As May is compost celebration month for west London we’ve been writing about home composting for the last few weeks. Getting started, feeding your compost bin and avoiding some basic problems. We know there are some things that can’t be composted at home in your garden and of course many homes in west London don’t have a garden so how can the nutrients from food we don’t eat be turned into something useful?
Most homes in our area have access to a food waste recycling service provided by their local council. Every month, hundreds of tonnes of food waste are collected from homes and then sent for composting at large commercial facilities.
Depending on which area you live in and how your food is collected it will be sent for composting either at an in-vessel composting plant in west London or Kent; or at an anaerobic digester in Hertfordshire or Northamptonshire. Here’s what happens…
The Anaerobic Digester
Generally where your food waste is collected in caddies from your home it is taken to local depots, loaded into larger containers and transported by road to the plant run by a company called Biogen.
The technology is called anaerobic digestion (otherwise known as AD), the food is broken down to produce biogas and biofertiliser. This process happens in a sealed, oxygen-free tank called an anaerobic digester.
This picture shows the steps in the process.
Once your food waste arrives with all the other food, checks are made to make sure there’s nothing else in there such as plastic bags or tins or plastic packaging that can’t be composted.
The food is then closed in the digester and begins breaking down naturally to generate heat, gas and a liquid. The gas is described as a biogas and is used as a fuel in a CHP (combined heat and power) unit to generate renewable energy i.e. electricity and heat. The food waste collected from west London households is enough to power thousands of homes.
What’s left from the process is a nutrient rich biofertiliser which is pasteurised to kill any pathogens. You could think of it as really potent liquid fertiliser like the brands you buy to feed your garden or house plants. The biofertiliser is stored in large covered tanks ready to be applied twice a year on farmland as a natural fertiliser made from foods that would otherwise just be sitting in a landfill site generating the harmful greenhouse gas, methane.
The other location your food waste could go is the in-vessel composter.
Both food waste and garden waste are treated together in the process known as in-vessel composting (IVC for short). Yours may be processed at an IVC in either west London, operated by a company called West London Composting, or in Kent, by a company called Countrystyle Recycling Ltd. Everything arriving at the IVC plants is weighed and then the lorries tip their contents in a large reception hall. Just like at the AD plant the load is checked for non-compostable items, before being shredded ready to be composted.
The shredded organic matter is then put in enclosed containers called vessels. Once it’s sealed in there it stays for up to 10 days. The vessel is carefully monitored to make sure it heats up to at least 60⁰C for 2 consecutive days. There’s no need to add heat, the bacteria that naturally exists in organic matter starts breaking it down and heat is created as part of this natural process.
After a couple of weeks and of being hot enough the contents of the vessel is moved to another vessel. Just like with your home compost bin moving the material lets in more air to help the process and mixes up the material. This second vessel is also monitored closely and the same process happens.
After about another 2 weeks the compost is ready to mature outside. It’s piled up, gets turned every week and continues to break down. There are lots of uses for this compost. Depending on how it’s going to be used it may be shredded again to break down some of the big pieces. West London Composting, have a booklet setting out this process in more detail if you want to know more.
All composting plants have very strict rules to follow. Each type of technology is inspected and licenced to make sure that what’s produced is a good quality product and meets a minimum safe standard specification.
Compost celebration events
The compost being given away at our compost celebration events is from West London Composting. When you come to one of our events you might be able to see steam rising from the pile of compost because it’s still generating heat and if you hold some in your hand it’s often warm, especially if you dig in deep.
The events in Brent, Hounslow and Harrow will be taking place over the next two weeks. Come along and you could take home some great advice about home composting from our compost experts, information about your local recycling services and some of the wonderful quality compost created from food waste produced in west London.