污泥生質炭再利用為三氯乙烯降解異相催化劑之可行性研究

Abstract

由於普遍新建掩埋場困難，現有掩埋場之處理容量逐漸下滑使得都市下水污泥處理成本節節攀升，未來將有可能嚴重影響污水廠正常操作。都市下水污泥之目前已有之再利用方式分為土壤改良資材、能源再利用及建築材料再利用三個方向。但由於下水污泥之高含水率、高灰分、含有重金屬及機械強度低等特性，使得下水污泥再利用難以實用化。本研究運用取自迪化污水廠之乾燥污泥以200瓦、300瓦，及400瓦進行微波熱裂解30分鐘分解有機成分並活化金屬成分，提出將下水污泥以熱裂解產製生質炭為異相催化劑降解三氯乙烯之再利用方法。 污泥生質炭之物理化學特性在本研究中以近似分析(Proximate Analysis)、比表面積分析(Specific Analysis)、零電位點分析(Point of Zero Charge Analysis)，及金屬成分分析進行研究。近似分析中，污泥生質炭之灰分隨著微波功率提升而顯著增加，於乾燥污泥中僅占25.54±0.52%，200瓦生質炭中占36.37±1.08%，300W生質炭中占58.67±2.11%，400瓦生質炭中占63.96±0.50%。而比表面積則是自200瓦的0.879 ± 0.104 m2/g開始提升，在300瓦時達到最高值37.362 ± 3.227 m2/g，最後於400瓦些許下降至33.071 ± 7.219 m2/g。零電位點分析發現污泥生質炭的表面電性於pH為中性的環境下皆帶負電荷，200瓦、300瓦，及400瓦之零電位點分別為pH=3.86、5.46，及5.31。金屬成分分析的結果顯示微波裂解使生質炭中鋅、鐵、銅、鉛濃度提升，而汞含量因高溫揮發而自生質炭中去除。透過掃描式電子顯微鏡-能量色散X射線光譜儀，本研究發現300瓦及400瓦裂解之生質炭表面出現均勻分布之微小圓球狀顆粒，可做為潛在催化位址促進三氯乙烯之降解。 為分析污泥生質炭於不同pH值條件下降解三氯乙烯的能力，污泥生質炭於pH=3.13±0.25、4.75±0.15，及6.79±0.15進行降解實驗。研究結果發現200瓦生質炭僅於pH=3.13±0.25具有約50%去除率，而於pH=4.75±0.15及6.79±0.15條件下對三氯乙烯降解能力較低。300瓦及400瓦生質炭則於pH=4.75±0.15及6.79±0.15依然有50%以上的去除率。連續批次實驗則證明在五次的降解實驗後400瓦生質炭之降解能力並未顯著的受到影響。 毒性特性溶出試驗(Toxicity Characteristic Leaching Process)的結果中觀察到微波裂解功率的增加與鋅、鉛的溶出具有正相關，但對於鐵、銅、鉻卻是負相關。表示微波裂解程序有助於鐵、銅、鉻三種金屬的穩定化。考慮到污泥生質炭將應用於透水反應牆之用途，將毒性特性溶出試驗之結果與台灣地下水污染管制標準進行比較。結果顯示除鉛微幅超標以外之金屬溶出濃度皆低於地下水汙染管制標準，然而於中性情況下其溶出狀況應低於標準值。 本研究以實驗數據為基礎進行生命週期評估(Life Cycle Assessment)探討污泥生質炭再利用為三氯乙烯降解催化劑之生命週期對環境造成的衝擊，比較三種功率製備之生質炭並分析此程序中可能的衝擊熱點。三種功率製備之生質炭中，以200瓦生質炭所造成的環境衝擊最大，此結果主要是因為其處理效率於中性情況下較低，造成生質炭投入增加。300瓦生質炭所造成的總環境衝擊略低於400瓦生質炭，其原因為300瓦生質炭製備時所耗費的能源較少，且於中性情況下對三氯乙烯的降解能力與400瓦生質炭相近。熱點分析結果發現微波裂解階段的能源使用為最大的環境衝擊來源。敏感度分析的結果顯示降解階段的污泥生質炭使用量為最敏感之參數，而裂解階段的能源使用為亦為十分敏感之參數。這些結果顯示，污泥生質炭的使用量、能源效率或是電力結構變化都將顯著的影響環境衝擊的分析結果。 本研究提出以微波裂解都市下水污泥為催化劑處理三氯乙烯的方式進行再利用，為都市下水污泥開啟新的再利用途徑。使用都市下水污泥的優勢在於不需再經過任何披覆程序即可有足夠的金屬成分可作為活性組分，中性條件下依然表現出一定的降解能力。毒性特性溶出試驗的結果亦證明污泥生質炭於現地使用對土壤造成的重金屬污染風險不高。綜合以上結果，污泥生質炭作為三氯乙烯降解催化劑是具有潛力的再利用方案。

Sewage sludge has become a serious problem of wastewater treatment plant for increasing cost of landfill. Land amendment, energy recovery, and construction material substitution are major ways to reuse sewage sludge currently. Heavy metal content, high water content, and low mechanic strength decreased the value of sewage sludge in these applications. This research provides a new concept to well utilize the nature of high ash content in sewage sludge to produce catalyst for pollution degradation. Pyrolysis was conducted by single mode microwave device with 200W, 300W, and 400W power level. Characteristics of sewage sludge derived biochar (SSDB) was investigated by proximate analysis, specific area analysis, point of zero charge analysis, and metal content analysis. For proximate analysis, ash content in SSDB was increased with microwave power absorption. Ash content in raw sludge, 200W SSDB, 300W SSDB, and 400W SSDB was 25.54±0.52%, 36.37±1.08%, 58.67±2.11%, and 63.96±0.50% respectively. Specific area of 200W, 300W ,and 400W SSDB was 0.879 ± 0.104 m2/g, 37.362 ± 3.227 m2/g, and 33.071 ± 7.219 m2/g respectively. All SSDB was found negatively charged at neutral pH. In metal analysis, iron was the highest metal content as high as 42 mg/g in SSDB produced at 400W. Zinc was the second highest metal content in SSDB. Copper, chromium, cadmium, lead, mercury, and nickel were also found in SSDB. Concentration of copper, lead, zinc, and iron occurred during microwave induced pyrolysis. However, mercury was only found in raw sludge and SSDB produced at 200W because of evaporation in high temperature. Surface morphology and chemical composition were observed by SEM-EDX. The surface of 200W SSDB was found relatively smooth which also can be proved by the result of specific area. Inorganic aggregation evenly distributed on 300W SSDB and 400W SSDB was observed. The aggregation can possibly react as catalytic active sites. SSDB was activated by 20mM of hydrogen peroxide to degrade TCE in aqueous phase. Degradability of SSDB was tested at different pH value for 2-hour reaction. SSDB produced at all power level was able to remove TCE effectively at pH=3.13±0.25. The C/C0 of 200W, 300W, and 400W SSDB was 0.45, 0.17, and 0.23 respectively. C/C0 of SSDB produced at 300W and 400W is 0.33 and 0.45 of TCE at pH=4.75±0.15 respectively. For neutral pH=6.79±0.15, 300W and 400W removes 0.4 and 0.52. Total hydrocarbon (THC), mercury emission and toxicity characteristic leaching process (TCLP) was conducted to investigate possibility of secondary pollution in SSDB life cycle. TCLP result shows that only lead concentration in leachate slightly surpassed groundwater pollution control standard in Taiwan. However, lead leaching at neutral pH is much less than acidic condition. Considering pH value of groundwater, SSDB is a safe material to apply on land. The result of life cycle assessment shows that environmental impact caused by 200W SSDB was the highest. Detailed environmental impact hotspot was analyzed. Electricity consumption was found the major source of impact in SSDB life cycle. Sensitivity analysis identified SSDB input in degradation stage and energy consumption in pyrolysis stage are the most sensitive parameters. Sewage sludge is a mixture of organic and metal content. This study applied microwave induced pyrolysis to reform organic content to fixed carbon support, and also activate metal content. The ability to degrade TCE at neutral pH and limited metal leaching make SSDB promising to use in situ remediation.