Harvesting technique could make algae a viable biofuel

Algae produce an oil that can be used to create a useful biofuel. Biofuels made from plant material are considered an important alternative to fossil fuels and algae, in particular, has the potential to be a very efficient biofuel producer.

Until now, however, there has been no cost-effective method of harvesting algae and removing water from it so that it can be processed effectively.

The technique from Sheffield University builds on previous research in which microbubbles were used to make algae blooms denser and consequently easier to harvest. However, removing the water so the algae could be harvested remained problematic.

Now, a team led by Mark Zimmerman in the Department of Chemical and Process Engineering at Sheffield University believes it has solved the problem.

Zimmerman told The Engineer: ‘What we’ve found is that we can separate the microalgae from the water or harvest it using microbubbles that are created by a fluidic oscillator.

‘A fluidic oscillator switches flows rapidly from one outlet to another, using feedback to do so with no moving parts. It is like an opening and closing mechanical valve that results in pulsing flow.’

Zimmerman explained that the process is much cheaper than attempting to make microbubbles through an industrial process known as dissolved air flotation, which generates bubbles that are too turbulent to harvest algae.

The fluidic oscillating system developed by Zimmerman’s team uses up to 1,000 times less energy to produce the microbubbles and, in addition, the cost of installing the Sheffield University microbubble system is predicted to be much less than existing flotation systems.

‘Our bubbles are made under laminar flow and we use practically no more energy than is required to make the interface of the bubble,’ said Zimmerman.

As a result of the low energy input, the bubbles rise very slowly, which is crucial as it means the algae particles can attach themselves to the bubbles more easily.

Two chemicals added to the liquid in the process — a flocculant and a coaggulant — help the algae bond to the rising microbubbles.

‘The idea is to create a surface on the algae particles that is hydrophobic so the microbubbles are attracted to it,’ said Zimmerman.

When the bubbles and the particles reach the surface, the flocculant and the coaggulant keep the algae in a fixed layer.

The blanket of algae can then be skimmed off the surface with something such as a belt skimmer, said Zimmerman. ‘In the lab, we use a knife,’ he added.

At the moment, the technique has only been demonstrated on a laboratory scale, but Zimmerman believes it can be scaled up relatively easily.

The technology is likely to have applications in lakes that have suffered from eutrophication. This occurs when a build-up of nutrients causes algal blooms to form and is often attributed to farming fertilisers entering water bodies.

The team has been in talks with Ken Shu, a scientific adviser to the Chinese government, to set up pilot-scale trials on remediating algal blooms in eutrophied lakes in China.

‘China has demographic drinking-water problems,’ said Zimmerman. ‘They’re running out because the lakes that used to be used for drinking water are all eutrophied with algal blooms.’