammonia. Carbon captured by the crystals could then be buried underground or utilised.

This technique could cut emissions related to oil sands by a quarter, the researchers say. The sieves could also work in coal gasification plants, which currently use solvents to scrub CO₂.

“Passing the steam used to extract oil through a crystal ‘sieve’ could cut emissions by a quarter”

Long-term stability and creating reliable, leak-free seals between crystals are still challenges, however. “We must prove we can go from a rock to a piece of equipment that really works,” says Ku. He hopes to have a pilot plant running in Alberta in two years.