Properties of sludge generated by the treatment of fluoride-containing wastewater with dicalcium phosphate dihydrate

Abstract

Fluoride-containing wastewater is discharged from various industrial sites, including semiconductor, plating, and metal-treatment plants. To remove the fluoride, we treated wastewater with dicalcium phosphate dihydrate (DCPD, CaHPO4·2H2O) to form fluorapatite [Ca10(PO4)6F2] crystals. We investigated the fluoride removal efficiency and the properties of the resulting sludge. For comparison, we also treated wastewater using conventional aluminum coprecipitation. The DCPD used in this study was a by-product of the gelatin industry. Nanoscale precursor particles were induced on the DCPD by treatment with warm water. After treatment of fluoride-containing wastewater in batch experiments, the amount, particle size distribution, settleability, and filterability of the resulting sludge were measured to assess the usability of DCPD. We found that the amount of chemical additives required for the DCPD method was less than that for the conventional method. Additionally, the amount of sludge produced using the DCPD method was small and the final fluoride concentration was below 5 mg L−1. In settleability tests, the sludge produced by the DCPD method was precipitated within 10 min. In filterability tests, most DCPD sludge was separated within 5 min. The settleability and filterability of sludge from the DCPD method were better than those of the conventional method. The release of fluoride from the DCPD sludge was below the levels stipulated by Japanese soil pollution regulations. Consequently, the sludge was considered unlikely to cause soil pollution, thereby facilitating landfill management. We concluded that DCPD can be used effectively to treat fluoride-containing wastewater and that the resulting sludge is unlikely to cause environmental damage.