廚餘以超音波預處理對厭氧醱酵產氣之影響探討

本研究以兩套兩相式厭氧醱酵模場進行平行試驗，透過超音波技術破壞廚餘並使廚餘內有機物質釋放至水相中，增加生物降解性促進水解及酸化過程中之生化反應，探討廚餘經超音波處理是否有助於縮短停留時間，提高厭氧生物系統穩定性，並提升沼氣產量，進而提高發電量。模場試驗共分為三個階段，第一階段先以生廚餘作為進料基質，第二階段使用熟廚餘，第三階段為生/熟廚餘混合(1：1)。為了解其物化特性，先對其個別進行特性分析，再對不同超音波處理時間下的生物分解特性進行探討，對擁有不同特性廚餘各別進行甲烷產氣潛能試驗(BMP test)。 BMP test溫度控制在55℃，轉速為30 rpm，並每日紀錄產氣。模場使用的超音波探頭由工研院材化所提供，材質為鈦合金及不鏽鋼，超音波設備參數為功率500W、頻率20 kHz、反應體積為4L。移動式模場由一標準40呎貨櫃改建，主要設備包含超音波設備、破碎機、混料暫存槽、2座酸化槽(各1 m3)、2座醱酵槽(各4 m3)及進出料所使用的單軸螺旋幫浦，試驗基質為生/熟廚餘，水力停留時間17天(酸化槽+醱酵槽)，進料TCOD濃度控制在50,000 mg/L，每日進料體積各300 L，每週進料5天。由BMP test研究結果得知，經超音波前處理後生廚餘SCOD增加率從3.90%至51.00%；熟廚餘SCOD增加率從6.67%至53.33%；生/熟廚餘(1：1)混合階段SCOD增加率從3.13%至62.51%，所有種類廚餘皆可以有效提升水解速率，且在一定時間內超音波時間與水解速率成正比，但產氣提升效果會因廚餘內含物質有所改變。生廚餘產氣效果最高可提升13.85%，熟廚餘產氣效果最多降低9.67%，生/熟廚餘產氣效果最高可提升16.01%。生廚餘及生/熟廚餘產氣效果隨著水解速率的提升而增加，但熟廚餘產氣效果卻出現下降的趨勢，熟廚餘產氣的下降可能是與超音波處理後熟廚餘中同時存在天然蛋白質與變性後蛋白質，兩者之間的相互作用被認為是大分子聚集體，相當難於生物降解。在模場試驗生廚餘階段，經超音波實驗組SCOD平均增加率為14.27%，產氣提升率可達20%，顯示生廚餘透過超音波前處理有利於提升水解效率、縮短停留時間及提升沼氣產量，可能與生廚餘中較高含量的纖維素有關。生/熟廚餘階段SCOD平均增加率為16.98%，但未發現產氣提升效果；熟廚餘因本身經過烹煮及調味使其易於酸化，若再經超音波水解，易造成酸累積於醱酵槽，導致系統不穩定甚至酸敗現象發生，因此不建議熟廚餘使用超音波前處理。超音波水解技術雖對生廚餘的水解能力有很高提升作用，但生廚餘本身含水率高，較難將COD負荷提高，所以可透過複合基質(生+熟廚餘)共醱酵來維持系統之穩定性。

關鍵字

厭氧醱酵；廚餘；超音波；清潔能源

並列摘要

The purpose of the study was to design two sets of two-phase anaerobic digestion pilot plant for the parallel experiments. It is hoped that, through the ultrasonic technology organic substances in the kitchen waste can be released into the water phase and promote the biochemistry of hydrolysis and acidification, and eventually increase the biodegradability. In this study, whether the ultrasonic treatment of kitchen waste can help shorten the hydraulic retention time and improve the stability of the anaerobic biological system, and increase the biogas production and power generation were investigated. The pilot plant study was divided into three stages. In the first stage, we used raw kitchen waste as the feed, and in the second and third stages, we used cooked kitchen waste and a mixture of raw/cooked kitchen waste (1:1) as the feed. In order to understand the properties of the kitchen waste, basic characteristics of the kitchen waste were established. The Biochemical Methane Potential (BMP) test was then used to investigate biological decomposition characteristics under different ultrasonic treatment times of the kitchen waste. The temperature of the BMP test was controlled at 55℃, the rotation speed was 30 rpm, and the gas production was recorded daily. The ultrasonic probe used in the pilot study was provided by the Institute of Industrial Technology Research Institute (ITRI-MCL). The material is made of titanium alloy and stainless steel. The specifications of the ultrasonic equipment are: 500 W power, 20 kHz frequency, and 4 L reaction volume. The mobile pilot plant was built in a standard 40-ft container, with the main units include ultrasonic equipment, grinder and mixing tank, 2 acidification tanks (1 m3 each), 2 fermentation tanks (4 m3 each), and 2 progressive cavity pumps for feeding and discharging materials. The test substrate was raw and/or cooked kitchen waste, with the hydraulic retention time of 17 days, and the feed TCOD concentration was controlled at around 50,000 mg/L. The daily feed volume was 300 L for each system, and both systems were fed 5 days a week. 　　According to the BMP test results, it was found that, after the ultrasonic treatment, the increase rate of SCOD for raw kitchen waste was from 3.90% to 51.00%; and for the cooked kitchen waste was from 6.67% to 53.33%; and for the raw/cooked kitchen waste mixture was from 3.13% to 62.51%. Ultrasonic treatment can effectively increase the rate of hydrolysis, and within a certain period of time the ultrasound treatment duration is directly proportional to the rate of hydrolysis, but the gas production improvement effect will be dependent on types of the kitchen waste. The gas production effect of raw kitchen waste was increased up to 13.85%, but the gas production effect of cooked kitchen waste was reduced by 9.67%, and the gas production effect of raw/cooked kitchen waste mixture was increased by 16.01%. The gas production effect of raw and raw/cooked kitchen waste mixture increased with the increase of the hydrolysis rate, but cooked kitchen waste did not. Instead, the gas production effect of cooked kitchen waste had a downward trend. The decrease in gas production from cooked kitchen waste may be related to the presence of natural protein and denatured protein in the cooked kitchen waste after ultrasonic treatment. The interaction between the two is considered to be a macromolecular aggregate, which is quite difficult to biodegrade. In the stage of the raw kitchen waste of the pilot study, the average increase rate of SCOD in the ultrasonic treatment system was 14.27%, and the gas production increase rate could reach 20%, indicating that the ultrasonic treatment of raw kitchen waste was beneficial in the improvement of the hydrolysis efficiency, shortening in the retention time and in the increase of biogas production. This might be related to the higher content of cellulose in the raw kitchen waste. The average increase rate of SCOD during the raw/cooked kitchen waste mixture stage was 16.98%, but no gas production improvement effect was found. The cooked kitchen waste was cooked and seasoned, and this makes it easy to acidify. The hydrolysis process was sped up by ultrasound, and this will easily cause acid to accumulate in the fermenter, possibly resulting in system instability or even failure. Therefore, it is not recommended to use ultrasound treatment for cooked kitchen waste. Although the ultrasonic hydrolysis technology has a high effect on the hydrolysis ability of raw kitchen waste, the water content of raw kitchen waste is high, and it is difficult to increase the COD load. This can be overcome by applying the composite substrate (raw + cooked kitchen waste) and to maintain the stability of the system.