Use of sedimentation ponds for removal of metals from ash transport waters

Abstract

AbstractThis study investigated the feasibility of using existing ash sedimentation ponds at coal‐fired power plants as treatment reactors to reduce concentrations of metals in bottom ash and fly ash transport waters by precipitation and/or adsorption onto iron oxyhydroxide. Intrapond treatment offers economic advantages over construction of a plant for treatment of ash pond effluent. Bench‐scale experiments were conducted with bottom ash and fly ash suspensions to investigate pH control and removals of aluminum, manganese, and arsenic under different chemical conditions, including addition of ferric chloride to promote adsoption on iron oxyhydroxide. The effects of pH and iron addition on ash‐water partitioning of the metals of interest were studied via batch experiments. In addition, the ability to control pH and promote removal of manganese in an intrapond treatment process was investigated using semi‐continuous, bench‐scale, pond‐simulation reactors. Results of the reactor experiments indicated that the pH could be maintained in range of 8 to 8.5 by dosing the influent with the proper amount of base as determined by batch titration, and that by doing so the influent manganese concentration of 2ppm could be consistently lowered to less than 0.2 ppm. The effects of accumulation of settled ash were examined in the reactor studies and with the use of a coupled hydraulic/sedimentation model for ash ponds. Overall, results of this study suggest that concentrations of most metals of interest in ash pond water can be kept low with control of pH, in situ precipitation of iron oxyhydroxide, and effective sedimentation of particles. Further examination of the intrapond treatment approach is warranted.