Contamination in Compost: Why Plastic Doesn’t Belong

Let’s talk compost. When done right, it’s nature’s way of recycling garden waste into rich, healthy, nutrient-rich soil. But when done wrong? Contamination can turn a good thing into a real mess.

One of the biggest culprits? Plastic.

At Coastal Recycling, we accept around 60,000 tonnes of source segregated green garden waste collected from across the county. Plastic often ends up in garden waste bins and can play havoc with our composting process.

Composting: A Natural Process With No Room for Plastic

Composting is a delicate, biological process. Organic materials (think grass cuttings, hedge trimmings and fallen leaves) break down naturally with the help of microbes, oxygen, and moisture. 

But plastic? It doesn’t play by the same rules.

Plastic doesn’t break down it breaks up into microplastics. These tiny particles don’t disappear. They stay in the soil, polluting the earth and even finding their way into food crops. That’s bad news for farmers, wildlife, and ultimately, for us.

The Contamination Problem

At Coastal Organics our composting facility, we see it all the time: compost bins filled with good intentions but topped with plant pots, plastic bags and food wrapping.

Our team remove as much contamination as possible but if we miss it and it enters our shredding equipment, the tiny plastic particles are hard to detect.

It only takes a small amount of contamination to ruin an entire batch. Once that plastic is in there, the compost becomes ineffective and can’t be used on farms or in gardens. We can put the compost through a rescreening process, using a machine called a wind sift to remove as much as possible. In some cases, the entire load has to be sent to a landfill, defeating the purpose of composting entirely.

Coastal compost is PAS100 certified, meaning it has to go through external laboratory testing. Tests include checks on all types of contaminants including plastics, stones and sharps, the amount in which the green waste has broken down and subsequent response of plants being grown in the compost, any presence of weeds or pathogens and heavy metals, such as Mercury.  This stringent testing means we are confident in the high quality of the compost we produce – and why it’s even more essential to keep those pesky plastics away from your garden bin.

Common Plastic Offenders to Watch Out For

• Plastic carrier bags – including those labelled as “biodegradable” can cause issues.
• Food packaging – salad bags, ready-meal trays, or wrappers are mistakenly thrown in.
• Coffee cups and lids – many look like cardboard, but they’re often lined with plastic.
• Plant pots, plant labels and seedling trays – handy in the garden, but not compost-friendly.

What Can You Do?

Composting right starts at the bin. Whether you’re at home composting or using district council  scheme, here’s how to keep things clean:

• Stick only to garden waste – hedge cuttings, grass, leaves, bark and small branches (less than 10cm in diameter)
• Keep all kinds of plastic, metal, glass and rubble OUT
• Compostable packaging, including bin liners, should not be placed in your garden waste container as it is not suitable for our ‘open windrow’ composting process.  Many forms of compostable packaging take longer than garden waste to break down.  If you are home composting, check with the manufacturers to check how long they take to break down.

The Bigger Picture

Dropping anything other than garden waste into your garden waste bin might not seem like a big deal,  but it has big consequences. It affects the quality of the compost we produce, the health of our soil, and the success of recycling schemes across the UK.

At Coastal Recycling, we’re committed to creating clean, high-quality compost and we need your help to do it. Together, we can protect our soil, reduce waste, and keep plastic where it belongs (hint: not in your compost bin).

Got Questions?

We’re always here to talk rubbish. (Literally.)

If you’re not sure what can or can’t go in your garden bin, just ask! Or check out our Green Garden Waste Guidance for more tips on keeping it clean and green.