At its Sarina distillery, near Mackay in North Queensland, CSR Distilleries ferments molasses to produce ethanol for Australian and export markets. One of the waste products of this process is a liquid known as dunder.
The distillery, which CSR purchased in 1972, discharged dunder directly into the tidal creek systems adjacent to Llewellyn Bay, within the Great Barrier Reef. As the local population grew and community expectations of environmental performance increased, CSR implemented a trade waste system, based on spray irrigation onto ploughed evaporation plots. This system operated satisfactorily during dry weather, but problems were experienced during the tropical wet season.
For example, biological oxygen demand levels in run-off from the plots were high, resulting in low oxygen levels in the local creek system. Discolouration of water sources was also a problem, and the onset of warm humid weather occasionally caused maggot infestation within the plots, leading to high levels of house flies on and around the plots. These, plus odour problems, put CSR ‘on the nose’ with the local community.
Although the local community valued the distillery as a major employer, CSR and its employees wanted to improve the community attitude towards the distillery's operation and to improve the condition of the spray irrigation site.
Several options to improve the method of dunder disposal were considered, including:
- incremental improvements to the existing spray irrigation system;
- installation of an incinerator to evaporate and burn the dunder, to generate steam and produce a granular
- installation of an ocean outfall pipeline;
- installation of an anaerobic digester to generate methane and reduce the biological oxygen demand of the
- construction of a new distillery using new technology; or
- cessation of distillery operations altogether.
In examining these options and the economics of the plant, CSR concluded that any dunder disposal operation had to generate savings or revenue sufficient to justify the building of a new plant. Otherwise, it would have to close.
The new plant option was considered in detail, with particular attention to the possibility of reducing water consumption and hence concentrating the dunder into an economic by-product. The solids component of dunder has always been recognised as a significant source of potassium, with potential for use as a soil fertiliser. However, the high water content of dunder resulted in a low concentration of potassium, and consequently widespread use of dunder as a fertiliser source was limited due to transport economics. Consideration of the new plant option focused on producing fertiliser as a viable by-product.
Cleaner production initiatives
During the 1980's, CSR built and operated a small-scale (one-tenth capacity) treatment plant to test a new continuous fermentation technology called Biostil. Laboratory trials indicated that the process would increase ethanol yield, but more importantly, it would effectively double the dunder concentration and reduce steam consumption.
To further improve the process, particularly for the handling of trade waste, the pilot plant was used as a model to determine how to increase the dunder concentration further. By modifying the pilot plant's operation to a semi-continuous mode, the plant successfully produced dunder at four times the original concentration.
After further research, CSR constructed a new distillery at Sarina, using a two-tank, semi-continuous Biostil fermentation system, coupled with the latest technology in heat reuse distillation system.
Advantages of the process
The plant was a world-first for Sarina and achieved:
- conversion of dunder to a by-product called Biodunder, which was sufficiently concentrated to be used as a
- canefield potassium fertiliser;
- elimination of odour and contamination of local waterways;
- increased plant productivity;
- improved quality of ethanol produced by the plant;
- reduction in steam usage within the plant by 30%; and
- reduction in water consumption by 70%.
The new plant commenced trial operations in May 1989, and moved into fully operational mode almost immediately. The distillery is fully automated, and is operated by one person using computer-controlled equipment.
Biodunder has been accepted by cane and other farmers as a valuable potassium-rich fertiliser and 100% of the Biodunder product at the Sarina distillery is now recycled into this market (see Graph below). Previously, only imported fertiliser was used by the cane farmers in the area. CSR, through its Sarina distillery, is now working with the sugar industry to improve and add further value to the Biodunder by adding nitrogen and trace elements.