| 摘要: | 磷污染是導致全球水體優養化的最重要因素之一，它威脅著全球水生生態系統，並可能通過產生藍藻毒素導致巨大的經濟損失甚至損害人類健康，因此，從廢水中回收磷不僅是水生環境保護的必要條件，也是人類可持續發展的必要條件。本研究預計使用流體化床結晶技術，從厭氧消化脫水濾液中以磷酸銨鎂結晶的方式回收磷，並探討均相結晶其介穩區的範圍，由於生產磷酸銨鎂的成本大多來自鎂的使用，大約佔整個技術成本的75 %。因此本研究期望能使用海水作為替代鎂源，評估其可行性，最後探討最佳流體化床的結晶條件。
首先使用粒徑分析儀來分析磷酸銨鎂結晶過程中顆粒變化，定義出其介穩定區的範圍，以避免在結晶過程中產生過多的微小晶體影響結晶效能，並使用反應曲面法(Response Surface Methodology, RSM)進行最佳結晶條件之預測。結果顯示在Mg2+濃度為1.62 mmol/L時，合成純廢水的介穩定區落在pH 7.53~6.39之間，而加入合成海水的結晶介穩定區pH值則落在7.34及6.87之間。透過Visual MINITEQ進行溶液中沉澱物成分分析，可以發現溶液中不只有磷酸銨鎂還有羥基磷灰石的成分，但是由於合成海水中的Ca2+離子濃度偏低，所以溶液中大部分的沉澱物成分應為磷酸銨鎂，透過反應式計算可以得知在Mg/P比0.5的時候，溶液中將會形成111.9 mg的磷酸銨鎂，而羥基磷灰石僅會生成22.4 mg。以反應曲面法預測之最佳結晶操作條件為pH 9.5 及Mg/P比2.5，反應時間66分鐘的情況下可以達到最佳的磷去除率(95.4%)以及結晶率(85.7 %)，在均相結晶的實驗中，反應時間66分鐘時可以達到94.5 %的磷去除率及80.1 %的結晶率，非常接近反應曲面法預測的95.4 %磷去除率以及85.7 %結晶率。
SEM分析結果表明，當水中含有懸浮固體時，懸浮固體會被包裹在晶體中，導致磷酸銨鎂結晶變差，使離子不能緊密地結合到晶核中，此外在收集和乾燥過程中，部分的懸浮固體可能會從顆粒表面脫落，導致顆粒表面出現裂紋和形狀不規則，篩分析也表明實際廢水的結晶，由於懸浮固體不是均勻的從晶核中成長，導致晶體中固體含量差異大，此亦為形成的磷酸銨鎂晶體粒徑多樣性之原因之一。
EDS 分析表明，實際廢水結晶中 Mg、N、P 和 O 的原子百分比分別為 6.3 %、4.7 %、5.0 % 和 52.70 %，表明其組成與磷酸銨鎂的化學計量十分相似，然後根據EDS分析的平均原子百分比，經過計算磷酸銨鎂的純度為72.3 %，其餘的化合物歸因於磷酸鎂的形成，而且可以發現Ca2+的原子含量僅有0.1 %，Ca2+離子未造成影響可以歸因為(1)本實驗將操作條件控制在適合的介穩定區，在此條件下反應器內適合磷酸銨鎂結晶，所以沒有明顯與其他離子產生化合物。(2)本研究的Ca/Mg 摩爾比僅有0.2，較低的Ca2+離子濃度所以未發現有明顯影響。(3)本實驗高的N/P 摩爾比，增加了磷酸銨鎂結晶的動力學，並最大程度抑制了Ca2+離子的影響。
篩分析的結果顯示，試驗10天後實際廢水所得粒徑主要分布於1.18-2.00 mm，比例高達70.2 %，而粒徑0.85-1.18 mm也有25 %的比例。實際廢水的粒徑展現了尺寸的多樣性，水中的懸浮固體亦為原因之一。然而合成廢水則展現了高度一致的粒徑大小，1.18-2.00 mm的晶體高達98.9 %，剩下小於0.30 mm的晶體可以歸因於乾燥過程中剝落的晶體。
;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. |
