Electro-dialysis is a membrane separation process that uses an electric potential difference to move charged ions across selectively permeable membranes. This method is effective for removing dissolved ionic species from water, which includes heavy metal ions like mercury and can also be used for ionized organic pollutants like phenol (depending on pH). Sedimentation and filtration primarily remove suspended solids. Biological oxidation can degrade organic pollutants like phenol under suitable conditions but is less effective for removing heavy metal ions like mercury.