| 摘要: | 由於近年磷礦開採量的上升，磷回收之議題逐漸受到重視。在污水廠中厭氧消化程序具有高磷與高氮之成分，但含有大量懸浮固體與有機物影響，容易使回收成效不佳。因此，藉由前處理之方式優化磷回收之品質，是未來實廠應用所需之程序，但透過薄膜過濾進行前處理時，容易遇到積垢與結垢之問題，導致過濾效率不佳，進而使成本上升及成效的降低。
因此，本研究使用合成脫水濾液進行無機物結垢潛力與磷保留率實驗，透過批次實驗與反應曲面法探討實廠濃度範圍之最佳條件，並利用長期薄膜過濾實驗以比較及驗證最佳條件與最差條件之結垢特性差異。
透過實廠廢水之水質分析中顯示，脫水濾液之pH為7.9±0.1、正磷酸鹽濃度為128.1±8.2 mg/L、氨氮濃度為771.3±68.1 mg/L、總固體濃度為1,163±80.1 mg/L及TOC為56.2±10.3 mg/L。厭氧消化上澄液之pH為7.6±0.6、正磷酸鹽濃度為170.7±42.0 mg/L、氨氮濃度為871.2±102.2 mg/L、總固體濃度為33,193±166.2 mg/L及COD為698.9±21.6 mg/L，二者相比，由於厭氧消化上澄液之懸浮固體與有機物較高之特性，較不易進行薄膜前處理與磷回收，而脫水濾液含有較少之懸浮固體且仍具有較高之磷濃度。因此，整體而言，脫水濾液具有較佳之回收潛力。
在批次實驗中使用不同pH值與鈣離子濃度進行MFI0.2與磷保留率之實驗，根據結果顯示當pH越高會造成越高MFI0.2與越低磷保留率。在pH 6－7中具有較低MFI0.2與較高磷保留率，主要因為產生沉澱反應之pH區間範圍約在pH 7.5－12，在低於該範圍時，不易有顆粒產生。在pH 7.5－9時，隨著pH的上升，溶液中離子積高於各類磷酸鹽化合物之KSP，如：磷酸銨鎂、羥基磷灰石等，並產生顆粒物，進而導致結垢產生。同時發現隨著pH與鈣離子濃度由低到高，MFI0.2、粒徑與可能產生之沉澱物種類皆逐漸上升，磷保留率則逐漸下降。
固定pH值，並在磷濃度1.5－4.5 mmol/L(46.46－139.37 mg/L)、鎂離子0.5－1.5 mmol/L(12.1－36.5 mg/L)及鈣離子0.75－2.25 mmol/L(30－120 mg/L)條件下使用反應曲面法進行規劃與分析，得到最佳解為當磷濃度1.5 mmol/L、鈣離子濃度0.75 mmol/L與鎂濃度1.5 mmol/L之狀態並預期出其反應值MFI0.2為-0.3、磷保留率為100 %，代表結垢情形不易產生，磷酸鹽呈現溶解態，對於後續磷回收為最佳狀態。最差解為當磷濃度4.5 mmol/L、鈣離子濃度2.25 mmol/L與鎂濃度1.5 mmol/L之狀態，預期反應值則為MFI0.2為 3.59、磷保留率為94.8 %，代表有結垢產生，磷酸鹽也發生沉澱反應，造成磷保留率下降，該狀態容易使在前處理時無機物沉澱出顆粒物造成薄膜結垢之現象，產生磷酸鹽保留率不佳。此外，長期薄膜過濾中，與最佳條件比較，最差條件之阻力明顯上升，其中，在薄膜過濾進行中，容易先導致孔堵塞再形成濾餅層，造成TMP迅速上升。在阻力分析中發現最佳條件之孔阻力占比為12 %，最差條件之孔阻力占比為30 %，因此，最主要導致脫水濾液無機性結垢之原因為不可逆結垢。
;Due to the increase in the amount of phosphate mining in recent years, the issue of phosphorus recovery has gradually received attention. The anaerobic digestion process in sewage treatment has high phosphorus and high nitrogen components, but it contains a lot of suspended solids and organic matter, which is easy to make recycling ineffective. Therefore, optimizing the quality of phosphorus recovery by pretreatment is a required procedure for future applications. However, when membrane filtration is used for pretreatment, it is easy to encounter the problem of fouling and scaling, resulting in poor filtration efficiency increasing costs, and reducing effectiveness.
In this study, the fouling potential and phosphorus retention rate experiments were carried out using synthetic sludge dewatering filtrate. The batch experiment and response surface methodology were used to explore the optimal condition for the concentration range of the actual wastewater treatment plant, Moreover using long-term membrane filtration experiments to compare and verify the difference in fouling characteristics between the optimal and worst conditions.
The water quality analysis of the wastewater from the actual wastewater treatment plant showed that the pH of the sludge dewatering filtrate was 7.9±0.1, the phosphate - phosphorus concentration was 128.1±8.2 mg/L, the ammonium - nitrogen concentration was 771.3±68.1 mg/L, and the total solids concentration was 1,163±80.1 mg/L and TOC was 56.2±10.3 mg/L. The pH of the anaerobic digestion supernatant was 7.6±0.6, the phosphate - phosphorus concentration was 170.7±42.0 mg/L, the ammonium - nitrogen concentration was 871.2±102.2 mg/L, the total solid concentration was 33,193±166.2 mg/L and the COD was 698.9±21.6 mg /L, compared sludge dewatering filtrate to anaerobic digestion supernatant, due to the higher suspended solids and organic matter in the anaerobic digestion supernatant, it is difficult to membrane pretreatment and phosphorus recovery, while the sludge dewatering filtrate contains less suspended solids and still has a higher phosphorus concentration. Therefore, the sludge dewatering filtrate has better recovery potential.
In the batch experiment, different pH values and calcium concentrations were used to do MFI0.2 and phosphorus retention experiments. According to the results, higher pH results in higher MFI0.2 and lower phosphorus retention. In pH 6-7, it has lower MFI0.2 and higher phosphorus retention rate, because the pH range of the precipitation reaction is about pH 7.5-12. When the pH is lower than this range, particles are not easily generated. When the pH is 7.5-9, with the increase of pH, The IAP in solution is higher than the Ksp of various phosphate compounds, such as magnesium ammonium phosphate, hydroxyapatite, etc., and the precipitate is generated. This leads to membrane scaling. At the same time, it is found that with the increase of pH and calcium concentration from low to high, MFI0.2, particle size, and possible types of precipitates all increase gradually, while phosphorus retention rate gradually decreases.
When the pH is fixed at 7.5 and analyzed by the response surface methodology under phosphate - phosphorus concentration 1.5-4.5 mmol/L (46.46-139.37 mg/L), magnesium ion 0.5-1.5 mmol/L (12.1-36.5 mg/L) and calcium ion 0.75-2.25 mmol/L (30-120 mg/L), the optimal condition is obtained when the concentration of phosphate - phosphorus is 1.5 mmol/L, the concentration of calcium is 0.75 mmol/L and the concentration of magnesium is 1.5 mmol/L, and the reaction value MFI0.2 is expected to be -0.3, the phosphorus retention rate is 100%. It means that the scaling situation is not easy to occur, and the phosphate is in a dissolved state, which is optimal for subsequent phosphorus recovery. The worst condition is that when the phosphate - phosphorus concentration is 4.5 mmol/L, the calcium concentration is 2.25 mmol/L and the magnesium concentration is 1.5 mmol/L, the expected reaction value is MFI0.2 of 3.59 and phosphorus retention rate of 94.8 %, which means membrane scaling. The precipitation reaction causes the phosphorus retention rate to decrease. This state is likely to cause the inorganic compound to precipitate out of particles during pretreatment, cause membrane scaling, and poor phosphorus retention rate. In addition, in the long-term membrane filtration, compared with the optimal conditions, the resistance of the worst conditions increased significantly. Among them, during the membrane filtration process, it is easy to cause the membrane pores to block first and then form the filter cake layer, resulting in a rapid increase in TMP. In the resistance analysis, it was found that the pore-blocking ratio of the best condition was 12%, and the pore-blocking ratio of the worst condition was 30%. Therefore, the main reason for the inorganic scaling of the dewatering filtrate was irreversible scaling. |
