Determination and abatement of methanogenic inhibition from oleic and palmitic acids

Abstract

Long chain fatty acids (LCFAs) are the intermediates from anaerobic digestion of lipid-rich waste, and limit the digestion even at millimolar concentrations. The digestion characteristics of unsaturated and saturated LCFAs, represented by oleic and palmitic acids, are investigated. Consequently, palmitic acid was far less inhibitory than oleic acid. Palmitic acid was almost not degraded before 20 d, and the subsequent methane was also produced at a lower rate, below a half for oleic acid. The heterogeneity between methanogenic sludge and palmitic acid was the likely reason. The abatement methods of soluble calcium addition and bentonite addition were synchronously compared to evaluate the effectiveness in upgrading the anaerobic performance of LCFAs. Limited improvement for palmitic digestion was observed. In contrast, calcium addition was more effective than bentonite addition in shortening lag-phase time, reducing reagent dosage, and decreasing solid concentration of digestate. Delaying the addition time of calcium salt had a negative effect on the abating the inhibition caused by oleic acid. The methanogenic activity decreased exponentially with the addition time of calcium salt, following the equation: Methanogenic activity (%) = 91.0e−t/942.5+24.8e−t/6.2−15.9.