What really happens to the organic waste you put in your compost bin

In many communities, residents are asked to separate food waste into bins for municipal composting. 

In theory, composting can create a circular economy of food: it is grown, it is eaten, the scraps are thrown out and turned into fertilizer sold to farmers, who grow more food. But most consumers are unaware of what the composting process actually involves. 

There are two kinds of municipal composting: aerobic (with oxygen) and anaerobic (without oxygen) digestion. In both cases, micro-organisms break down biodegradable material into compost that can later be used as fertilizer.

Aerobic digestion, also known as traditional composting, is the more low-tech, and is done in jurisdictions such as Metro Vancouver, Nova Scotia and Montreal (although the city has approved plans to build two new composting facilities, one of which will use anaerobic digestion).

In its simplest form, traditional composting requires making a pile of wet organic matter 一 made up of yard waste, food scraps and occasionally sewage 一 and waiting several months for the waste to break down into humus (which is compost that's mainly used for conditioning soil). This is something you can do in your backyard.

With anaerobic digestion, food waste is put in an oxygen-free environment, instead of being piled up and left in the open air. Food in the digester creates methane, which can be captured and used as a source of energy.

Dianne Saxe, the former environmental commissioner of Ontario who now leads the consultancy Saxe Facts, said that, if left to the landfill, food waste contaminates groundwater and releases methane, most of which is not used for energy.

Instead, the methane — a potent greenhouse gas — is released into the atmosphere.

Anaerobic digestion

Anaerobic digestion is the less common method among municipally owned and operated organic waste facilities in Canada, but it's being used in the country's biggest city: Toronto.

The facility on Disco Road collects up to 45 per cent of Toronto's organic waste and converts it into something renewable. The air inside the facility is thick and heavy. The smell of rotting garbage is so strong you can feel it permeating your skin and sticking to your clothes.

During a recent visit, Derek Sawyer, supervisor of organic processing in Toronto, pointed to a towering pile of compost bin waste, all stuffed into bags of various colours and sizes.

The waste, all residential, sat wrapped in layers of plastic before it was piled onto conveyor belts by a large, green excavator, where it will be taken to a large, loud machine called the hydro pulper.

The hydro pulper is a large, whirring machine that uses water to remove non-compostable materials, known as contaminants. Sawyer said contaminants can comprise up to 18 per cent of every batch.

Those contaminants include items that residents are asked to put in the green bin, and that many believe to be compostable —and although they're accepted in the Toronto's green bins, this includes parts of disposable diapers and sanitary napkins. 

The light materials — plastic bags, diapers, compostable forks and straws — float to the top of the pulper, where some are shredded and others are skimmed away by a giant claw machine. The screening isn't perfect — small shreds of plastic are still visible in the resulting compost — but it gets the amount of plastic low enough to meet standards.

The heavier contaminants — glass, metal, dirt and shells — fall to the bottom and caught in the catch basin of the hydro pulper before being removed. Even items that could be recycled, such as metal and glass, end up as garbage.

Recyclables head to dump

The organic waste becomes a sludge, ground up and sopping wet, before being  brought to large tanks called anaerobic digesters. In the digesters, micro-organisms, in the absence of oxygen, break down the material to produce the pre-compost stage — called digester solids — and release biogas, a mix of methane and carbon dioxide. 

The digester solids are then sent to a third-party company — in this case, All Treat Farms, about two hours west of Toronto —  to be heated, aerated and turned into usable compost.

The biogas is filtered out with big fans, and used to mix and heat the anaerobic digesters, as well as burned to keep the facility warm during the winter months. Some of it is also used to heat the digester tanks in the facility. 

Disco Road currently burns the remaining gas. In the future, the City of Toronto hopes to sell it as a more renewable and sustainable type of fuel used for heating and vehicles. 

All liquids — liquid digestate, rainwater and all other water used on-site — is collected and purified so that it can supply most of the facility's water needs, mainly for drinking and to clean instruments.

Aerobic digestion

Aerobic digestion creates nutrient-rich fertilizer without diverting partially decomposed waste to a different facility. 

It can still be a complicated, multi-step process, where staff measure how much water, air, carbon and nitrogen-rich materials go into the pile.

Each ingredient does its part in the decomposition process:

• Oxygen oxidizes the carbon, which sparks the process.

• Carbon produces the energy and heat needed. 

• Nitrogen encourages the growth and reproduction of micro-organisms.

• Water maintains the microbes needed to decompose.

The process is started by shredding yard waste, adding water and regularly stirring or turning the pile. Bacteria and fungi that need oxygen to survive manage the decomposition by converting the materials not only into compost, but also into heat, carbon dioxide and ammonium. 

This method can be shortened from months into just two to three weeks.

With both aerobic and anaerobic digestion, the final product — compost — is sold to farms, plant nurseries, grocery stores and garden centres. 

But the majority of Canadian organic waste still ends up in the landfill. Ontario alone throws out two million tonnes a year, the Ontario government reports.

According to Saxe, looking at organic waste as a resource can help raise awareness and build an industry based on the principles of the circular economy.