A yeast factory in Toowoomba was faced with the problem of finding a disposal option for its high strength dunder. The dunder was the waste from the yeast propagation vats and was normally disposed to sewer. The waste had direct entry via the sewage works to Gowrie creek and subsequently the Murray-Darling river system. The factory undertook a waste audit and management assessment process and came to the conclusion that the dunder could be applied to land as resource recovery of the dunder's nutrients. The timeframe to devise a resource recovery plan was short and failure would have led to the factory closure.
A research project was initiated with three objectives:
1. to devise a theoretical framework (method) to "fast track" the assessment of dunder characteristics and match them to possible land users;
2. to use the framework to do an environmental hazard assessment, determine possible impacts and outline rehabilitation and monitoring procedures specifically for high application rate accidents; and
3. to test the efficacy of the dunder as a nutrient source in an agricultural production system, outline environmental impacts and monitoring procedures specifically for low level repeated application rates.
A framework was outlined that was a hybrid of the standard land treatment assessment and the Australian mining industry environmental risk management (ERM) process. The process was prescriptive and transparent.
A field trial to test the conclusions about the environmental hazards was undertaken. The trial used on two soils types (sandy clay loam and a clay loam). A randomised complete block design with three replicates and five treatments of 0, 2, 6, 12 and 24 litres dunder per square metres was used. The dunder was applied weekly for four weeks and the plots sampled for dry matter production and plant uptake of N, K, Na, Ca and Mg. Soil samples in 100 mm increments were taken to 400 mm depth and analysed for C, N, K, Na, Ca, Mg content, electrical conductivity and pH. A further three weeks of dunder was applied and the plots soil sampled again. The environmental hazard trials identified the hazards of nutrient balance (mainly potassium), soil salinity, soil sodicity, short-term anaerobic soil conditions and impeded soil surface infiltration. All impacts were short term and simple control measures, rehabilitation procedures and monitoring protocols were sufficient for environmental protection. The trial showed the dunder to be a K fertiliser.
The agricultural system trials tested the efficacy of the dunder in comparison with mineral K fertiliser and the environmental implications of low level applications of dunder. The dunder was tested on a cereal and legume hay production system, cabbage and tomato crops and permanent pastures on a Sodosol and a Vertosol. The trials showed the dunder to behave as a substitute for mineral K fertiliser. There was neither adverse impact on the plants nor the soil characteristics. The trials showed the dunder could be applied as a surface application and single basal dressing. The monitoring requirements were shown to be the same as standard agricultural practice in the region.
The risk assessments and trial data identified the dunder as a soil conditioner/fertiliser. The dunder is now dispersed to a variety of grazing and farming properties within a 40 km radius of the factory and is now no longer disposed to sewer and the Murray- Darling river system. Overall, the methodology and its indicated outcomes have general application to the assessment of any waste that may be land disposed for nutrient resource recovery. The cyclic and iterative process used in this study progressively focuses the planning and research for land disposal from a broad environmental assessment to individual crop and land units. The use of this approach minimises risk of environmental impacts and maximises the integration of the waste with land use practices.