THINK OF THE WORD “raceway” and images of fast cars and chequered flags probably spring to mind. But for Associate Professor Tryg Lundquist, an environmental engineer at California State Polytechnic University, the term conjures up a very different image; one of a green algal slurry flowing through a complex maze of snaking ponds.
And this raceway doesn’t burn fuel. It creates it.
Welcome to the future of wastewater treatment as Lundquist sees it. Current methods of wastewater treatment are costly and energy-intensive, so Lundquist believes that by using algae to treat wastewater, treatment plants could save on costs, reduce greenhouse gas emissions and recover key nutrients for reuse.
As a by-product, he says, the algae could be used to produce biofuels, a move that could see wastewater treatment plants go from energy consumers to net energy producers.
“You can use less electricity, you can recycle the water, the nitrogen and the phosphorus, you can keep mixing the algae into a fuel, and it keeps rolling on in an endless cycle,” Lundquist says.
The key to treating wastewater with algae is raceways, small-scale artificial rivers consisting of a shallow network of looping ponds through which wastewater is passed. A paddlewheel gently pushes the water around the course, mixing the algae in as it goes.
The raceway design maximises exposure of algae to sunlight and fresh air, accelerating growth.
Lundquist has been engineering raceways for more than two decades. He says they are cheap to build and operate, yet they are highly efficient at treating wastewater. And the algal biomass produced is large enough to make the generation of biofuels viable.
Producing biofuels from algae is not a new concept, nor is using them for treating wastewater. But the idea of combining wastewater treatment with fuel production could be just the thing to remove the price barrier that has so far prevented the algal fuels industry from getting off the ground.
“People have been trying to produce biofuels from algae since the 1940s, but the problem remains that the cost is much too high,” says Professor Michael Borowitzka from the Murdoch University Algae R&D Centre.
The price of algal oil produced from purpose-built algal ponds ranges from around US$250 a barrel up to US$500 a barrel. To be an attractive alternative to petroleum, the price needs to be around US$100 a barrel.
As Borowitzka sees it, “The challenge to bring the cost down is still very, very large.”
But if production costs were offset by the capital and operating costs of wastewater treatment plants, algal fuels could be produced at a much cheaper price, says Dr David Batten, an industrial ecologist with The Temaplan Group and formerly with CSIRO’s Energy Flagship.
Batten estimates algal oil could be produced using wastewater for less than US$160 a barrel (US$1 per litre).
He adds that enough algal fuel could be produced from wastewater to potentially replace up to 10 per cent of Australia’s current diesel use. More populated countries, such as the USA and China, would have the capacity to produce even greater quantities of fuel.
“It would be good for the wastewater companies to start weaning themselves off the more costly mechanical methods and start thinking about algae as a solution,” he says.
How much of a solution algae can be will ultimately depend on land availability. “The downside of it is the land usage. You need a lot of land for a solar based system like algae,” says Lundquist.
Land is needed to accommodate the large network of raceway ponds needed to grow the algae. For example, a plant treating a modest four million litres of wastewater per day would require some five hectares of land. Sydney Water, for comparison, treats 1.5 billion litres of wastewater a day.
The land needs to be located close to the treatment plant, otherwise the electricity and transport costs to move the wastewater around make the process non-viable.
Climate will also play a key role in determining algal productivity, with cold temperatures and limited sunlight in high latitude countries like Canada and Greenland not being amenable to the high algal growth needed for biofuel production.
The favourable climatic conditions in Australia and New Zealand, however, place both countries in a good position for capitalising on biofuels production from algae, according to Dr Rupert Craggs from the National Institute for Water and Atmospheric Research in New Zealand.
Raceways are already used to treat the wastewater from a number of dairy farm in New Zealand and Craggs is now assessing the technology for treating municipal wastewater.
Successful trials have already been run with pilot-scale raceways and now a full-scale raceway system is being trialled at a wastewater treatment plant in Cambridge, New Zealand. If successful, the raceways will be permanently adopted into the treatment system.
Craggs has so far focused on producing biogas (methane) from the algae, as he sees this as the most viable way to produce energy in the near-term. “It’s easy and it’s simple technology. Wastewater treatment plant operators understand it, so why go with anything different?”
Biogas can be used for heating and electricity generation, or it can be compressed for export into a natural gas pipeline or used as a transport fuel.
But there is also the possibility of producing liquid fuels from algae as well, which could be substituted into aviation and bunker fuels as well as other transport fuels. Biodiesel can be made by extracting the lipids from the algae with solvents, while biocrude oil can be made by subjecting the algae to high temperature and pressure in a process known as hydrothermal liquefaction.
The latter process has been successfully trialled with algae grown in pilot-scale ponds at a wastewater treatment plant in Kansas, USA. Using this technique, it was estimated the treatment plant could produce up to 1,500 L of high quality biocrude oil per day from algae.
While the amount of fuel that can be produced from wastewater is not going to be a large scale solution to fossil fuels, if realised, it will go some way to bringing algal fuels to the market in the near-term.
And it will be the icing on the cake for a technology that promises to improve the sustainability of the wastewater treatment industry and ultimately result in cleaner wastewater reaching our oceans and rivers.
As Lundquist sees it, “Some technology efforts don’t have any immediate pay-offs, but with algae we can be getting little pay-offs as we go.”
Read article at the ABC