| dc.description.abstract | Phosphorus (P) is one of the important nutrients for all forms of life. The phosphate rock depletion and the loss of phosphorus resources have made phosphorus recovery as high-profile issue. Currently, there are many mature phosphorus recovery technologies to recover phosphorus from wastewater and sludge, including chemical precipitation method , fixed bed crystallization method, fluidized bed crystallization method, and so on. Taiwan is developing reclaimed water plants, and the treatment of reverse osmosis (RO) rejected water has become a problem.
First, the difference in particle size distribution of crystallized phosphate in actual and synthetic wastewater was explored. Actual wastewater contains suspend solid, so it is impossible to accurately determine the pH of the primary nucleation through particle size. In this study, the particle size analysis cannot determine the metastable zone for actual wastewater.
The results of the jar-test experiment show that the highest phosphorus removal efficiency condition is Mg/P molar ratio 1 and pH 10, and the phosphorus removal efficiency is 87.7 %. Visual MINTEQ predicts that the optimal pH of magnesium ammonium phosphate crystallization is 9.5, and the optimal pH of the hydroxyapatite crystallization is 10. In this study, the expected product is magnesium ammonium phosphate, so the optimal condition selected pH 9.5. Mg/P molar ratio 2 as the optimal condition based on the magnesium and calcium removal quantity.
The phosphorus removal efficiency in the fluidized bed factor experiment are all higher than 86.7%. The optimal phosphorus removal efficiency condition is HRT 5 minutes and upflow velocity 20 m/h, and the phosphorus removal efficiency is 92.2%; the optimal phosphorus crystallization efficiency condition is HRT 10 minutes and The upflow velocity is 30 m/h, the phosphorus crystallization efficiency is 15.4%, and it is found that the phosphorus crystallization efficiency has a negative value. The reason is that colloid accumulates in the upper part, causing the phosphorus concentration of sample to be higher than the phosphorus concentration of inflow. The accumulation of phosphorus crystallization efficiency is significant at lower upflow velocity and higher HRT.
Long-term experiments verified the optimal conditions for the factor experiment and confirmed that sludge accumulation still occurred as the reaction time increased. Therefore, the fluidized bed operation was changed and the nucleation stage was added. Then, the optimal conditions were adjusted from the factor experiment. The residual phosphorus concentration of reflux and outflow is 10.31 mg/L, achieving the highest phosphorus removal rate and phosphorus crystallization rate of 74.5%.The system can withstand the phosphorus surface loading of 0.29-0.34 kg/m2h.
XRD analysis results show that it is hydroxyapatite, which is caused by the water quality condition of Ca/Mg molar ratio of 2.45, indicating that the high concentration of competitive ion calcium inhibits the formation of magnesium ammonium phosphate. The particle size of 0.6-0.85 mm accounts for the largest weight proportion, which is 68.4 %. The water content is 9.9 %, and the apparent density is 1.396 g/cm3. | en_US |