Nitrogen dynamics of amending coal gasification ash with organic industrial sludge: implications for dumpsite rehabilitation

Abstract

Coal gasification ash (CGA) is the waste by-product of the process of producing synthetic fuel from coal. Every year, millions of tons of this saline, alkaline waste are generated and although alternative uses have been found such as for brick-making purposes, transport costs make it unviable to reuse this bulky material beneficially, leading to the inevitable stockpiling of the waste product. It has thus become increasingly evident that onsite rehabilitation of ash dumps is the most workable waste management option. Use of sludge to amend ash dumps has been suggested since sludge could improve the physical and chemical properties of ash and aid the establishment of vegetation on these dumps. Sludge could add a range of necessary plant nutrients, especially nitrogen. Nitrogen in sludge, however, is bound up in organic forms which have to be mineralized into plant-usable nitrogen species subject to a wide range of physical and chemical processes and reactions. This thesis presents studies exploring nitrogen dynamics in CGA amended with organic industrial sludge. First, it sought to establish a limit above which sludge addition to CGA will be harmful to the environment, based on trace element solubility studies in a column leaching experiment. The study found that with respect to trace element release, up to 5% dry sludge addition may be safe for amending CGA. Incubation experiments aimed at investigating the rate and pattern of nitrogen mineralization from sludge in a CGA medium showed that more N was mineralized in fresh ash than in weathered ash and in soil. Most nitrification occurred in soil while most of the N mineralized in ash was in the form of ammonium, suggesting an inhibition of nitrifying organisms in the ash medium and implying that at least initially, plants used for rehabilitation of CGA dumps may take up N mostly as ammonium. Due to the build up of ammonium, the risk for ammonium leaching was investigated in batch experiments by reacting fresh and weathered CGA with ammonium concentrations in the range found in common sludge materials. Results showed high ammonium sorption in ash with more sorption occurring in weathered ash than in fresh ash. Chemical modelling analysis of leachates from a column experiment indicated that ammonium may also be retained by chemical precipitation as struvite in a sludge-amended CGA medium, but that this will be dissolved as the concentration of P reduces due to precipitation of calcium phosphate, and as pH and ammonium levels drop due to nitrification. Results from a volatilization experiment showed that ammonia volatilization is greater and more rapid in soil than in CGA, and that less nitrogen was volatilized in weathered than in fresh ash, a phenomenon attributed to the high ammonium sorption capacity of CGA which is enhanced by ash weathering. This thesis concludes that with respect to nitrogen, use of sludge to rehabilitate CGA dumps is feasible and when sludge rates do not exceed 5%, and with good management practices, environmental risks will be minimal. As the first in-depth nitrogen study of CGA-sludge mixtures, this study evaluated the risks and benefits of sludge amendment of CGA, proffered practical solutions to rehabilitation of CGA dump sites, and enhanced the understanding of nitrogen dynamics in alkaline, saline environments.