Tanneries produce different types of solid organic wastes, most of which contain some toxic chemicals. Anaerobic digestion of the waste can be used to produce biogas, which contains methane and carbon dioxide gases.

When the methane content of the biogas is more than 55% the gas can be used as a substitute for natural gas. This means that tannery waste can be used as a source of renewable energy, according to University of Canterbury final-year engineering student Amanda Kirk.

She is researching the project with Michaela Aspell under the supervision of University of Canterbury civil and natural resources engineer Dr Ricardo Bello-Mendoza and Peter McGuigan, manager of the university environmental engineering laboratory.

Different wastes may be more or less suitable for producing methane gas. Kirk said biochemical methane potential is a measure of the methane gas production of a given organic substrate under anaerobic conditions.

“Tanneries are looking for alternatives to deal with their waste problem. We are undertaking a study of the major organic wastes produced in Christchurch”, she said.

Results can then be used as an exemplar for the methane potential of wastes from other tanneries around NZ.

The five waste types being investigated are sludge pit waste, effluent screen waste, solvent degreasing sludge, drum waste and fleshings. The solvent degreasing sludge is particularly problematic for a tannery as it cannot be disposed of in landfills.

“Our tests result gives us confidence that waste will be able to produce useful energy for the tannery. We have also investigated using mixes of waste to achieve higher gas production”, Kirk said.

Mixes of the degreasing sludge and waste fleshings were able to produce up to 65% methane gas content. Mixing the waste types allows more waste to be used in the process, particularly important for reducing landfill disposal costs and using up the degreasing sludge, which is currently stored on site.

Research has shown there is potential for tanneries around NZ to produce useful renewable gas for onsite energy production.

“It is hoped this research will help tanneries in future meet most of its energy needs from its own waste’’, Kirk says.

Source: NZ Resources