Everyone knows that wastewater is full of "waste". But is this really the case, or is it just a matter of perspective.
He also asks Zhen He, professor in the Department of Energy, Environmental and Chemical Engineering at Washington University in St. Louis.
"Why are they waste?"
"They're organic materials," he says, "and those can provide energy in a number of ways." And then, it should not be forgotten, in wastewater there is the other precious resource. The most precious of all: water.
He's lab has developed a system that recovers both energy and water. A system that filters wastewater while creating electricity. The results of the first tests were published and presented as a cover article in the magazine Environmental Science: Water Research & Technology.
"Waste water" to whom?
Just because we don't like them doesn't mean that no one likes the waste materials in wastewater. For example, for bacteria they are food. 'Bacteria love them and can convert them into things we can use,' said Professor He. "There are two sources of energy that can be recovered from wastewater. One is biogas, the other is bioelectricity."
There are already ways to capitalize on bacteria to make energy from wastewater, but these methods often do so at the expense of water, which could be filtered and otherwise used. If not for drinking, at least for "gray water" purposes such as flushing and flushing toilets.
The Washington University lab took the two processes (filtering and power generation) and combined them into a kind of microbial fuel cell. Although perhaps the exact term is another.
A bacterial battery
It is actually a bacterial battery, which uses electrochemically active bacteria as a catalyst (where a traditional fuel cell would use platinum). In this type of system, bacteria are attached to the electrode: when wastewater is pumped into the anode, bacteria "eat" organic materials and release electrons, creating electricity.
To filter that same water, then, the researchers made sure that the same anode acted as a filter.
The anode is a dynamic membrane, made of conductive carbon fabric. Together, the bacteria and the membrane filter 80% to 90% of organic materials, which leaves the water clean enough to be released into nature or further treated for non-potable uses.
Casting office for waste water
A system that looks truly brilliant and complete, there is no doubt. But how did the choice of the best candidate bacteria to be part of this special battery come about? The researchers chose a mix of bacteria that had one thing in common: the ability to survive in an environment with zero oxygen.
"If there was oxygen, the bacteria would simply discharge the electrons into the oxygen and not the electrode," says Zhen He. "We let nature choose. Natural selection. The bacteria that survived the various tests were the chosen ones."
How much energia can produce such a system?
The amount of electricity created is not enough to power a city, of course. But in theory it is enough to help offset the significant amount of energy used in a typical wastewater treatment plant. In other words, it would be enough to make it energetically independent, and that's no small feat.
3% to 5% of the electricity is used for activities such as filtering waste water. Imagine what savings municipal purification plants could have. The "bacterial" process consumes about 0,5 KWH of electricity per cubic meter. Researchers on He's team aim to halve that, if not more.
In summary: a wastewater filtration plant that filters, recovers nutrients such as nitrogen or phosphorus for plants, and feeds itself. Do you still think it's just waste?