商業化生質柴油之生產使用精製食用油為原料，造成原料油占生質柴油生產成本之70%到85%；是以有必要尋找便宜且大量之料源以生產生質柴油。本研究利用次臨界水處理代表台灣不同工業之五種汙泥樣品以增加汙泥中可萃取之油脂量。結果發現反應溫度(175oC)為影響可用來與甲醇反應以生產生質柴油之可萃取中性脂質(增加2至4倍)之最重要因素。雖然汙泥樣品來源不同，但是所有汙泥油脂中主要之脂肪酸為C16及C18。
文獻中大多數以汙泥為原料生產生質柴油之研究都是利用需要長反應時間(約24小時)及高甲醇負載之酸催化轉酯化法。本研究探討利用在次臨界甲醇及醋酸中進行活性污泥之轉酯化，在250oC、使用85%甲醇及15%醋酸為溶劑且溶劑對乾汙泥之比為5mL/g之條件下，可在30分鐘內得到高甲基酯產率(30.11%，以乾汙泥為基準)。與在55oC下使用4%硫酸、25mL/g甲醇對乾汙泥比、需時24小時才能得到35%之甲基酯產率比較，本研究所用之方法避免使用礦物酸，且甲醇使用量及反應時間都顯著減少。

Commercial production of biodiesel mainly uses refined edible oil as the feedstock which constitutes about 70 % to 85 % of the overall cost of biodiesel. Finding cheap and abundant new feedstock for producing biodiesel is necessary. In this study, subcritical water treatment was employed to enhance the amount of extractable lipid of 5 wet sludge samples collected from a wide spectrum of industries in Taiwan. Reaction temperature (175 oC) was found to be the most important factor in increasing the extractable neutral lipids (by 2 to 4 times) which can be esterified with methanol to produce biodiesel. This study found that although activated sludge samples were collected from various sources; however, C16 and C18 fatty acids are the dominant components in all activated sludge oils.
Most previous studies reported in literature on biodiesel production from sludge were carried out by acid catalyzed transesterification that required long reaction time (about 24 h) and high methanol loading. Transesterification of activated sludge in mixture of subcritical methanol and acetic acid was investigated in this study. At a reaction temperature of 250 oC, high methyl esters yield (30.11%, based on dry activated sludge) can be obtained in 30 min by using a mixture of 85% methanol and 15% acetic acid as the solvent and a solvent to dry sludge ratio of 5 mL.g-1. Compared to the 35% methyl ester yield obtained by acid-catalyzed (4% H2SO4) transesterification which required 24 h at 55 oC and using a methanol to dry sludge ratio of 25 mL.g-1, the method developed in this study avoided using mineral acid, required significantly less methanol and much shorter reaction time.

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