Breaking It Down: Aerobic vs. Anaerobic Composting

Hearing the term aerobic might make you think of a cardio session at the gym, but it also applies to the two main methods of composting.

Aerobic refers to the breakdown – known as digestion –  of organic materials with oxygen, and anaerobic is without oxygen. These terms usually refer to large-scale composting at facilities, though smaller, backyard composting can be considered aerobic.

In Canada, aerobic digestion is used in most cities and communities that collect compost; it’s considered the more traditional, common method. With this, at a facility, compostable material is placed into a large pile and left for several months until it breaks down into humus, a substance used to condition soil. This is similar to at-home composting, where a pile of soil, food scraps, and some types of paper is put together and turned regularly until it breaks down.

With anaerobic digestion, organic matter is kept in an oxygen-free environment, and when it starts to digest, methane is produced, which can then be used for energy. The City of Toronto, Canada’s largest municipality with a compost collection program, currently uses anaerobic digestion.

There’s a lot of promise in the anaerobic method, as it can turn composted material into a new fuel source with wide-ranging applications. In fact, the City of Toronto has announced plans to capture the “biogas” (methane) from green bin waste and combine it with natural gas to power some homes and city-owned vehicles.

With this said, it’s important to understand that if the methane from anaerobic digestion is not captured or properly used, it can be problematic, as some of it may release freely into the environment. This is actually why composting is so important to begin with, because when food scraps are disposed of in normal garbage – not the ideal environment for digestion – they release methane, and their usefulness is squandered.

Both aerobic and anaerobic composting have their benefits, drawbacks, and useful applications. It’s not necessarily a matter of which is better, but rather, about aiming for higher diversion rates and better management of compost overall. This goes for both consumer engagement, and improved design of compostable products.

This need was seen in an examination of Toronto’s Disco Road compost facility, which processes up to 40% of the city’s green bin waste. While that number is pretty high, there’s still a sorting process that removes items said to be compostable, but aren’t, such as pieces of diapers, napkins, and cutlery. These things unfortunately end up in landfill, and we know that across Canada and elsewhere, diversion rates with compost are not as high as they could be.

To achieve the potential of both aerobic and anaerobic digestion methods, we need to improve the actual compostability of products, and continue to divert waste from landfill. Every food scrap and compostable item can be used for energy, or for our soil to grow more food – it just has to get there first.