| dc.description.abstract | Phosphorus is a non-renewable resource and one of the essential nutrients for living organisms. When excessive phosphorus is discharged into water bodies, it not only negatively impacts the ecosystem but also poses a risk to human health. In the face of significant phosphorus resource loss, how to effectively recycle and resource phosphorus while achieving a circular economy has become a highly focused issue. This study plans to use fluidized bed crystallization technology to recover phosphorus from dewatering filtrate of anaerobically digested sludge and explore how adjusting the pH value of the fluidized bed affects the crystal structure.
First, the study investigates the optimal pH range for crystallization in actual dewatering filtrate from anaerobic digestion. The pH range chosen for this study is between 8.0 and 9.5. After long-term crystallization experiments, the results show that at pH = 9, the best phosphorus removal rate was 64.4 % and the phosphorus crystallization rate was 56.4 %. Next, under the optimal pH range of 9, Mg2+ ions were added to achieve a Mg/P ratio of 1.2 for long-term testing. The results showed that the best phosphorus removal rate reached 89.8 %, and the phosphorus crystallization rate reached 85.4 %. Therefore, this study confirms that both pH and the Mg/P ratio have a significant impact on crystallization.
SEM results indicate that when suspended solids are present in the water, the crystal appearance is rough with irregular cracks. When organic matter is present, the crystal shape is rectangular and block-like. Additionally, a higher recirculation ratio and upward flow velocity result in smoother crystal surfaces. Therefore, the presence of impurities (suspended solids, organic matter) in the water and operational conditions (upflow velocity, recycle ratio) have a significant impact on crystal morphology.
According to the XRD results, during stages I, II, and III, the crystallized products are mixed crystals, including hydroxyapatite, magnesium phosphate, and ammonium magnesium phosphate. During stages IV and V, the crystallized products are ammonium magnesium phosphate and magnesium phosphate, and in stage VI, the product is magnesium phosphate. Additionally, in the XRD pattern of stage IV, a noticeable shift in the diffraction peaks is observed, which is caused by the presence of Ca2+ ions in the crystals. Additionally, sieve analysis results indicate that the crystal size collected in this study is mainly between 0.6 – 0.85 mm, while crystals larger than 1.18 mm are observed in stage III due to the high crystal nucleation density. | en_US |