As Australia moves away from traditional waste management methods and plans for a carbon constrained future, options for the production of energy from waste take on greater importance. If energy from waste technologies are to be widely adopted in Australia, three main issues must be addressed, Veolia’s Shaun Rainford told Enviro 2010 last week.

Those issues are: matching the technology to the volume of waste available; lifecycle cost-benefit analysis of the solution, including greenhouse gas impact; and consideration of the robustness of the technology.

Prior to recovering energy from waste, high energy components of the waste stream can be removed for recycling. Maximising the source separation of these materials (such as aluminium, steel and plastic) recovers the embodied energy present, avoiding the energy associated with the manufacture of new products from virgin materials.

But what about the post-recycling residual solid waste stream? Rainford analysed three options: anaerobic digestion, landfill gas recovery, and thermal treatment technologies. Each has differing characteristics in terms of the volume of waste required to make them viable, use of the output products, community acceptance, technology risk, cost and climate change impact.

Among the conventional, commonly utilised ‘proven’ technologies are anaerobic digestion in a controlled environment, with methane collected to produce electricity, collection of methane from landfills and thermal technologies such as traditional mass-burn incineration, use of refuse derived fuel in cement kilns and power stations.

There are also emerging thermal technologies, being pyrolysis of waste under heat with little or no oxygen, and gasification, a thermal and chemical reaction between waste and a gas (oxygen and/or steam and /or hydrogen). This includes new ‘plasma’ gasification technologies.

Rainford focused on the proven technologies and their application to residual solid waste in Australia.

Anaerobic digestion: Earthpower
Earthpower was Sydney’s first anaerobic digestion facility for source-separated food waste. Earthpower has been operating since 2003 but was only purchased by Veolia and Transpacific in 2007. T

It accepts solid and liquid food and organic wastes and produces methane as well as a fertiliser product for which there is a healthy market demand. The facility has a capacity of 80,000 tonnes per year, but is currently operating at 40,000 tonnes per year.

Rainford said the anaerobic digestion process works well, although it is complex and is being continually improved as more experience is gained. Compared to landfill technologies, the facility is complex to operate, leading to greater potential for process failures and sub-optimal operation.

The process cannot currently compete with landfill prices, and to be commercially viable requires a gate fee higher than landfill. There has been a marked improvement in rates over the past couple of years, as the previous operator was accepting material well below landfill prices and the waste received was generally poor quality. The NSW landfill levy has improved viability considerably.

The facility competes for feedstock with soil injection operations and stock feed manufacturers. These competing industries are less strictly regulated and less complex than Earthpower, and it is a constant challenge for Earthpower to try to maintain feedstock volumes.

Quality of feedstock is a major issue. The process does not handle contamination well, and continuous education of customers is required in order to reduce contamination in incoming waste streams. This is particularly an issue when councils introduce a separate organics bin for residents, as historically these have been plagued by high levels of contamination.

Labour and maintenance are major costs for Earthpower. Veolia and Transpacific are about to undertake a total cleanout of the digesters, after years of sediment buildup inside the large digester tanks. Significant investment is required to achieve our aim of increasing the throughput volume to 80,000 tonnes per year within the next two years.

The process only works for source-separated organic waste streams. Earthpower previously tried unsuccessfully to operate the facility using mixed wastes with high levels of contamination. The digesters blocked with raft and grit. A mechanical pre-sorting line was retrofitted to the front of the process, however this added cost and complexity with no real benefit.

One of the first actions Veolia and Transpacific took when they purchased the facility was to stop accepting mixed waste and concentrate on improving the quality of feedstock. This led to an immediate bottom line improvement.

Earthpower now houses three 1.3MW Deutz gas engines that produce electricity from the methane captured from the digesters. This renewable energy production, which is fed into the NSW grid, and the associated Renewable Energy Certificates created, are an important revenue source for the facility, and provide greenhouse gas reductions by displacing coal-fired power and avoiding methane that would have been released if the waste had been disposed in landfill.

Our experience with operating the Earthpower facility leads us to the following conclusions about the potential of anaerobic digestion technologies to treat waste in Australia:

• Anaerobic digestion is a well-proven technology for processing source-separated organic waste with very low levels of contamination;
• Anaerobic digestion facilities cannot yet compete commercially with landfill prices, and currently demand a price premium in order to be commercially viable;
• The technology is not robust enough to effectively deal with high levels of contamination or mixed waste streams; and
• Greenhouse gas benefits are derived from the avoidance of landfill methane and the production of renewable electricity; however the process is more electricity-intensive to operate than landfill, so life-cycle comparison shows that the results are comparable with best-practice landfill technologies.
  

• Bioreactor landfill technology is robust, in that it is not sensitive to mixed waste inputs and contamination;
• The technology for collecting methane from landfills and converting it to electricity is well proven and widely utilised;
• There is currently a healthy end market for the electricity and renewable energy certificates produced by these facilities, and the greenhouse gas benefits are comparable to those of more complex and more expensive alternative technologies; and
• Bioreactor landfill technology does not maximise the recovery of recyclable materials from the waste stream.