Treatment of wastewater from mining operations and refineries by precipitation which generates large volume of hydroxide sludge can coprecipitate toxic substances such as Cd, As, Pb, Se, etc. Results in our laboratory works showed that Ferro-Nickel Slag fibers (FS) in column bed served as excellent media in the rapid separation (2 mm/s linear velocity) of suspended low concentrated ultra fine sludge particles (SS) from precipitation. This can be explained by the DLVO theory based on the heterocoagulation phenomena between the FS fibers in column bed and the ultra fine particles. In the separation of low concentrated toxic substances such as Cd, Se and Pb in the wastewater, the above mechanisms were also applied but with FS fiber surface modifications. This was carried-out to low concentrated Cd solution (1 mg/l), which was rapidly separated by precipitation of Cd to ultra fine CdS particles by the addition of sulfide ions in the presence of 100 - 102 mg/l Fe or Zn. Rapid separation of 1 mg/l Cd2+ was also carried-out by application of ferrous sulfide surface modification to FS fibers. Other precipitation method was also applied to Pb solution (1 mg/l) using bioreactor with Sulfate Reducing Bacteria (SRB) on FS bed. In the rapid separation of SeO4 2-, the basic ferrous and ferric hydroxide surface modification on the surface of FS fibers was conducted. Favorable rapid separation results were obtained and as a conclusion, low concentrated toxic substances can be selectively removed suggesting that potential drastic change in wastewater treatment processes in the future.