都市下水污泥中 DEHP 之有效萃取及堆肥化處理技術之研究

Abstract

本研究於都市污水處理廠之污水、污泥中皆偵測出濃度不等之鄰苯二甲酸酯類化合物，尤其是鄰苯二甲酸-2-乙基己基酯 (di-(2-ethylhexyl) phthalate, DEHP) 為主要代表物種，此物質不但具危害性、生物難分解性、且為環境荷爾蒙 (endocrine disrupting chemicals, EDCs) 的一種。由於 DEHP 具高度疏水性與親脂性且其生物降解效率不佳，經過污水與污泥處理後仍有相當之濃度殘留或累積在脫水污泥餅中，欲將污泥餅做為綠農地利用等相關資源化途徑時，其中所含之危害性有機物則需再進一步分解與穩定化。堆肥法 (composting) 即為在好氧條件下，利用微生物分解有機物質且自發性的產生熱而達到去除致病菌的效果，經過快速分解 (active phase) 與腐熟化 (curing phase) 兩階段之後，其中的有機物質組成則更趨於穩定。污泥堆肥程序等相關研究進行之前，本研究之首要課題為建立污泥中有機污染物質之高效率且具可信度之萃取、分析方法；本研究利用超臨界流體 (supercritical fluid) 具有低黏滯性、低表面張力、高擴散性、高溶解度與不具毒性等優點，針對都市下水污泥進行萃取，並建立其最適化操作條件。除此之外，本研究亦利用固相萃取裝置 (solid phase extraction, SPE) 建立液相樣品前處理操作程序以期改善傳統液相萃取之缺點。研究結果顯示，污泥量 0.2 g、萃取壓力 202 bar、萃取溫度 80oC、流體流速 2.0 mL/min 並添加 0.5 mL 之 methanol 做為修正劑、靜態萃取與動態萃取時間分別為 1 與 15 min 為本程序之最佳操作值。透過建立固相與液相樣品的萃取分析程序，本研究即針對 DEHP 在污水處理廠中之濃度變化及其在各處理單元之流佈與去除進行探討。研究結果顯示，DEHP 於固相與液相中濃度差異甚鉅、且具有在固相中累積與富集的現象；然而 DEHP 於旋轉生物圓盤法處理單元之去除效率較傳統活性污泥程序為佳，且約有 30% 的 DEHP 在污泥喜氣消化單元被去除，此結果說明生物降解為 DEHP 在污水與污泥處理單元之主要去除機制。本研究最終之目的為採用槽式堆肥的方式 (in-vessel composting)，於固定初始含水率 (60%) 與瀑氣量 (10 L/min)、不同摻和比的條件下加速污泥中 DEHP 與其他有機物質之生物降解率，並探討堆肥過程中其他相關物化因子的變化，最後再透過反應動力模式描述污泥中有機物與 DEHP 於堆肥過程中降解之相關性。研究結果顯示於初始含水率 60%、污泥與木屑混合比例 1:0.5 (w/w)、曝氣量 10 L/min 等之操作條件下，堆肥樣品中之 DEHP 可於 18 天內達到 85% 的降解率，其中約有 60% 的 DEHP 在堆肥高溫期被降解，且 DEHP 之降解效率亦較 VS、TC、TOCw 等參數為佳，此結果亦顯示 DEHP 於高溫期之降解效率明顯高於中溫期，經過 18 天的堆肥化處理之後，污泥中的 DEHP 濃度已可被降解並符合歐盟制訂之農地利用等標準值 (100 mg/kg)。

As an end product of sewage treatment processing, it is obvious to note that the hazardous lipophilic organic matter present in the sewage tends to accumulate in the sludge. Di-(2-ethylhexyl) phthalate (DEHP), the predominant organic pollutant determined in the sludge cake, has the concentrations (105-333 mg/kg) above the limited value (100 mg/kg) of sludge land application as suggested by the European Union (EU). Because of its carcinogenic, mutagenic, and estrogen-like characteristics to biota, DEHP has been restricted as one of the endocrine disrupting chemicals (EDCs) on the lists of Japanese and the US governments, respectively. Therefore, the land application of sewage sludge needs considerable attention prior to land disposal.Supercritical fluid is low viscosity, low surface tensions, high diffusion ability, high solubility and non-toxic. DEHP in selected municipal sewage and industrial sludge samples with various characteristics were extracted and quantified by the supercritical fluid extraction (SFE) and gas chromatography/electron ionization detector (GC/EID).After the optimal extraction parameters of SFE were developed, the concentrations and the occurrences of DEHP in a municipal sewage treatment plant (MSTP) were assessed. Sewage and sludge samples were collected from each sewage and sludge treatment unit in Min-Shen MSTP during five times of sampling. The concentrations of DEHP in raw sewage samples and its removal in attached and suspended growth biological treatment systems of a municipal sewage treatment plant were investigated. The levels of DEHP in absorbed and dissolved phases of the samples were determined by SFE and solid-phase extraction (SPE) methods, respectively, followed by GC/MS quantification. The removals of DEHP both in dissolved and adsorbed phases were higher in RBC compared to ASP. This indicates that attached growth systems are more tolerant to DEHP. Almost 30% of DEHP was removed after aerobic sludge digestion. The result reveals that biodegradation of DEHP is the predominant degradation mechanism in sewage and sludge treatment processes. Finally, a complete mass balance of DEHP in the MSTP was worked out to explore the behavior of DEHP under various treatment systems of the MSTP.An effective, rapid and easily-controlled in-vessel composting process was used to convert DEHP-containing sludge into harmless end-product as soil fertility improvement for agricultural purposes. The investigations covered fundamental studies (lab-scale composting experiments) that allowed developing methods for operating the in-vessel sludge reactor. A suitable operating condition for the reduction of DEHP-containing sludge by in-vessel composting was therefore obtained. A single stage thermophilic phase was observed within two days where almost 60% of DEHP was degraded. At the end of composting, total DEHP degradation of more than 85% was observed in all conditions. These findings reflect that higher temperatures generated during composting was shown to be effective in degrading DEHP from sludge at a faster rate compared to mesophilic treatment. It was observed that DEHP could be degraded at a faster rate along with other organic matter of sewage sludge using in-vessel composting. Moreover, the concentrations of DEHP in the final compost were proven to meet the criteria of sludge land application as suggested by the EU.