UV/H2O2處理水中含碳及含氮有機前質與新興消毒副產物生成之影響

Abstract

水中有機物質之組成可能因來源、有機質濃度、分子量大小、結構、官能基組成以及消毒劑不同而對消毒副產物的形成產生影響。湖庫的優氧化、工業及生活污水的污染使原水水質惡化，自來水淨水場加氯處理後除了造成傳統含碳消毒副產物濃度上升外，具有較高健康風險的新興消毒副產物種如含氮消毒副產物也可在飲用水中發現，形成飲用水處理的一大挑戰。紫外光結合過氧化氫程序(UV/H2O2)藉由產生高氧化力的氫氧自由基，配合適當的操作條件設計可使有機物完全礦化，達成目標有機物質的去除及消毒副產物生成潛能(disinfection byproducts formation potential, DBPFP)降低的目的。 本研究採用UV/H2O2進行飲用水中有機物之氧化處理，探討原水中有機物組成、UV/H2O2程序對於消毒副產物前質之降解以及對消毒副產物生成之影響。研究選取不同來源有機物做為前質代表，分別以天然水體添加不同比例經生物處理家庭污水上層液之人工原水及以六種含不同分子結構含氮有機前質配置之水溶液為實驗對象，並藉由氧化產物有機物濃度監測、三鹵甲烷生成潛能(trihalomethanes formation potential, THMFP)、含鹵乙酸生成潛能(haloacetic acid formation potential, HAAFP）、含氮消毒副產物N-亞硝基二甲基胺生成潛能(N-nitrosodimethylamine formation potential, NDMAFP)之消長以評估UV/H2O2處理程序對有機物降解及消毒副產物控制之功效。 研究結果顯示以UV/H2O2程序處理受家庭污水污染之原水或天然有機前質之處理及降低消毒副產物之目標均可收到一定成效。但在水中可以重力沈降去除的顆粒性有機物質濃度過高時會影響氫氧自由基之氧化效率，在氧化不完全時將因粒狀有機前質被降解為粒徑小於0.45 µm之物質而使非氣提性溶解有機碳(non-pergeable dissolved organic carbon, NPDOC）、THMFP及HAAFP之濃度提高；溶解性有機氮(dissolved organic nitrogen, DON)的降解亦因粒狀有機物質的存在，降低氫氧自由基的氧化效果而無法達到良好成效。UV/H2O2處理程序中THMFP與HAAFP的變化趨勢顯示氫氧自由基同時扮演氧化有機物及消毒副產物前質反應之角色，在有機物氧化不完全時會增加其需氯量，造成含碳消毒副產物(C-DBPs)的增加。氫氧自由基的氧化作用同時增加處理後水中有機物質的親水性，導致HAAFP及含溴物種比例的增加。在天然水體添加10%廢水之氧化試驗中，經UV/H2O2處理NDMA前質降解反應主要發生在0 至30分鐘之間，約可降低50%的NDMAFP（由66 ng/L下降至34 ng/L），30 – 60分鐘間變化極微，顯示經過UV/H2O2程序後仍有部分NDMA前質無法被降解。 以目標含氮有機物所配置之水樣經過高壓（HP）及低壓（LP）UV/H2O2氧化處理後，依其不同結構及氮取代位置而呈現相異的NPDOC及DON降解趨勢。結構複雜的diltiazem及atrazine降解效率較低，分子末端具有胺基結構或含氮苯環者較容易被分解而產生氨氮，但位於分子中心的氮結構則較難以被UV/H2O2處理所降解。以LPUV/H2O2進行氧化處理時，由於少了高能量紫外光的直接氧化作用，僅能藉較低濃度的氫氧自由基的間接氧化目標有機物，使得其有機物的降解效率較HPUV/H2O2為低。以Dimethylamino propyl methacrylamide (DMAPMA)及diltiazm為例，在受到不同強度之UV/H2O2處理後其DBPFP隨氧化時間增加而呈現相異的變化趨勢，依DBPFP之變化趨勢顯示其氧化效率為DMAPMA優於diltiazem，而 HPUV處理效率又高於LPUV處理。前質分子結構末端具有三級胺官能基的有機物易與氯胺反應形成NDMA，且不完全的氧化會提高處理後水樣之NDMAFP，需提供充足的氧化劑或延長反應時間才可有效將有機物礦化及DBPFP降低，由氧化處理中間產物DMA的增加更可印證此趨勢。以目標含氮有機物所配置水樣進行之批次實驗顯示以DON推估NDMAFP雖有良好的相關性，但應用於實際水體時由於其有機物組成歧異性較大，單純以DON或DMA濃度推估NDMAFP可能會導致偏差。

The presence of various organic contaminants in water sources is of concern due to their direct threats to human health and potential to react with disinfectants to form carcinogenic byproducts including trihalomathanes, haloacetic acids and nitrosoamines in water disinfection process. Ultraviolet light coupled with hydrogen peroxide (UV/H2O2) is a powerful water treatment technology by forming highly reactive hydroxyl radicals. This study examined the relationships between organic matter compositions and disinfection by-product formation potential (DBPFP) by two major aspects. First, laboratory synthetic water with various compositions of organic carbon and nitrogen were treated with UV/H2O2 to evaluate its degradation efficacy. Second, both high-pressure and low-pressure ultraviolet light were applied to evaluate its efficacy for degradation of six selected nitrogenous organic compounds. After UV/H2O2 oxidations, corresponding disinfection byproducts (DBPs) formation potentials were measured. In laboratory synthetic water, it was found that carbon containing precursors were relatively easier to mineralize by UV/H2O2 treatment than the nitrogen containing compounds. UV/H2O2 processes successfully reduced the precursors of trihalomethanes (THMs) and haloacetic acids (HAAs); however, the treatment efficiency was lower for N-nitrosodimethylamine (NDMA) precursors. It was also observed that the degree of precursor removal was reduced when raw water was contaminated by domestic wastewater effluents. In comparison to untreated water, UV/H2O2 treated water produced a higher ratio of HAAs than THMs after chlorination. This suggests that a higher fraction of hydrophilic compounds was obtained after UV/H2O2 treatment. Raw water impaired by wastewater effluent also altered the formation and species distribution of DBPs, since higher ratio of HAAs and brominated DBPs were observed. With higher oxidation power and simpler molecular structure, target compounds resulted in better reduction of organic matters and DBP formation potentials (DBPFPs). However, insufficient contact time and oxidant doses could lead to a rise of DBPFPs in the early stages of UV/H2O2 reactions. A greater percentage removal was achieved for organic carbon than organic nitrogen after UV/H2O2 treatment, especially for complex compounds such as diltiazem. During the UV/H2O2 treatment, the intermediate products include tertiary amine, dimethyl amine (DMA) or DMA-like structures, which are N-nitrosodimethylamine (NDMA) precursors after disinfection. Furthermore, it was observed that using dissolved organic nitrogen and DMA to predict NDMA formation potential could lead to biased conclusions because of the complex nature of nitrogenous matters in aqueous environments.