Scavenging energy from your waste

04 March 2012

Interview with 

Dan Cooke, Viridor


Ben - According to government figures, we produce around 280 million tons of waste in the UK each year and while waste management may not seem glamorous especially when you consider that this includes dealing with over 90 million tonnes of animal slurry, it is essential. It's estimated that we could scavenge back as much as 5 terawatt hours of electricity from this waste per year if we use technologies such as anaerobic digesters.  

We are joined today by Dan Cooke from Viridor which is one of the UK's leading recycling renewable energy and waste management companies.  Dan, what are our priorities with waste management?  What's most important?  

Dan -   Well, first and foremost, what we need to be doing is preventing waste in the first place - i.e. reducing it, thinking about what we need to buy, et cetera, designing our waste so that goods and items are reusable first and foremost, and then recyclable.  Once it's hit the waste stream material then it's the priority for companies such as Viridor to recycle, recycle, and recycle, and then squeeze what energy we can out of what can't be recycled.  

Ben -   So what sorts of waste are we talking about here?  Can we take literally anything and make the best of it or are we still restricted to what we can actually recycle, what we can re-use?  

Dan -   There is always a limit in terms of what can be recycled, whether it comes down to environmental benefit in doing so or economic feasibility, and benefit in doing so.  So, while we can recycle probably much more than we are at the moment, there will always be residues that can't be recycled for either economic or environmental benefit that we have to still safely dispose of.  

Ben -   So, that would be a situation where for example, we have something that we could potentially claim back from our rubbish and turn back into nearly pristine material, but it might not be worth it either economically or in terms of energy purposes.  In which case, what happens to it?   rubbish

Dan -  Well, anything that can't be recycled currently either has to go for landfill and landfill are engineered holes in the ground where we can still recover some of the energy out of material that goes into the ground. When it breaks down the organic content in there, it produces methane gas and we tap that gas off and burn it for electricity in the same way as we do using anaerobic digesters.  So, material that can't be recycled goes to landfill for disposal mainly, although increasingly, we're looking to thermally treat it and still recover energy out of it.  

Ben -   We've mentioned anaerobic digesters.  I mentioned in the intro that we apparently could be able to claim what seems to be an enormous amount of energy back from waste using these bits of kit.  What are they and how do they work?  

Dan -   Well, an anaerobic digester as the name suggests, digests organic material in the absence of oxygen, so that is anaerobically.  It is a good technology for a relatively narrow amount of the total waste stream - i.e. it has to be organic material, it has to be pre-segregated or separated out, and turned into a slurry.  We then feed it into tanks where it's kept - well, you add water to it, feed it into tanks as a slurry and you have to have the right temperature for the bugs to start working, and the bugs start bubbling up in the absence of oxygen.  They produce methane.  We then tap that methane off and burn it through lean burn gas engines to produce power.  It's already been used for quite a long time in the sewage industry for example and increasingly, we're using it within the standard waste industry for pre-separated food waste, kitchen waste, organic waste, and garden waste.  If we can keep that waste stream separate then it's a good source of energy.  

Ben -   It sounds like a lot of preparation that you have to go into in order to actually be able to put the stuff in the digester in the first place.  How does it compare in terms of the amount of energy to what we get out from just tapping the energy from - as you mentioned earlier - landfill and tapping the gases that come out naturally?  

Dan -   Well, it is more effective than that in the long run if you can keep the proportion of waste -  and we're talking probably about 15 to 20% of normal household waste and certainly, more from catering waste in that type of establishment - so there are some commercial waste streams that are particularly suited to it.  And it's always going to be more efficient, if you can segregate out those waste streams, to separate it out and get gas and energy from it in that way, rather than mixing in and trying to extract it once you've put it into a landfill site.  

Ben -   And with regards to the scale and positioning, what do you think is the best approach?  Should we have one enormous one in the middle of the country that burns off all of our waste or should we have lots of little ones, perhaps backyard size ones or ones in villages, small towns, maybe even one at your local supermarket that just gets rid of your kitchen waste?  What scale should we be looking at?  

Dan -   It's very much horses for courses. There are a hundred different things you have to look at when you're locating any waste treatment facility.  One, you have to look at the waste streams, where they're coming from, how they're collected, whether they are already separated, or whether you're going to have to separate some of the materials out. And generally, there are economies of scale to be had by reasonably scaled facilities rather than lots and lots of little ones.  They'll cost more, you have to find balance to keep them running properly, to keep them running efficiently, and therefore, they take a lot of care and attention in the operation of them, the environmental monitoring of them to make sure you're doing it properly and not causing further environmental issues for example as you go along.  So, all of those things together mean that probably, centrally and sensibly located facilities dealing with waste streams from a fairly decent sized regional area are better than hundreds of little community facilities.  

Ben -   And what happens to the other products, presumably the electricity that you generate can just go back into the national grid and go to wherever it's needed. But the other products, all the excess water that you've needed to use and what's left, which I believe is called the "digestate"?  Where does that all go?  

Dan -   Again, that depends on the setup of the kit that you're operating.  The energy as you say, the electricity, is fed into the national grid.  There is some heat in the form of hot water, you can produce and use on-site if you've got anything else on-site that could utilise such things. The water has to go in a controlled system and usually, it's re-used over and over again if it can be.  Otherwise, it has to be safely discharged to sewer for treatment again, further treatment. The digestate itself again will very much depend on the material that you fed into the plant.  What we're doing as a company - Viridor - in different parts of the country, we're producing a compost-like material that can be used for landscaping, soil blending, that type of thing.  in other areas, it's actually dried out and then used as fuel or added into a fuel mix for thermal treatment where we'll squeeze even more energy out of it.  So again, it very much depends on the waste streams that we're talking about.  

Ben -   So clearly, there's quite a lot to be thought about, but they seem to be a very useful, very efficient bit of technology, and that is Dan Cooke from Viridor.    


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