Politicians and scientists from all over the world are gathering this week in Stockholm to discuss global and local problems concerning water management. One of the many topics on the agenda is how to organise more sustainable systems for wastewater treatment.

Wastewater treatment is no minor issue because treatment plants are huge energy consumers. First, a lot of power is needed to “air” the bacteria that break down the organic components in the water. Second, it takes a lot of energy to get rid of the remaining sludge. In 2010, Flanders needed 733 gigawatts of electricity to power its water treatment plants. That’s about 3% of the total electricity use.

However, at Ghent University’s Laboratory of Microbial Ecology and Technology (LabMET), scientists are working on a ambitious solution: the so-called microbial fuel cell, or MFC. The theoretical concept behind it is far from new: In 1911 scientists wondered if they could lock up certain types of bacteria in tubes and then place them in wastewater. In the ideal scenario, the bacteria would take out electrons from the pollutants and give them to a external circuit – to generate electricity. Apart from cleaning the water, then, MFCs would actually produce electrical power. And water treatment plants, each powered by thousands of connected MFCs, would therefore become self-reliant. In theory.

Thanks to rapid design improvements in MFC technology during the last decade, theory is becoming more and more a practice. The power output of a single MFC, for example, has been multiplied by a factor of 10 since the turn of the century.

The Matrix

Korneel Rabaey, head of LabMET and an absolute pioneer in MFC research, compares the operation of an MFC with the movie The Matrix, in which humans are hooked up to machines to provide electrical power. “MFC’s work in a similar way,” he says. “What we do is use certain micro-organisms that can be connected to devices to generate an electrical current. The current can be used to generate power.”

Before he returned to Ghent University – his alma mater – in 2011, Rabaey worked in Australia, where he was a guest professor at the Advanced Wastewater Treatment Centre of the University of Queensland. It was there that he set up the first MFC pilot plant in the world to generate electricity from wastewater.

Rabaey and his Australian colleagues targeted a major wastewater producer in the area – Foster’s brewery in Yatala. “In Australia, there is much more attention for wastewater-related issues because they have already suffered some extreme droughts,” explains Rabaey. “The government knows that a good water-treatment system can be a vital element in a sustainable economy where clean surface water is regarded as a scarce commodity.”

So how did the MFC plant at the Foster’s brewery do? “Very well,” says Rabaey, “although we didn’t break records in producing electrical power. But for us the major goal was to study how we could scale-up this technology to support larger discharges of wastewater. While most researchers make tiny reactors, we have been building larger systems and investing much time in making them work really well.”

The Wastewater Treatment Centre’s reactor consists of 12 modules; each one is a three-meter-high tube with carbon brushes on the inside that serve as the anode. The wall of the tube is a membrane that facilitates the transport of electrons to the outside of the cylinder, which consists of cathode-carbon brushes clamped to a stainless steel mesh.

“The goal of the pilot facility was to remove at least five kilograms of organics per cubic metre of reactor volume per day,” explains Rabaey. “We succeeded in doing that, by which we achieved a power production of up to 500 watts continuously. But actually power is always the secondary target. In this first phase, we wanted to clean the wastewater in an energy-efficient way and without generating much sludge. That’s where the real benefit is.”

Chemical yield

Since Rabaey installed the MFC plant at Foster’s five years ago, he and his colleagues have learned that it would be more beneficial to use the plant not only to produce electricity, but also to “yield” valuable organic and inorganic chemicals, like bio fuels, hydrogen gas and methane. Rabaey: “We have calculated, economically speaking, that the production of these chemicals yields 20 times more than the production of electricity. So thanks to this pilot project, we discovered a more profitable trajectory on which to base further research.”

So can MFC technology help in making wastewater treatment in Flanders more sustainable? Definitely, thinks Rabaey. “And it could save us a lot of money. If you look at wastewater treatment, this is an area where we spend money and use energy. If we can install a technology that just saves money, then it’s making money. We don’t have to make it pay for itself, we just have to make it better than what people are currently using.”

But Rabaey also says that scaling-up these systems for use in domestic water treatment is a long-term goal and that the more immediate need is to develop pilot-scale reactors for industrial locations like food-processing facilities or in remote parts of the world that lack central waste treatment facilities – like the Foster’s brewery in Australia.

www.labmet.ugent.be