What Happens at a Composting Facility

Welcome to part 3 of this not-very-glamorous trilogy on what happens to your household waste. In parts 1 and 2, I reported on my visit to a recycling sorting centre and a landfill site. Part 3 is all about what happens to your food and garden waste (spoiler alert: it gets composted!).

So what happens to your green bin?

There are basically two different processes – open windrow composting and in-vessel composting – depending on whether we’re dealing with garden waste only or mixed garden and food waste. The reason there are two methods is that compost made from food waste has to comply with Animal By-Products Regulations. This may sound kind of boring, but the rules are there to protect the environment and human health.

Compost bag

In-vessel composting (IVC)

This is what happens to bins containing food waste as well as garden waste.

Once the collection lorry reaches the waste management centre, it dumps all of the waste in a big tipping hall. Here, everything is shredded to measure less than 40cm before being transferred to giant tanks (“vessels”) where the natural bacteria in the waste get to work breaking everything down, just like in a home compost bin. During this stage the decomposing waste reaches very high temperatures, which means that diseases are killed off and the compost that comes out will be sterile.

The waste in each vessel is carefully monitored and turned frequently so that the bacteria are fed with plenty of oxygen. Towards the end of the process each batch is screened to remove any plastics or other contaminants that have found their way into it. All in all it’s about an 8-week process resulting in a high-quality compost.

At our local facility, the so-called soil improver is not only sold on to businesses, but is also available for locals to come and collect free of charge. All you need is a spade and a bag!

Open windrow composting

This is the process used for garden waste that doesn’t contain food waste. It’s very similar to in-vessel composting except that the waste isn’t kept in tanks.

First of all, anything that shouldn’t have been in the green waste bin has to be removed. Next, the waste is shredded and placed into “windrows” – long piles of material that allow the air in. Sometimes they're outside and sometimes they're inside a big warehouse to protect them from the rain.

Just like with in-vessel composting, microorganisms then break the waste down. They need oxygen in order to do this, so the waste is turned frequently. With temperatures in the composting pile reaching around 60°C, the bacteria can turn the entire lot into compost in about 2-4 months.

The final step in the process is to screen the compost to make sure there’s nothing in there that shouldn’t be. If the compost still contains large pieces, they can be put through the process again until they comply with requirements.

Why can’t I just put my food waste in the bin?

We talked in part 2 about how your general waste goes through a materials recovery process before being landfilled. If there’s food in there, it can hinder this process and cause problems that add time and cost money. Composting food instead is far less wasteful and means we aren’t burying so much of our rubbish. On top of that, the compost produced can help enrich our soil and keep it healthy.

As mentioned in various other blog posts, the system varies hugely depending on where you live. If you’re lucky enough to have garden and food waste collections in your area then please make sure your green waste doesn’t end up in general rubbish. If your local council doesn’t collect food waste, why not try composting at home or, if that’s not possible, campaigning for food waste collections to be set up?

Sack of compost

Find out more about green waste composting:

https://www.recyclenow.com/recycling-knowledge/how-is-it-recycled/garden-waste

http://www.wrap.org.uk/collecting-and-reprocessing/organic-waste/composting/open-windrow-composting

http://www.wrap.org.uk/content/vessel-composting-ivc

 

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