Long-chain fatty acids, major derivatives of lipid hydrolysis, are well-known inhibitors causing serious problems in biologically anaerobic systems treating fat containing wastewater. In this work, the synthetic wastewater containing major LCFA was digested in single-phase system, a UASB (upflow anaerobic sludge bed) reactor and two-phase system composed of an acidogenic reactor using a CSTR (continuously stirred tank reactor) and a methanogenic reactor using a UASB reactor.
LCFA did not induce the significant toxic effect in both of the single-phase and two-phase systems at up to LCFA mixture of 3,000 mg COD/L (1,040 mg/L, 3.78 mM). In the single-phase system, the removal efficiencies of COD and LCFA were above 90 % and the methane conversion was above 75% of a theoretical value. In the acidogenic reactor of the two-phase system, about 5∼15% of unsaturated LCFA was saturated, however it did not affect on inhibition. The removal efficiencies of the two-phase system were the same levels as those of the single-phase system and, at maximum, 25% of LCFA became scum layer at the concentration above 2,500 mg COD/L.
Sludges from both continuous systems were used as the inoculum in batch test for the examination of the inhibitory effect of LCFA. LCFA inhibited acetate degradation more than propionate degradation. Unsaturated LCFA were more toxic than saturated ones on the methane production from VFA. However, the beta oxidation of itself was done more quickly than that of saturated ones.