31 Mar 2010

Waste not, want not

What happens to your rubbish these days? It’s a very good question, and the answers aren’t quite as straightforward as you might hope for. Over the past decade, we have all been more or less trained to separate out our waste. Most local authorities now have collections for a variety of products like paper, glass, plastics and aluminium cans. And most now try to separate organic waste (food, cardboard, garden clippings, etc) from ordinary, indestructible household waste (packaging mostly in my house at least).

Now the non-organic, non-decomposable waste used to go into landfill mostly, at least it does in Cambridgeshire, my council, although it can be incinerated to produce usable power and heat. In contrast, the green, bio-degradable waste was composted.

Cambridgeshire County Council has just spent £42 million on a gigantic waste sorter cum compost heap known as a Mechanical Biological Treatment Plant (PFI money) and you can read about it here. It’s so state-of-the-art, that it can extract the organic material from the black bin waste, and they estimate that something like 80% of the waste currently being landfilled will end up going through this MBT plant. Good news? Maybe.

Look a little bit more closely. There is an interesting sentence at the end of their guide to MBT which states: Due to the fact that the input is mixed waste, the material produced cannot be used by farmers or the public, but we hope to use the compost-like output of the MBT plant for quarry restoration, growing energy crops or as a fuel. In other words, they haven’t really got a use for all this compost yet.

Organic matter left in landfill slowly undergoes anaerobic digestion which release methane very slowly (can take up to 100 years), whereas the stuff that is fed through the MBT plant undergoes aerobic digestion very quickly. Now aerobic digestion produces CO2 instead of methane, but unfortunately both of these happen to be greenhouse gases. I know methane is something like 300 times more potent but the landfill digestion process seems to last about 300 times as long, so do they not more or less cancel each other out in terms of climate sensitivity?

So now instead of having holes in the ground filling up with untreated waste, we now have all this expensively treated but low-grade compost looking for a home because they are not allowed to give it away to farmers or the public (something to do with the old BSE scare, methinks). And they may end up incinerating it! Which, of course, they could have done in the first place without treating it.

Still, it’s a fine looking building, the size of three football pitches, and stuffed full of trommel screens, tilting vibrating plates, electro magnets, eddy current separators, not to mention targeted air jets and waste grinders. I wonder how much energy it consumes?


  1. I'd sooner have x tonnes of organic material decompose aerobically into CO2, fast, that the same x tonnes decompose anaerobically into 300 greater greenhouse-effect CH4, unstoppably for the next 300yrs!

    Can you burn compost, even if unwanted? The point is, it's almost completely oxidised already - that what composting is.

    This kind of super-sorting, which AFAIK is a great byproduct of Australian mining tech, is v gd news (apart from worries about energy consumed). As China plans to get the west over a barrel by starving it (us) of rare raw materials, 100% recycling can't come soon enough - this is but an early step towards that.

    Such sorting/recycling is bound to be energy-intensive, as it's working back 'uphill' against entropy. That can only be done by energy input from somewhere, at expense of increased entropy somewhere else (that's a definition of all living things).

    Plants do it by stealing solar energy from the universe and failing to return it; animals do it by eating plants, whether fresh or fossilised.

    It's all OK, as long as future sorting facilities get their energy from the sun, not from fossil-burning, nor even from burning their own product (unwanted compost!).

    BTW, that's two future-vital things to come out of Australia - Permaculture (father of the Transition movement), and super-sorting.

  2. It's a good point about burning compost. There won't be any residual carbohydrates to burn!

    So what do you do with it if you can't put it on the soil? Quarry restoration? Isn't that just a fancy term for landfill?

  3. More of a topping than a filling I think!

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  5. This is a problem throughout the waste management industry: When does a 'waste' stop being classified as waste? And therefore stop being covered by waste handling regulations (e.g. someone wanting a bag of composted material would need to be licenced for waste transfer...). In order for the material to be a 'product' it would need to reach a quality standard (PAS 100 in this case i think).

    Lots and lots of LCA work on waste management. Burying stuff in the ground is almost always about the worst thing you can do with waste, hence all the EU regulations which are steering us towards things like MBT.

  6. Burying non-compostable waste that can't be (or isn't yet) sorted and recycled, is far better than burning it for power.

    Think of landfills as quarries of raw materials that will be highly valued in future, as supplies of virgin material - hydrocarbon, mineral and metal - dry up.

    Burn it and it's gone forever. That's the flaw in the cement industry's boast that it's steadily reducing its fossil content - that's almost entirely because they're instead burning our recoverable future stockpiles of valuable resources.

    Burying is a very good option.

  7. The UK seems determined to invent 50 eccentric ways to sort or collect or dispose of waste which are different from the ways adopted by the council next door.

    My impression is that across the Channel householders must sort their waste (if they don't in Switzerland they can be fined for littering!) and the council does what's best with different wastes roughly as follows

    rotting vegetables or garden waste: shred it and digest it to methane (as a few UK towns do) and sell the compost and gas
    paper: recycle
    metal: ditto
    glass: ditto but far more sensible to go back to returnable bottles
    plastic and miscellaneous - incinerate the lot because sorting and recycling over 10 different meterials is near-impossible (the UK pretends the plastics we put in the wheelie bin to be recycled are recycled, well I'd love to know what really happens to them)