Wednesday, 27 March 2013

Recycling: Green waste, and Food waste

Recently I managed to get an invite to go on a tour of my local council's food and green waste recycling facilities.

It was very interesting!

Obviously, compost is one of my specialist subjects, so I was interested to see how it would be done commercially, and if there were any tricks of the trade to be learnt.

The tour was of two different facilities: the first was mostly garden waste, with some food waste,  being composted in the "normal" fashion for use as agricultural soil conditioner/fertiliser, and the second was food waste only, being processed through an anaerobic digester to produce methane (converted into electricity) and organic liquid fertiliser.

First on the agenda: in-vessel composting.

This is where the contents of the Brown Bins (in my part of Oxfordshire, green garden waste goes into brown bins, whereas cardboard/glass/plastic recycling goes into green bins. Illogical, huh?) go, along with some of the food waste that we put out in corn starch bags.

The process is surprisingly simple, and very similar to that of normal composting, but on a rather larger scale, and it happens correspondingly faster.

The trucks arrive from Oxford City Council and Cherwell District Council, go over the weighbridge, reverse into the receiving hall, and tip their load out. The trucks then leave, being weighed again as they leave, so the company (Agrivert) can calculate how many tons of waste they are processing.

The load is put through a giant shredder/mixer, that orange thing, then moved out of the receiving hall into a series of long roofed tunnels to get up to heat.

They use what I would call a loader - like a tractor, but with a big metal bucket on the front to scrape up, move, and tip out the material.  They have three or four of them, and they do all the moving of materials on the site.

As you can see, there is hardly any mess or leftover bits around - the whole site, I have to say, was remarkably clean and well-kept. And not because they had visitors - the site was operational the whole time we were there, and we all had to wear high-vis jackets, and were warned to keep alert for moving vehicles at all times.

The second stage is the tunnels: not rounded poly-tunnels, but long rectangular pens, with flat roofs that open up on struts to allow the loaders to get in and work unimpeded.

Here we are, at the row of tunnels, and you can get a general idea of the scale of them from the small size of Brian, site supervisor, opening one of the doors further down the row for us.

You can also see the strut arrangements above the tunnels, which allow the roof panels to be opened.

The freshly mixed and chomped waste is loaded into the tunnel, the roof is closed, the doors are closed, and they are left to get up to heat.

To help this, air is blown in from underneath, to oxygenate the heaps, and excess liquid is drained off.

We all  know that compost heaps generate heat, and these ones do it on a rather larger scale - when the door was folded back, a fog of steam wafted out! Several of the tour members held their noses in somewhat cissy fashion, but there was no unpleasant smell at all, even though these heaps had only been stewing for a few days.

 Here's an unhelpful photo of the inside of one of the tunnels: just a heaving, steaming mass!

As you can see, not much rotting has taken place yet, you can clearly see the shapes of twigs etc, and the white stuff is just fungus, as you would expect in something warm and dark.

Of course, they don't have the usual brandlings or Tiger Worms in these piles  - it is far too hot for livestock. The rotting process is purely heat-driven, and all the heat is generated by the rotting action. Clever, huh? All they have to add is the oxygenation: air is free, and the fans are surprisingly low-powered, just 5 kilowatts per tunnel, and they only run when the sensors indicate that they are needed.

The tunnels have to achieve a certain temperature (60 degrees) for two days, and the tunnels are closely monitored to ensure that this happens. The material then stays in the tunnel for a surprisingly short time - 7-10 days - before the tunnel is opened, and the heaps were moved out from the other end into the next phase, the second set of tunnels.

The second set of tunnels were identical to the first, but only half as many of them: of course, as with domestic composting, the material reduces rapidly in volume.

After another fortnight in the second set of tunnels, the material is scraped out again by the loaders, or "loading shovels" as they call them, and stacked in long rows (referred to as "windrows" ) in the open yard, to continue the rotting process.

When they are "done", the material then gets loaded through an industrial sized sieve - ah, said our group, at last the expected conveyor!  The loaders tip the stuff into a hopper, it runs up a conveyor then through a rotating drum with a coarse 1" mesh.  The "fine" stuff - relatively fine, not what you and I would call compost, but this is agricultural material, not potting compost! - drops through, the rough stuff goes to the end, where air is blown up through it to get out the contamination, ie the plastic bags and other rubbish.

Deborah, our tour guide, told us about the ridiculous rubbish that people leave in their garden waste bins - not just the *sigh* expected plastic bags, and the occasional plant pot, but even huge things such as barbecue sets. Why, while we were there, we saw a wheel from a barbecue, any amount of plastic bags, and the arm of an office swivel chair.

Why? Why? Are people really that stupid? Apparently, "yes", to go by what we saw. Garden waste recycling is not like the council-run rubbish dump, with a dozen blokes stood around watching, able to shout "Oy!!" and fish out items that have been put in the wrong container. This is a commercial operation, and they can't afford to have people picking over the incoming waste. Anyone reading this and thinking "huh, well they jolly well ought to pay someone to pick out the rubbish, it shouldn't be allowed to get onto our fields, grumble grumble"  should realise that to do so, the cost of rubbish collection - currently "free" in amongst our council tax - will rise significantly. And no-one wants to pay directly for rubbish collection, do they?

So please, spread the word that we, the general public, should be a bit more careful about what we put in our garden waste bins.  As it is, the plant has one member of staff watching as each truck unloads, to check for contamination, but we can all do our bit to help.

Getting back to the process, the sifted material is then heaped up, ready to be collected by local farmers and spread on the land.  We ran our hands through it (well, those of us like me that don't mind getting our hands dirty) and agreed that it was quite coarse, but still good stuff. It had a lovely texture, just dry enough to be pleasant and light, moist enough to hold an impression when squeezed.

Which, incidentally, is my definition of Perfect Compost.

The finished product goes mostly to agriculture locally. Deborah told us that it goes to farmers within a tiny radius, less than 10 miles, which is wonderfully eco. They are even prepared to give some of  it away free, to allotment groups or charitable gardening groups, a comment which promoted a lot of interest amongst the group!

So, lessons learned from the day?

Firstly,  already known, Don't Contaminate Your GreenWaste Bin!!  Think about all the things that you would hate to find in your bag of multi-purpose compost, bought from a garden centre: glass, bits of plant pot and plastic bags, plant labels, stones, etc. Then don't put them in your garden waste bins. Spread the word.

Secondly, Size Matters.  On a small domestic scale, you need brandlings and time - 6 months to a year, to make good compost.  When done on this sort of scale, it can happen in mere weeks. But don't expect to achieve this timescale at home.

Thirdly, Simple Is Best.  This recycling plant is wonderfully efficient, with minimal handling of materials, minimal staff, no mess, clean concrete yards, no steps or ledges for material to catch on: they even used the excess soil left over from levelling the site to make a nice earthen/grassy bank around the site, to avoid offending the eyes of the occasional passing motorist.

Next time: anaerobic digesting - which is what happens to the things we put in our kitchen waste caddies: and then, part 3, what happens to our recycling wheelie bin waste!


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  1. Very interesting Rachel. I use the Fife council stuff on our community village garden and it's really good. Has some nutrient value as slow release with trace elements as well. The P and K are released in first season and N much slower to release in 2nd and 3rd years.
    Bob Bilson

  2. Hi Bob, round here they use it direct on the fields, thus keeping the "compost miles" to an absolute minimum.

    I'm interested to know how you assess the nutrient value of the stuff that your council produce - it must vary hugely, depending on what went in to it, I would have thought?

  3. Rachel, I think the results come from a 1 off report for Fife council so they could give fertility advice to farmers. I did some earlier work with similar results on estate compost manure leafmould mushroom compost. I sent samples to Scottish Agricultural College Penicuik and published in pgg mag July 2009. I've always been interested in this and the definitive work was done by the late Lawrence D Hills of HDRA, now GardenOrganic. There is a booklet pub about 1975 by him called fertility without fertilisers. Hope this is useful. Bob

  4. Hi Bob, I didn't realise you were a fellow PGG member! *waves enthusiastically*

    That's very interesting, and no, I wasn't aware of the work of Mr Hills - I shall look him up, forthwith!

    Mind you, you've neatly highlighted my point - even in the case you mention, the report was done at one point in time, but there is no guarantee that the same results would arise if the tests were repeated today, is there?

    However, your reply is so interesting that I've now been inspired to write a new post on the subject - watch this space!


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