From plate to plant

It's a tall order to welcome over one million visitors a year to a world-famous tourist attraction and watch your waste line. But with the help of a huge on-site composter, the Eden Project in Cornwall, UK, is now converting around 90% of its food waste into compost for its impressive collection of plants. Hannah Bullock reports...

Aerial view of the Eden Project, Cornwall, UK

The Eden Project, an ecopark in Cornwall in the UK, recently passed a milestone: 100 tonnes of food waste has been diverted from landfill since it started composting kitchen scraps and leftovers from its cafés in 2005. Nicolette Knight, Waste Operations Manager at Eden, describes the process of turning it into a valuable resource as 'Eden's version of alchemy'.

Performing alchemy

At the heart of the operation lies a 30 m³ in-vessel composter – the Neter, the largest model ever manufactured by Swedish company Susteco. At the time of installation, it was one of only three in the world, the others as far afield as a school in France and a fishmarket in Japan.

Eden's Neter, specifically designed and manufactured for use on the site, was initially installed as a living experiment to research whether such in-vessel composters are a viable way of treating organic waste without the risk of leachates and emissions of potent greenhouse gas methane.

A two year trial, funded by the BOC Foundation for the Environment and the Combined Universities of Cornwall Research and Development Programme, delivered positive results, with no methane detected on any of the sampling dates. The composter has remained a permanent fixture at Eden.

It is also helping relieve pressure on landfill sites in Cornwall. The UK target is to halve the amount of biodegradable municipal waste sent to landfill by 2013, compared with 1995 levels – though experts have warned that the country is not on track to meet this.

'Eden is in a unique position to be at the forefront of this great experiment,' says Knight. 'Not only do we have enough visitors and plants creating the volume of material needed to make a composter of this size work, but we have specialist waste, horticulture, science and maintenance teams, all of whom play a vital role in producing and developing top quality compost.'

The magic ingredients – how it works

• Food scraps from the kitchens and cafés – everything from vegetable peelings to leftover pasties – are separated into food waste bins by staff and visitors and are then fed into the Neter at the waste compound on the perimeter of the site.
• The waste operations team adds green waste from the gardens, as well as cardboard fluff – from a local animal bedding company – to absorb excess moisture and aerate the compost.
• Rotating blades chop the raw material as it enters the machine.
• During the 80 day aerobic composting process the contents reach more than 60°C and reduce by 75% in volume.
• Eden gardeners use the product as a nitrogen-rich soil enhancement.

It sounds simple enough, but the waste team have perfected the recipe over several years, learning that too much of certain foods – such as lemons and onions – can kill the bacteria which digest the organic matter. They've also found that the system works best with a good proportion of raw kitchen waste along with leftover processed food. So more tasty homemade ingredients on the menu in Eden's cafés doesn't only mean happier customers, but healthier bacteria, too.

James Clark preparing a winter bed in the Global Garden at the Eden Project. Spreading compost from the Waste Neutral Composter

The team are developing their know-how with users of similar composters, such as Dartmoor Prison, which runs a Big Hanna, Susteco's smaller version of the Neter. 'This was one of the first machines of this type in the world, so as we learn how to produce the best compost we're building knowledge that other communities and organisations can use in their own composting,' says Knight.

A valuable product

The deep brown fine dust that the Neter produces can either be mixed with soil or used as mulch. 'From a horticulture perspective we're really pleased with the results we've seen,' says horticulturist Darren Topps. 'And using compost made from waste to grow new plants is a great illustration of the Eden philosophy.' Knight adds that 'plants go crazy' for the powerful stuff, so gardeners wait until the last frost has passed in case they have a growth spurt.

This isn't Eden's first attempt at composting. Before the visitor attraction opened in 2001, the gardening team created 83,000 tonnes of soil, by mixing waste from the former china clay pit with green compost, to turn the site into a landscape capable of sustaining plants from all over the world. The gardening team also use a conventional compost heap to break down waste from pruning and clipping into soil.

An educational tool

What's unique about Eden's food composting process is the way visitors are exposed to the journey of their food from the plate through to the plant. 'It starts as they sort waste from their trays into different containers, and ends as they pass the composting plant, which has been deliberately sited near one of the public car parks,' explains Knight.

Composting is just one element of Eden's Waste Neutral philosophy, through which it aims to buy in the same weight of recycled materials as it sends for recycling or disposal. The idea is to help boost the market for recycled goods by adding one more vital element on to the 'reduce, reuse, recycle' motto: reinvest.

Hannah Bullock is campaigns and communications manager at the Eden Project.
e-mail: hbullock@EdenProject.com

• Eden, home to the two biggest greenhouses in the world, welcomes one million visitors a year to the Cornwall-based tourist attraction.
• Since fully opening in March 2001, 11 million people have visited Eden and it has generated £900 million for the local economy.
• The Eden Trust, which runs The Eden Project, is an educational charity that develops educational programmes and events that highlight the vital link between man and the natural world, and encourage us to live in more sustainable ways.