Turning Rubbish into Hydrogen
In the UK, we throw away over 7 million tonnes of food every year, the majority of which goes to landfill. But thanks to recent advances in microbial biotechnology, this waste could become a valuable future source of energy in the form of biohydrogen...
Kat - Now, anyone who's had a rather whiffy bin will know that bacteria can generate gases, but how are you actually making bacteria generate hydrogen from food waste?
Mark - The first thing to say about biohydrogen from waste is that it's difficult to answer that question straight, because it is still an area of research and everybody has different ideas about the right way to do it. There are lots of different bacteria that will do it. The capacity to make hydrogen is virtually ubiquitous among the microbial world and there are really special organisms that we have singled-out which are really good for the process.
Generally, you can say it'll either work in the dark or it'll work in sunlight. So if you just take a pile of potato peelings or something, and you seal it in a container so that the air can't get in, the bacteria will begin to consume the sugars and use up the oxygen, so it'll go anaerobic. Enzymes in there called hydrogenases will switch on and then it will begin to fill that chamber with hydrogen.
In the process of doing that, it'll also make some CO2 and some organic acids. Those are what you're smelling. It's not the hydrogen that you can smell, because hydrogen itself is odourless. but those dark organisms, as they break down their carbon sources, produce a lot of things like acetic acid (vinegar) or butyric acid which is very, very smelly, but it's actually completely harmless. It's even added to some foods for extra flavour.
The other thing that you can do is take those organic acids and feed them to a second kind of bacteria. These ones live on sunlight, they are my favourites. These are the purple bacteria and what they do is use those organic acids and react them using an enzyme called nitrogenase. And if you set up the reactor just right and give it sunlight, then it will make a lot of more hydrogen.
Kat - And so, you've got this bacteria in a canister and then you're taking the things from them. How do you actually trap the hydrogen because, presumably, there may be other contaminating gases in there? Is this really good source of clean hydrogen?
Mark - Well funnily enough, it's actually one of the cleanest ways to get hydrogen. Everybody knows now that hydrogen is the fuel of the future. It's got fantastic advantages and people are looking at having to scale up the hydrogen production industry, because people are developing fuel cells left, right and centre, so they need lots of hydrogen for it. Everyone agrees that that hydrogen is going to come from natural gas to begin with, through the traditional process of steam methane reforming. The problem with that chemical process is that it generates a lot of carbon monoxide which is a contaminant in the gas and it's especially important as a contaminant if you're going to use the hydrogen in fuel cells, especially the kind that we used in experimental fuel cell cars. Whereas, as we eventually move towards biohydrogen, that problem will disappear because the gas is inherently pure. It doesn't contain carbon monoxide to begin with. The only contaminant really is CO2 and it comes out really 90% hydrogen or more in the first place, and CO2 is very easy to separate from hydrogen.
Kat - And so, what kind of waste are we talking about here. I mean just generally, I'm an avid composter so every week, my little bin of compost is collected by the council, but what kind of food waste could you use? Could it literally be anything?
Mark - It should be anything. Obviously, the more organic it is, the better. It shouldn't contain too many things like the packaging, the plastics, the tins - those won't do any good because the bacteria can't break them down. The thing to understand about composting is that what it really boils down to is allowing the natural degradation to occur, but with lots and lots of aeration; getting oxygen in there. That encourages the aerobic respiration to happen, to get all that carbon to turn into CO2 as quickly as possible. And the only useful thing that you get from composting is heat and as everyone will know, compost gets warm. You can sometimes find a cat sitting on them!
But if you take the same waste and seal it in a pot so that your air doesn't get in there, then it'll either go to make methane, or if you do under certain controlled conditions, it'll make hydrogen and organic acids. We think that, from examining the biochemistry and the stoichiometry of these reactions, if you can take that dark hydrogen reaction, use the organic acids from that to feed to purple bacteria then we should get about more than two times as much energy value as what you would get if you will let it react through normal digestion to methane. So making hydrogen will give you more energy than making methane.
Kat - So this all sounds brilliant - we can stick our bacteria and our rubbish in a pot and we'll get hydrogen out of it. How close are we to actually scaling this up? What sort of challenges have you still got to overcome before this is suitable on an industrial scale?
Mark - Well I think one of the main challenges is that the market isn't there for biohydrogen. People at the moment are much more used to liquid fuels. This is good for us in a way because the algal biofuel people are blazing the trails for us, developing solar reactors which we will then be able to use to make biohydrogen. One of the main issues is getting those reactors developed up to a level where they can be used economically and efficiently, and where they can be produced in very large quantities, very cheaply.
Kat - Where are you at? What do you think is the most exciting things that you've come up with lately in your research?
Mark - Well, it that just happens that we've just had something hot out of the lab a couple of weeks ago which is a real development. We've added 75% to photosynthetic productivity. We've had this project running with the Manchester Photon Science Institute about using one sunbeam to drive not one but two different photobioreactors containing two different kinds of organism. And the way that works is that, as we know, sunlight is composed of a whole rainbow, a whole spectrum of different wavelengths, and the purple bacteria that make hydrogen happen to like a particular part of that spectrum, whereas green organisms like the algae that make oil or spirulina that make nice health foods that you can buy in health food shops as a food supplement, prefer different parts of the spectrum. So all we need to do is find a way to separate the sunbeam into those two beams and direct each at a separate reactor and we'd be able to double the productivity that you get from a single sunbeam. We set up this project with the University of Manchester and they told us about dichroic mirrors which turn out to be absolutely perfect for the job. And so, we found that we can add 75% to the photosynthetic yield.
Kat - That sounds absolutely fantastic! That's Mark Redwood from the University of Birmingham.