Phosphate removal from water using zirconium-based mesoporous materials

The element phosphorus (P) is essential to all life (e.g. plants, animals and bacteria) and is a key ingredient in fertilizers to sustain high crop yields. However, the lifetime of exploitable reserves of natural ore deposits containing P is estimated to range from the next few decades to several hundreds of years, since P is intensively used in agriculture and other industries. The use of P yields large amounts of P-containing wastewater. P discharged from municipal and industrial wastewater treatment plants as sludge and water is approximately estimated to be 40,000 tons P per year in Japan. Hence, the wastewater can be considered as an alternative source of P to phosphate rock. Since the release of P-containing wastewater into water bodies leads to deterioration in water quality, such as eutrophication, P in the wastewater also has to be removed from an environmental point of view. Therefore, substantial attention has been paid to the efficient removal and recovery of P from wastewater and then recycling of P. P can be removed from both raw wastewater and P rich side-streams in wastewater treatment plants by chemical precipitation with addition of Fe, Al(III) or Ca salt, biological processes, adsorbents and ion exchangers. Considering the reuse of the P recovered from the wastewater, concentrations of impurities, such as heavy metals, toxic organic materials and pathogens, should be very low. Hence, the solution used for the phosphate recovery should be clear. In addition, high phosphate concentration is beneficial for effective phosphate removal. Therefore, we applied zirconium sulfate-surfactant micelle mesostructure (ZS) as an ion exchanger to recover orthophosphate from P rich water. Although the use of ion exchangers for P recovery is relatively expensive, they have advantages of easy operation, high selectivity, high efficiency and no sludge production over other processes to obtain concentrated phosphate solution in high purity. Since a typical influent concentration of total P in sewage wastewater ranges from 5 mg-P/L to 10 mg-P /L, ZS was applied to the wastewater at higher phosphate concentration, such as reject water from an anaerobic digestion process. This thesis started with characterization (ion exchange capacity, isotherm, effects of pH and interfering ions, elution capacity) of ZS. Then ZS powders were immobilized on polymer matrix to avoid decomposition of ZS structure in regeneration process. Polymer coated zirconium sulfate-surfactant micelle mesostructure (P-ZS) was used for recovery of phosphate in artificial solution and ground water containing phosphate of about 10 mg-P/L. Furthermore, we tried to obtain phosphate rich solution without suspended solids from excess sludge generated in a membrane bioreactor (MBR) by anaerobic digestion with an anaerobic MBR (AnMBR).