揮發性脂肪酸對上流式厭氣污泥床反應器表現之影響

Abstract

揮發性脂肪酸在有機物生物轉化成甲烷和二氧化碳的過程中扮演著重要的角色。本論文探討揮發性脂肪酸的組成和進水有機容積負荷 (organic loading rate) 對上流式厭氣污泥床(upflow Anaerobic Sludge Blanket, UASB)反應器表現之影響。 在第一部份實驗中，UASB反應器以相似成分的乙酸，丙酸和丁酸進料。化學需氧量(COD)濃度為20 g COD L-1，操作有機容積負荷介於0.82 和10.4 g COD L-1 day-1。實驗發現揮發性脂肪酸的COD移除率隨有機容積負荷的增加而下降，由99%下降至83%。甲烷產率(methane yield)理論值百分比由38.6%上升到86.5%。這顯示了提高有機容積負荷能更有效的使揮發性脂肪酸轉化成甲烷。 在第二部份實驗中，進料組成分別改成具有相同COD濃度之純乙酸，純丙酸和純丁酸。實驗發現以純乙酸和純丁酸進料的UASB反應器相較於純丙酸有較高的沼氣產量（9940 and 9730 mL day-1），甲烷組成（90%及77.5%）和COD移除率（78.6%及97%）。而COD濃度為20 g COD L-1的純丙酸對生物反應器呈現非常嚴重的抑制效果，甚至造成反應器失效。碳平衡指出在相同的COD濃度下，純乙酸較純丁酸有更多的碳進入反應器，而在丁酸進料的反應器中有10.5%的碳形成微生物質量(biomass)，這使得最終以丁酸進料的污泥揮發性懸浮固體(volatile suspension solid)濃度較乙酸進料的污泥有13.1%的增加。而乙酸進料的反應器所產生的高純度甲烷源於高pH值的出水對沼氣中二氧化碳的吸收。 產甲烷活性測試證實了以乙酸進料的污泥缺乏產乙酸菌來降解丁酸。乙酸和丁酸的降解能有效的幫助降解丙酸。反應器的甲烷產量可以藉由提高有機容積負荷增加2.4到4.2倍。而除了以混合揮發性脂肪酸進料的反應器外，其他反應器的COD移除率都能通過產甲烷活性測試來預測。

Volatile fatty acid (VFA) is the most important intermediate product in the bioconversion of organic matter to CH4 and CO2. This study investigated the effect of VFA on the UASB digester performance based on the organic loading rates (OLRs) and the VFA composition. UASB digesters fed by similar VFA of 20 g COD L-1 were operated at OLR between 0.82 and 10.4 g COD L-1 day-1. The chemical oxygen demand (COD) removal efficiencies were between 83% and 99%, and methane yield increased from 38.6% to 86.5% of the theoretical value. The results showed an elevated effectiveness in converting mixed-VFA substrate to methane at the increased OLRs. By feeding pure acetate, propionate or butyrate substrate in the same COD to the UASB digesters, the acetate- and butyrate-fed digesters showed higher biogas production (9940 and 9730 mL day-1), methane composition (90% and 77.5%) and COD removal (78.6% and 97%) than pure propionate-fed reactor. Propionate substrate exhibited severe inhibition leading to system failure at a feed concentration of 20 g COD L-1. Carbon balance showed higher amount of carbon in acetate substrate fed to digester than butyrate substrate. 10.5% of carbon fed to butyrate-fed digester was converted to biomass and made an 13.1% increase of VSS concentration in butyrate-fed sludge compared with acetate-fed sludge. High purity of methane in acetate-fed digester was due to dissolution of carbon dioxide in high pH effluent. Specific methanogenic activity test proved that the acetate-fed sludge was lacking of acetogen to degrade butyrate. Degradation of propionate is feasible while combined with the degradation of acetate and butyrate. Methane production of the digesters can be further increased from 2.4 to 4.2 times. The COD removal of digesters can be predicted well by SMA test except for the mixed acid-fed digester.