Garden School:


Garden School:
Teaching this week: Rose pruning (as always!) and leaf mold.

Monday, 29 April 2013

Composting II - Food Waste Recycling

After viewing the in-vessel composting site, we drove off a few miles to Cassington, to Agrivert's purpose built, high-tech, anaerobic digester facility.

Sounds impressive? It was! This is where the bulk of the area's food waste goes, and is situated in peaceful farmland just off the A40. You'd never know it was there....

The process is "anaerobic", ie not using oxygen, because it takes place inside the tanks, not in the open air: if you remember the garden waste processing, that was an aerobic process, just as our domestic composting is: it needs oxygen in order to work. Very different systems!

The waste arrives by the truckload - Debra, our tour guide, told us that it's not just our household food waste, but also food waste from restaurants, supermarkets, and milk processing waste as well. Before entering the reception area, Debra warned us that it might be a bit smelly... but actually, it really wasn't that bad.

I can imagine that on a hot day it would be a bit ripe, but the process is very swift: the trucks pick up the food waste from our streets, bring it straight here, empty out and return for more, so it's not as though it's sitting around for any length of time.

It all arrives in the unloading bay, where the trucks reverse up and tip out their contents into a large pit. At the base of the pit are various mechanisms to move the "stuff" along and up a conveyor belt, where the contamination is removed, and the good stuff is macerated.

Here's a photo looking down into the pit - no, it really wasn't as smelly as you would have thought. And no, I wouldn't want to fall in.

Once again, the process is surprisingly simple: this arrangement moves the waste from the pit, through the "chomper" and through a de-contamination unit, which picks out any plastic or other unwanted items.

The contaminants are moved away on a slow conveyor, and the "good stuff" appears as a liquid slurry, lovingly referred to as "soup" by the team.

The "soup" is then piped into a series of large metal vessels, where it is heated and sterilised. And how are the heaters powered? Aha, we'll come back to that.

Just a word before I go on to the rest of the process - corn starch bags. Familiar with them? Our supermarkets sell them, to go inside our indoor food caddies, so we can bag up our food waste and put it in the food bin outside, which is then regularly emptied.

As I was looking at the contamination conveyor, I couldn't help noticing that most of it was pale green plastic. I asked Debra - "corn starch bags", she replied. "Are they not processed along with the food waste?" I asked. "Oh no," she replied, "we have to remove them. They go off to be incinerated along with the other contaminants."

What!!

When the council first told us that we would be putting our food waste in separate bins, the leaflets said we can only use compostable bags made of corn starch.   Although we can use a small amount of newspaper to line the bin, if we really, really wanted to. But corn starch bags were the way to go.

Ever since, I have been buying expensive corn starch bags, and resenting the fact that I barely half fill a bag in a week, and I have to empty my indoor caddy long before it is full. (because it starts to smell, of course.)

Now I find that not only are the corn starch bags not necessary, but they are actually a contaminant, and have to be carefully removed!

As Debra told us: "Just put the food into the bin, loose. You don't need bags. Tip it out every day into the outside bin. When that's been emptied, just rinse it out! That's all I do - there is no need to use bags."

As each council seems to differ, I should explain that in the Vale, we have a small kitchen caddy - just about the right size for a corn starch bag, huh - which is emptied into a larger outside caddy, which is put out for emptying every week. Some other councils have different arrangements, but basically we all have to have some sort of pot indoors, which is tipped into the outdoor collectable one. Debra says, don't bother with bags, just empty the kitchen caddy or pot out into the outdoor one, leave the stuff loose: and if, when it's been emptied, it gets a bit smelly, rinse it out. She agrees that newspaper can be used, but not too much of it. No need to wrap every item individually!

Well, that's convinced me: once I've used my existing stock of corn starch bags, I will in fact stop using my kitchen caddy, and will go over to the system my friend in London uses: she has a tub on the worktop for peelings and scrapings, and as soon as the meal is prepared and/or cleared, she just tips it straight out into the outside bin. It never gets smelly indoors: and every so often she runs the emptied outdoor bin under the outside tap, quick rinse and that's that.

So, back to the process:

The sterilised - or is as pasteurised? ok, the "heat treated" soup is then piped into a series of truly enormous round above-ground tanks, looking somewhat like the old-fashioned London gas storage tanks:  you know, those ones where the rounded tops would rise, indicating how full they are.

These tanks don't change shape, but we were told that they have a gas-permeable membrane inside them that sits on the surface of the soup to prevent it forming a crust, while it digests - quite literally, just as our stomachs do, using nothing more than bacterial action. No chemicals, no stirring, no additives, just ongoing bacterial action. Agrivert had to buy the initial "dose" of bacteria, but once it got going, apparently it is self-sustaining, just like our own stomachs.

The process gives off quantities of methane, which is piped away and into the gas turbine engine, where it is used to create electricity.

Now - and this is the really clever bit - some of that electricity is used to power the plant, and to heat up the steriliser vessels! How neat is that!

After something like a hundred days, the "soup" has moved through all of the large above-ground tanks, and what is left is a non-smelly, utterly organic, wonderful liquid fertiliser, which is pumped into tankers with spreader attachments, and off it goes to local farmers, to grow our crops for us.

I am so impressed with this facility, I can't tell you:  ok, it cost a lot of money to set it up, but once in operation it quietly takes all our horrible food waste, and turns into fertiliser to be used within a very few miles of the plant, and as a by-product it makes enough electricity to power the entire operation, and enough left over (sold back to the Grid) to power, ooh, what was it? 4,200 homes - a whole village, in effect.

It uses no noxious chemicals, no imported or added materials, a minimum of machinery, a minimum of staff, is just about completely smell-free, and although it is fair to say that the gas turbine is quite a noisy bit of kit (well, it is HUGE!) we were able to hold normal conversations while standing outside it, waiting our turn to go inside to look. So, not even noisy, then.  OK, quite a lot of trucks coming and going, but that is inevitable, as we all insist on throwing so much away.

Personally I waste as little food as possible.  In fact, I would say that I waste very little actual food, my food waste is normally vegetable peelings, and once I get my wormery set up again - lost them all in the cold winter, drat - I probably won't have anything much to put out at all. But I'm very pleased to  know that what little I do waste is going to be processed into useful commodities in such a very efficient way.

And as soon as I have finished up my current roll of corn starch bags, I will never use one again!

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