Ozonation also produces fewer disinfection by-products than chlorination. Treatment facilities included ultrafiltration, modification of plumbosolvency, disinfection and chlorination. Disinfection by chlorination can be problematic, in some circumstances. The treatment process involves clarification, sedimentation, filtration, chlorination and fluoridation. In the case of chlorination, one of the chlorine atoms replaces a hydrogen atom. Methods for purification include filtration and the addition of chemicals, such as chlorination. Hexachloroethane is a byproduct of many industrial chlorination processes. Liquid phase chlorination and oxidation of toluene are the main routes. Allyl chloride is produced by the chlorination of propylene. Methods include electrolysis, wet chemical chlorination (aqua regia), and wet chemical parting. Other methods for the production of "p"-cresol include chlorination of toluene followed by hydrolysis. The water quality is considered to be excellent, needing only chlorination and fluoridation treatment. Copper(II) chloride is prepared commercially by the action of chlorination of copper. Common polymer-analogous reactions are in case of polyethylene crosslinking, chlorination and sulfochlorination. Today's process is a combination of chlorination and electrolysis.