Fast composter for city waste

Researchers from the Environmental Biotechnology Cooperative Research Centre (EBCRC) have successfully tested a, self-sustaining process that rapidly composts municipal solid wastes.

Sydney based biotech company Organic Resource Technologies (ORT) developed the DICOM process and is now attempting to commercialise it. CRC researchers are helping the company develop a more complete understanding of the microbiology involved.

The anaerobic phase of the process yields a steady supply of methane, that fuels an electrical generator to provide process heat to sustain the high operating temperature of the anaerobic phase.

Lee Walker, a Phd student with the CRC, said municipal wastes and household rubbish initially go through a mechanical sorting process to remove non-degradable materials.

The organic fraction is then loaded into a sealed, aerobic reactor for five days of controlled aeration. Aerobic microbes - bacteria, actinomycetes and fungi - generate heat as they replicate, raising the temperature in the reactor to around 55-60C.

After five days, the hot, decomposing material is exposed to anaerobic conditions, where facultative anaerobes from the first aerobic phase switch to anaerobic decomposition, aided by heat loving Archaea microbes, which generate methane gas to fuel the generator, in a parallel, self-sustaining cycle.

The anaerobic, thermophilic phase is the centrepiece of the process - as a result of the anaerobic phase, composting times can be reduced to only 19 days, according to Walker. A microbe-rich liquid floods the reactor

Walker said the DiCOM process offers multiple advantages over current municipal composing systems, which are becoming increasingly common as cities attempt to reduce the volume of wastes going to scarce landfill sites.

"It operates in a sealed reactor, which allows us to collect the methane as a fuel to power the plant," he said. "There's no waste from the process - a typical compost heap loses leachates to the soil, but in ours, all the nutrients stay in the system.

After seven days, the microbe-rich fluid is drained and recycled into the next charge of organic wastes in a parallel reactor, while the drained wastes undergoes another phase of aeration.

"At the end, you're left with a compost that is reasonably rich in nutrients," Walker said.

He said his PhD project is trying to determine the composition of the microbial flora of each stage, and to understand the contributions of the different species to the decomposition process -- with the prospect of improving it by manipulating the microbial flora, or the environmental conditions inside the chambers.

Based on positive results from the pilot-scale plant, Organic Resource Technologies is now building a full scale demonstration plant for Perth's Western Metropolitan Regional Council, capable of processing 20,000 tonnes of municipal solid waste.

If the demonstration plant is successful, ORT will build a full-scale commercial plant capable of processing 55,000 tonnes of municipal solid waste a year. ORT has longer-term plans to export its expertise, and license its IP in the international market.