Trace element removal from ash dam waters by nanofiltration and diffusion dialysis

Liquid waste streams in coal-fired power plants contain a wide spectrum of trace elements, most of which originate in the coal and remain in the fly ash or bottom ash when the coal is burned. Electric power authorities seek methods for removing toxic elements from such streams. The use of nanofiltration and diffusion dialysis for removing boron and fluoride from ash dam water is described in this article. Negatively charged Nitto Denko NTR-7410 and Filmtec NF40 nanofiltration membranes gave better results than amphoteric Toray UTC-20HF and UTC-60 membranes. Membrane performance depended on pH, the best results being obtained when the pH of the ash dam water was in the 3–4.5 range. Diffusion dialysis membranes with negatively charged functional groups gave better results than membranes with positively charged functional groups.