STOCKPILED MILL MUD/ASH: ENVIRONMENTAL IMPLICATIONS AND CHANGES IN NUTRIENT VALUE WITH AGE

A field trial was established near the Broadwater sugar mill in NSW to assess the environmental impacts and changes in nutrient value of stockpiled mill mud/ash. Stockpiles of mill mud/ash were constructed and instrumented so that both runoff from the stockpile and leachate moving through it could be measured and chemically analysed. Regular sampling of the stockpiles for up to 350 days was used to assess nutrient changes with age. Mud and mud/ash stockpiles are very permeable with 31% to 48% of rainfall passing through as leachate. Runoff was negligible. Leachate concentrations of the mineral forms of N and P are high but the amounts lost were small, although there is potential for some environmental risk from P in the early period of stockpiling. Very high biological oxygen demand (BOD5) levels in the un-attenuated leachate are of concern and have implications for the siting of stockpiles. The excess liquid initially draining from the stockpiles had the greatest oxygen depletion potential. Even though there were losses of N (22% to 33%) and P (6% to 16%) from the stockpiles, the loss of mass due to carbon mineralisation resulted in a slight, although non-significant, nutrient enrichment for N and a significant nutrient enrichment for P (22% to 32%). Information from this study will assist the industry in developing best practice guidelines for stockpiling mud and ash by-products, which are an important component of the overall distribution and recycling process.