Assessing Mechanical Improvement of Coal Ash Using Enzyme Induced Carbonate Precipitation.

The long-term storage of coal ash in impoundments is associated with various stability concerns. To address these concerns, microbial induced calcium carbonate precipitation (MICP) using ureolytic bacteria has been proposed. The resulting coal ash when treated with MICP has improved strength and reduced compressibility, indicative of more stable material. However, inoculating the coal ash material with ureolytic bacteria results in significant filtering of the bacteria, which in turn results in non-uniform calcium carbonate precipitation. To overcome these concerns, urease enzymes are used to treat the coal ash through enzyme induced carbonate precipitation (EICP). The uniformity and degree of mineralization using EICP is compared to MICP through a series of soil column tests. The degree of mineralization is assessed by measuring the shear stiffness of the treated coal ash using shear wave velocity, as well as the mass of precipitated calcium carbonate. The results of the soil columns indicate that the same level of mineralization was achieved for both EICP and MICP treatment methods; however, the EICP method achieved more a uniform distribution of mineralization throughout the column. Furthermore, the shear strength of the treated coal ash is assessed under undrained simple shear loading. The results indicate that both EICP and MICP treatments resulted in comparable shear strengths. Based on these preliminary results, improving the stability of coal ash impoundments using ureolytic enzyme induced mineralization is feasible and provides a more uniform level of treatment throughout the deposit.