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KMID : 0383620030240000073
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Bulletin of Environmental Sciences
2003 Volume.24 No. 0 p.73 ~ p.82
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Solubilization of Organic Material in Mesophilic and Thermophilic Anaerobic Digestion
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Kim Moon-Il
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Abstract
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Most research in anaerobic digestion has been focused on the methanogenic step, while not much attention has been paid to the hydrolysis and acidogenesis steps, when solid organic substrates are converted into mostly volatile fatty acids (VFAs). Better understanding of these steps can lead to the improvement of digester stability due to physical separation of phases, increase in the concentration of soluble organics, and optimization of biological nutrient removal (BNR) processes. The purpose of this study was to evaluate the effect of pH and inorganic nutrient supplementations for hydrolysis and acidogenesis at both mesophilic (35 ℃) and thermophilic (55 ℃) temperatures. Solubilization of a synthetic sludge and primary sludge was observed in batch operation for the evaluation of the pH effect. pH was uncontrolled in one reactor and controlled at 4.5, 5.5, and 6.5 in the other three reactors for the synthetic sludge at both temperatures. For primary sludge, pH was controlled at 65. at mesophilic temperature. When synthetic sludge was used, the greatest degree of solubilization was with the pH controlled at 6.5 and the pH of the uncontrolled reactor dropped to 3.4 at both temperatures. At this low pH, hydrolysis and acidogenesis were severely retarded. Acetic and n-butyric acids were primarily formed with lesser amounts of propionic acid at both temperatures. lactic acid was produced the earliest as an intermediate and as the reaction went on, short chain VFAs were produced as final end products with a decrease in lactic acid. The higher the pH, the earlier this trend happened. For the controlled reactors at pH 6.5, the soluble COD production and the VSS reduction peaked in 4 days at 55 ℃ while it took about 11 days to do the same at 35 ℃. During the linear SCOD production period and at the pH of 6.5, the hydrolysis rate of the thermophilic reactor was greater than that for mesophilic, while thermophilic conditions appear to be more sensitive to pH than mesophilic ones for both the hydrolysis and the acidogenesis. At the optimum pH of 6.5 based on previous experiments, primary sludge was also used as a substrate for the solubilization/acidification at the mesophilic temperature. Until 5 days, the reactor removed VSS with a corresponding production of VFAs and SCOD. Among measured VFAs, acetate and propionate were the main VFAs produced. After 5 days the reactor started reducing VFAs indicating that the reactor performed as a methanogenic phase. Therefore, at the pH of 6.5, optimum operation time of solubilization/acidification appears to be 5 days. Additional experiments for the effect of inorganic nutrient supplementations for solubilization at both temperatures were conducted. The results strongly implied the importance of inorganic nutrient supplementations for the hydrolysis and the acidogenesis at both temperatures.
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