利用反應曲面法改善磷酸銨鎂結晶法以回收厭氧消化出流廢水中的磷

Abstract

本研究致力於探討最具經濟效益之磷酸銨鎂結晶 (Struvite crystallization) 操作程序，並藉由中央合成設計及反應曲面法 (CCD-RSM) 找出最佳化鎂鈣比、曝氣指數與磷酸銨鎂晶種投加量，以獲得於厭氧消化出流廢水進行結晶的最高之磷回收效率與最低的鈣回收率。初步試驗與CCD實驗中使用合成廢水模擬養豬厭氧消化出流廢水以探討磷酸銨鎂結晶程序的最佳化操作參數，再根據此結果來進行實際廢水結晶實驗。CCD實驗結果顯示，提高鎂鈣比、曝氣指數與晶種濃度等三項操作參數可得到良好的磷回收率與較高的結晶動力常數。隨著鎂鈣比與晶種濃度的增加，也降低了磷酸鈣形成的機會而導致鈣回收率下降，同時提高磷酸銨鎂在沉澱物中的的比例。然而於廢水中曝氣提升pH值，雖然有利於磷酸銨鎂結晶形成，過大的曝氣量卻也導致劇烈的磷酸鈣反應產生大量沉澱，進而降低磷酸銨鎂在沉澱物中的比例。將實驗結果進行反應曲面的最佳化分析後，可得到在鈣鎂比為2.3、曝氣指數為4.77與晶種濃度為475 mg/L的操作條件下，能達到最高之磷回收率與最低鈣回收率，於實際廢水中進行最佳化條件實驗，也支持此結果。總括本研究成果，可知以低曝氣強度之氣提法可取代鹼劑以提高廢水pH值，而磷酸銨鎂結晶產物也可回收作為晶種再利用，因此主要成本來源僅有調整鎂鈣比之藥劑，顯示於高鹼度與高氨氮濃度的養豬厭氧消化出流廢水進行磷酸銨鎂結晶法為經濟且可行的磷回收方法。

The objective of present research was to evaluate a cost-effective approach on the recovery of phosphorus through struvite crystallization. Central composite design of response surface methodology (CCD-RSM) was employed to investigate the effects of Mg/Ca molar ratio, aeration index and struvite seed crystal concentration on the performance of struvite crystallization with synthetic wastewater. The results show that the PO43--P recovery efficiency and the kinetic rate constant both increased with all the three factors (Mg/Ca molar ratio, aeration index and struvite seed crystal). The Ca recovery efficiency reduced and struvite purity increase with increase in Mg/Ca molar ratio and the struvite seed crystal concentration. Although the aeration in solution had positive effect on struvite formation, but excessive aeration would cause Ca recovery efficiency to increase and make the percentage of struvite decrease in production. The maximum PO43--P recovery efficiency and the minimum Ca recovery efficiency was achieved at optimized conditions with Mg/Ca molar ratio of 2.3, aeration index of 4.77. Seed crystal concentration of 475 mg/l is enough to improve struvite purity of precipitation and shorten the crystallization time. Partial precipitated product can be reused as new seed for MAP crystallization.