Like a giant human stomach, the anaerobic digesters at the Cronulla Wastewater Treatment Plant in Sydney produce solids and gas. Unlike a giant human stomach, the gas is put to good use fuelling a generator which contributes electricity to power the plant.

One way to get more gas out of one end would be to add more food at the other, and that’s a theory Sydney Water started testing in May this year.

So far, things are going well.

Waste food is collected from seven local fruit and vegetable retailers, including unsold produce, outer leaves pulled from cabbages and lettuce and anything else discarded in the backrooms as fare is prepared for display. Yesterday’s fruit and veg is then mixed with sewage sludge and fed to the digesters, where things bubble along to release about 60% methane and 40% carbon dioxide.

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“The digester has a very stable base of waste water solids, which doesn’t change too much,” says Sydney Water service planning lead for energy Phil Woods. “We’re adding about 10% feed of the food waste. It’s only a relatively small proportion of what’s in the digester now.”

The resultant biogas fuels an 830kW generator, and the aim is to increase generation to 6GWh from the current 5GWh from biogas produced without the addition of food scraps.

Waste heat from the generator goes back to maintaining the temperature of the digesters.

Hard to digest

Nothing ever goes exactly to plan, however. “With this sort of thing when you start it’s a case of expect the unexpected,” says Woods. For a start, the supply of scraps can be uneven, and fibrous gunk from celery sticks has been clotting up the odd artery here and there. When you’re relying on what others throw out with the trash it’s hard to expect too much, but Woods is working with suppliers to ensure minimal contamination. “We’re not quite there yet,” he says.

There is no argument about the economics of the process, however. “We can be competitive or beat the market in terms of existing options of disposal of that waste,” he says. Apart from the biogas used for generation the solids are dispatched for use as agricultural fertilizer.

If all goes well with the pilot program the process may be extended to other sewage treatment plants around the country, he says, and the team is also conducting deeper research on the quality of the biogas and digested solids at a site in Shellharbour, NSW.

“There is a good understanding of doing this around the world in terms of it being a productive way of dealing with waste but the science around it is a little bit limited, in terms of detailed scientific research,” he says.

A balanced diet

Any plant with an anaerobic digester can be adapted to take food waste, Woods says. As to building new sites, it will always be a challenge to get approval in heavily populated urban areas. It’s far easier to add digester capacity at an existing site. He expects the Cronulla trial will be running at 100% “very soon”.

“What we’re hoping to get out of these pilots is a real hands-on experience and a much better understanding of how to deal with different types of waste so that we offer this service as a much larger scale in the near future,” he says.

At the launch in May NSW Minister for Primary Industries, Lands and Water Niall Blair said the project will also save 150,000 wheelie bins of fruit and vegetables a year from landfill. “That’s 600 wheelie bins a day, five days a week,” Niall said.

Environment Minister and Member for Cronulla Mark Speakman said renewable energy produced from food waste will generate more than 60% of the energy the plant needs to run, “which is enough to power a third of homes in Cronulla for a whole year.”

“Fruit and vegetable waste which is typically driven many kilometres away for landfill will also now stay in Cronulla,” Speakman said. “This means fewer trucks travelling long distances and a saving of 90,000 kilometres each year.”

The three-year trial is jointly funded by Sydney Water and the Office of Environment and Heritage’s Sustainability Advantage Program.