Driven to reduce impact: Australia’s most efficient waste management facility
Owned and operated by the Southern Metropolitan Regional Council (SMRC), the waste management facility in Perth, Australia has reduced landfill by more than 100 000 tonnes per year. This remarkable environmental achievement is due in part to the unique drive solution used on the facility’s digesters.
The seven Perth councils that make up Australia’s Southern Metropolitan Regional Council (SMRC) are understandably proud of their waste management. Their high-quality compost is increasing the yield of local farms, and the equipment that produces it is the most efficient in Australia. Each year, 5000-6000 domestic and international guests visit the area to learn and to be inspired.
But things were not always this good. As recently as 1998, the SMRC was using traditional waste management methods and sending 87% of its domestic waste to landfill. Determined to improve the situation, the council set its sights on state-of-the-art technology.
SMRC representatives, including CEO Stuart McCall, travelled the globe in search of advanced reclamation methods for a new waste management facility. Among their discoveries was a unique form of in-vessel composting, encountered by McCall in Atlanta, USA. “The key advantage,” McCall says of the technique, “is that it doesn’t rely heavily on people changing behaviour. It turns about 2/3 of the waste people put into the green bin into compost.”
Having selected in-vessel composting as the method of choice, the SMRC designed a facility of overwhelming size. In addition to a materials recovery plant for recycling and a processing plant for green waste, the facility was to feature the second largest in-vessel composting plant anywhere in the world.
A question that remained, however, was how to drive the facility’s four massive digesters. The rotation of the digester shells, each 70 metres long and 4.6 metres in diameter, would be the critical mechanism in turning raw waste into compost. Girth-gears and electromechanical motors seemed an obvious choice, but analysis showed that these would be prone to excessive wear.
“The fact that the waste changes volume and density at various stages of its travel through the digesters means the digester shell deflects and does not remain completely straight,” explains Kevin Jones, project manager for the SMRC. “This would create a lot of wear and tear on a conventional girth-and-pinion drive, because continuous full-face contact would be impossible to achieve.” In other words, a traditional drive system would mean increased downtime and a heavy dependence on spares.
Instead, the SMRC chose a less conventional solution based on hydraulic motors from Hägglunds. Hydraulic drives, which do not require the foundations, gearboxes or clutches of electromechanical systems, both simplify construction and offer a wide range of operational advantages. For example, they feature a low moment of inertia and can be run with infinite variations in speed.
For the SMRC’s facility, Hägglunds delivered sixteen of its hydraulic motors. These were grouped four to a digester, along with tyres to produce a friction-drive solution. Each tyre was set in contact with two trunnion rollers, to be driven in turn by two hydraulic motors.
Since the facility’s inauguration in 2004, the results have spoken for themselves. The friction-driven trunnion rollers, whose shafts are self-aligning, provide full-face contact that drastically reduces wear and tear. According to project manager Kevin Jones, “Our most recent measurements and predictions indicate that the driven tyres will last 194 years!”
Likewise, the Hägglunds solution has protected the facility from unplanned stops in production. Because the drive system automatically senses load shifts and directs power to the motor that needs it most, each digester can be operated with as few as two motors. “If one or even two motors are out of service,” says Jones, “production can still continue at a lower throughput. Damage a girth gear, and the digester would be down for months while a replacement is manufactured.”
But above all, the drive system has helped the SMRC get the most out of its composting technology. The council expects to maintain plant efficiency at 84% over the long term, compared with an estimated girth-gear drive efficiency of 70-75% at best. “Over 20 years,” Jones predicts, “this will account for huge savings in electricity.” Already today, the plant’s energy consumption is 40% less than originally estimated.
Overall, the new facility has achieved a waste recovery rate of 70%, which is nearly 20% better than that of its closest rival. The SMRC has the most efficient waste management in Australia, having reached the facility’s design capacity already in its first full year of operation. Says CEO Stuart McCall, “Next year we’ll increase production to 135 000 tonnes, which should see us produce almost 80 000 tonnes of compost.”
The impressive figures have been noticed in Australia and elsewhere, which helps to explain the high number of visitors to the SMRC each year. As word spreads further, McCall is convinced that the facility will be copied both nationally and internationally.
Yet even so, the SMRC is far from resting on its laurels. McCall is focused on the future, and has set his sights on a far more difficult target. “We’re proud and happy that we’ve been able to reduce landfill by more than 100 000 tonnes per year,” he says, “but within the next 20 years, we’re hoping to get rid of landfill altogether.”