At Veolia’s Bulla Organics Facility in Melbourne’s north-west, food and garden organics are saved from methane generation at landfills and transformed into high-grade compost.

Words by Chris Sheedy

This article was originally published in the November 2024 issue of create with the headline “Closing the organic waste loop”.

There is a common belief that organic waste is fine to send to landfill. After all, it would simply break down over time. However, sending nutrients back into the soil and resulting in a healthier natural environment is the
preferred option.

Not only does placing organic waste in landfill miss a powerful opportunity for resource recovery and circularity in waste management; it also poses serious environmental risk.

When organic waste decomposes anaerobically, or in an environment of low oxygen – as is the case when the waste is buried – methane is released as a result of the breakdown process. Methane, compared to carbon dioxide, can be 26 times more potent in terms of greenhouse gas emission.

In an engineered composting facility such as Veolia’s Bulla Organics Facility in Melbourne, the composting occurs in an aerobic environment.

Image: Veolia

The facility, constructed in 2011-12 after Veolia tendered for the processing of organic waste from 11 local councils in the north-west of Melbourne, is centred around 14 enclosed vessels into which the organic materials are placed. The vessels are purpose-built with a forced aeration system.

“The floors have a series of pipework underneath that forces air through the organic mass,” Mark Globan, Veolia’s General Manager, Resource Recovery, told create“There’s a tarpaulin that goes over the top that encloses the system, and a door at the front.”

“One of the key desired outcomes with the engineering of the facility was to have a system that sampled the oxygen inside the vessel.”
Tony Strobbe

Within the vessels – each measuring 24 m long, 4.2 m wide and 3.6 m high – sensors measure temperature, moisture and oxygen saturation.

“We need to get up to greater than 55°C for more than 72 hours to meet the Australian standard,” Globan said. “We hold it for a little bit longer and at a slightly higher temperature, just for comfort. If the sensors say it’s getting too hot, we’ll force air through to bring it back down.”

A vital ally

That airflow is vital in preventing the engineered aerobic system from becoming anaerobic and releasing harmful methane into the atmosphere.

“One of the key desired outcomes with the engineering of the facility was to have a system that sampled the oxygen inside the vessel,” said Tony Strobbe, Veolia’s South-East Organics Manager. “Once you’re below 10 per cent oxygen, you’re heading towards anaerobic.

“There’s a fail-safe system that ensures the oxygen is always above ten per cent. That’s calibrated every hour, on the hour. If required, oxygen is pulled from the surrounding air, and once the oxygen in the mass is at an acceptable level, the system goes back into recirculation mode.”

Research for and development of the facility dates back to the late 1990s and early 2000s, when Veolia ran a similar but smaller plant in Dandenong.

“We originally ran two vessels and eventually built up to five,” Strobbe said. “We had to engineer these facilities to suit the Australian market. The design in later plants has been very similar. We’ve just improved on the original and fixed some problems.”

Aeration holes in the floor are roughly 600 mm apart over the full, 24 m length, with each hole measuring 18 mm in diameter. A false concrete floor made from 200 mm double mesh combines with concrete walls of 150 mm single mesh to allow liquid to escape onto a floor that slopes at one end.

“Without this, the liquid would sit in the base of the pile and convert the process into an anaerobic state,” Globan said. “Removing the liquid allows the process to remain in an aerobic state.”

The plant runs 14 small vessels instead of fewer, larger ones, Strobbe said.

“If you have a very long vessel and something fails – if you’re unable to keep the mass above 55°C for 72 hours, for example – that’s a major failure. With a batch system, you engineer out that risk.”

Sorting and grinding

When trucks loaded with food scraps and green organic waste arrive at the facility, they pass over a weighbridge before dumping their valuable cargo onto a pad.

Fine materials are separated out from larger materials, which are taken to a sorting station where a team of six to eight staff members sort the acceptable materials from the non-acceptable ones.

“Sometimes people will throw out their garden clippings, and sometimes they will throw out their entire garden pots,” Globan said. “Every so often, we’ll find a sledgehammer or another garden tool – all sorts of stuff.”

Large items such as tree branches are sent through a grinder before being remixed with the fine products and sent into the vessels for processing.

Leave it to the microbes

The temperature of the mass must be kept slightly above 55°C to enable micro-organisms to decompose the various organic materials without causing the release of methane.

“If the mass is too hot or cold, the process stops,” Globan said. “If you don’t have the optimal conditions in terms of oxygen, the microbes decide the environment is not working for them, and once again the process slows or stops.”

If that occurs, the material must be removed and the process restarted, or the mass must be allowed to remain in the vessel for twice as long as it previously would have.

“Finding the optimal temperature in the cooler months might take a day or two longer than it would in spring or summer,” Globan said. “No matter the time of year, we don’t use heated air. The heat itself is generated by the microbial activity. Our job is then to create the environment in which it can remain above 55 degrees, and to help it cool down if it goes too much higher.”

Waste to wealth

From more than 90,000 t of feedstock, approximately 40,000 t of high-grade, nutrient-rich compost and 20,000 t of mulch
is produced.

The products are sold back into the market, including to garden centres, landscaping companies, civil construction firms, horticulture and viticulture businesses, and broadacre farms.

In doing so, the facility not only prevents organics going to landfill. It also improves soil health and increases stored carbon in the soil where its products are used.

While the Bulla Organics Facility is not carbon neutral, it utilises solar panels to offset some of its energy requirements. Its carbon emissions are expected to further reduce as renewable energy options come online.

“On the waste hierarchy, at the very bottom of the inverted triangle, is disposal of waste via landfill. If there’s one thing we really want to avoid, that’s it.”
Mark Globan

Sustainability and community outcomes are front-of-mind for the facility, whose staff pride themselves in the role they play in creating a circular waste management environment.

“Having the composting in an enclosed environment also significantly reduces any risk of odour impact, which is important for the local communities,” Globan said. “But most importantly, we’re removing organics from landfill.

“On the waste hierarchy, at the very bottom of the inverted triangle, is disposal of waste via landfill. If there’s one thing we really want to avoid, that’s it. Bulla Organics Facility makes that possible.”

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