Abstract: Provided is a wastewater treatment technology (in particular, pretreatment technology) that, in comparison to existing technologies, can reduce the quantity of sludge generated as a result of processing wastewater that contains fluorine and silicon. Before adding a calcium compound to wastewater that contains fluorine and silicon and performing coagulation-sedimentation treatment on said wastewater, as a pretreatment, sodium hydroxide is added to the wastewater (first reaction tank 1), sodium silicofluoride is precipitated, and the precipitate sodium silicofluoride is removed via solid-liquid separation (first sedimentation tank 2).

Abstract: An object (first object) of the invention is to provide a waste water treatment technology (pretreatment technology in particular) which makes it possible to recover fluorine at a high recovery rate as highly pure calcium fluoride, without diluting waste water containing fluorine and silicon, i.e. to recover fluorine from high-concentration waste water containing fluorine and silicon. Waste water containing fluorine and silicon is supplied to a pH controlling tank 1 and sodium hydroxide (NaOH) is added thereto, with the result that sodium silicate is precipitated. Thereafter, the waste water is supplied to a solid-liquid separator 2 so that solid-liquid separation of sodium silicate is conducted.