化學前處理於聚乳酸與沼液厭氧共消化之影響

Abstract

聚乳酸是目前市場上公認最有潛力取代現今石化塑膠之生質塑膠。因其生物可分解性及其與塑膠相仿之機械特性使得綠色循環經濟為主流價值的社會對此材料越加重視，同時市場的需求也逐漸增加，而伴隨著聚乳酸製品需求的提升，聚乳酸廢棄物處理的重要性也受到大眾的關注。 本研究之目的為聚乳酸製品與豬糞尿水沼液進行厭氧共消化的可行性分析。首先探討聚乳酸粉末前處理之最佳時間，由乾物質量損耗試驗及化學需氧量和溶解性化學需氧量的變化試驗得到最佳水解時間為24小時。接著以生化甲烷潛能(BMP)測試找出最佳的聚乳酸及沼液之混和比例以及聚乳酸前處理方式。由試驗結果得知於中溫操作的環境下(37°C, Mesophilic)聚乳酸以及厭氧沼液之最佳混合比例為3:1，其20日之生化甲烷潛能測試可得單位甲烷產量為10.4 mL CH4/g VS added。此外。於相同之混和比例，分別測試不同化學藥品之聚乳酸前處理，由結果得知，以0.5M NaOH，80°C加熱處理24小時之聚乳酸有較佳之產氣表現，單位甲烷產量為139.4 mL CH4/g VS added。 再由上述的實驗結果，以24小時的水解時間及3:1的混和比例，在水力停留時間(HRT) 10天及進流TS 8%下進行連續式進出流試驗。分別測試在不同的化學前處理及操作溫度(37°C, 55°C)下，聚乳酸和沼液共消化的進出流效能。每日取樣分析pH、TS、VS及COD。氣體則每日讀取產氣量及分析氣體成分。實驗結果顯示，於中溫操作環境下，以0.5 M NaOH及0.5 M HCl處理之聚乳酸及沼液共消化之甲烷產率分別為1.17 L/L/d、1.08 L/L/d，與未處理聚乳酸及沼液共消化之甲烷產率比較，分別高出25%及15%。於高溫操作環境下，以0.5 M NaOH及0.5 M HCl處理之聚乳酸及沼液共消化之甲烷產率分別為1.38 L/L/d及1.18 L/L/d，與未處理聚乳酸及沼液共消化之甲烷產率比較，分別高出41%及20%。無論是中溫環境或高溫環境之連續式進出流處理，經NaOH處理之聚乳酸皆比HCl處理之聚乳酸能夠與厭氧沼液有更佳的厭氧產氣表現。而高溫操作明顯比中溫操作有較佳的產氣表現。

Poly-lactic acid (PLA) is known as a promising material to take the place of traditional petroleum products due to its biodegradability and similar properties to the petroleum-based plastics. Therefore, the treatment of the used PLA products cannot be overemphasized owing to its growing demand of the market. The aim of this study is to utilize PLA products to anaerobically co-digest with the digestate of swine manure. First of all, we found out the optimal pretreatment time of PLA by performing the experiment of mass loss ratio and chemical oxygen demand. By the results, we concluded that 24 hours of pretreatment of PLA was optimal. Biochemical methane potential (BMP) test was performed to optimize the most appropriate mixing ratio of the PLA and the anaerobic digestate as well as to find out the best pretreatment method (acid or alkaline) of PLA. The results showed that at mesophilic temperature (37°C), the optimal mixing ratio of the PLA and the digestate was 3.0, with a yield of 10.4 (mL CH4/g volatile solids added) for a duration of 20 days BMP test. Moreover, under the identical operational conditions, alkaline pretreatment demonstrated better performance. It was found that heating PLA with 0.5 M NaOH for 24 hours would produce 139.4 (mL CH4/g volatile solids added). The results of the BMP test showed that the pretreated PLA can serve as an appropriate carbon source for the anaerobic digestate of swine manure. A continuous operation experiment was followed up at mesophilic and thermophilic conditions. At low hydraulic retention time (10 days) and high concentration (TS=8%) operation conditions, we discovered that PLA pretreated with 0.5M NaOH performed better from gas production and removal efficiency aspects. At mesophilic temperature, the methane production rate of NaOH pretreated PLA co-digestion with the digestate is 1.17 L/L/d, which is 25% more than the co digestion of the untreated PLA and the digestate; and the methane production rate of HCl pretreated PLA co-digestion with the digestate is 1.08 L/L/d, which is 15% more than the co digestion of the untreated PLA and the digestate Moreover, at thermophilic operation, the methane production rate of NaOH pretreated PLA co-digestion with the digestate is 1.38, which is 41% more than the co digestion of the untreated PLA and the digestate; and the methane production rate of HCl pretreated PLA co-digestion with the digestate is 1.18 L/L/d, which is 20% more than the co digestion of the untreated PLA. Operation temperature plays an important role for NaOH pretreated PLA. The rise of the gas production and removal efficiency showed the hydrolysis properties of PLA in alkaline environment.