| dc.description.abstract | Phosphorus pollution is the most important factor leading to global water eutrophication. It threatens aquatic ecosystems worldwide, and may lead to huge economic losses and even damage to human health through the production of cyanobacterial toxins. Therefore, recovery of phosphorus from wastewater is not only a necessary condition for aquatic environmental protection, but also a necessary condition for sustainable human development. This study try to use the fluidized bed crystallization technology to recover phosphorus from anaerobic digestion dewatered filtrate by crystallization of magnesium ammonium phosphate, and to explore its metastable zone for homogeneous crystallization. Since most of the cost of producing magnesium ammonium phosphate is mainly influenced by the cost of magnesium source which can contribute up to 75% of overall costs. Therefore, this study expects to use seawater as an alternative magnesium source, evaluate its feasibility, and finally explore the optimal fluidized bed crystallization conditions.
Firstly, use a particle size analyzer to analyze the particle changes during the crystallization of magnesium ammonium phosphate, and define the range of its metastable zone, to avoid the production of too many tiny crystals during the crystallization process and affect the crystallization efficiency. Use the Response Surface Methodology (RSM) to predict the optimal crystallization conditions. The results showed that when the Mg2+ concentration was 1.62 mmol/L, the metastable zone of synthetic wastewater was between pH 7.53 and 6.39. The pH of the crystallization metastable zone with the addition of synthetic seawater is between 7.34 and 6.87. Using Visual MINITEQ to analyze the composition of the precipitate in the solution, the results show that there are not only ammonium magnesium phosphate but also hydroxyapatite in the solution. However, due to the low concentration of Ca2+ ions in the synthetic seawater, most of the precipitate in the solution should be magnesium ammonium phosphate. Through the calculation of the reaction formula, it can be known that when M/P ratio 0.5, 111.9 mg of magnesium ammonium phosphate will be formed in the solution, while only 22.4 mg of hydroxyapatite will be formed. The optimal crystallization operating conditions predicted by the RSM are pH 9.5 and Mg/P ratio 2.5, and the best phosphorus removal ratio (95.4%) and crystallization ratio (85.7%) can be achieved when the reaction time is 66 minutes. In the experiment of homogeneous crystallization, 94.5% phosphorus removal ratio and 80.1% crystallization ratio can be achieved when the reaction time is 66 minutes, which is very close to the 95.4% phosphorus removal ratio and 85.7% crystallization ratio predicted by the RSM.
The results of SEM analysis show that when the solution contains suspended solids, the suspended solids will be encapsulated in the crystals, so that the ions cannot be tightly bound into the crystal nucleus, resulting in poor crystallization of magnesium ammonium phosphate. Suspended solids may be detached from the particle surface causing cracks and irregular shapes on the particle surface. The sieve analysis also shows that the crystallization of real wastewater, because the suspended solids are not uniformly encapsulated in the crystals, resulting in different suspended solids content in the crystals and on the surface, so the formed magnesium ammonium phosphate crystals have different size diversity.
EDS analysis showed that the atomic percentages of Mg, N, P and O in the real wastewater crystals were 6.3 %, 4.7 %, 5.0 % and 52.70 %, respectively, indicating that the composition is very similar to the stoichiometry of magnesium ammonium phosphate. According to the average atomic percentage of EDS analysis, the purity of magnesium ammonium phosphate is calculated to be 72.3 %, the rest of the compounds are attributed to the formation of magnesium phosphate, and it can be found that the atomic content of Ca2+ is only 0.1 %. The absence of Ca2+ ions can be attributed to (1) this experiment controls the operating conditions in a suitable metastable region, and the reactor is suitable for crystallization of magnesium ammonium phosphate, so there is no obvious compound with Ca2+ ions. (2) The Ca/Mg molar ratio in this study was only 0.2, and no obvious effect was found because of the lower Ca2+ ion concentration. (3) The high N:P molar ratio in this experiment increased the kinetics of crystallization of magnesium ammonium phosphate and suppressed the effect of Ca2+ ions.
The results of sieve analysis showed that the particle size of the real wastewater was mainly distributed in 1.18-2.00 mm after 10 days of the experiment, with a proportion of up to 70.2%, and the particle size of 0.85-1.18 mm also had a proportion of 25%. The particle size of real wastewater shows a variety of sizes, which can be attributed to the suspended solids in the water, in addition, during the collection and drying process, part of the suspended solids may be detached from the particle surface, resulting in cracks and irregular shapes on the particle surface , because the suspended solids are not uniformly wrapped in the crystals, resulting in different suspended solids content in the crystals and on the surface, so the formed magnesium ammonium phosphate crystals have different size diversity. However, the synthetic wastewater showed a highly consistent size, with up to 98.9 % of the crystals of 1.18-2.00 mm, and the remaining crystals smaller than 0.30 mm could be attributed to the exfoliated crystals during the drying process. | en_US |