BACKGROUND: Linoleic acid, which is a major derivative generated from hydrolysis of vegetable oils, is often found at high concentration in food processing and kitchen wastes. This fatty acid could be introduced into an anaerobic system treating piggery wastewater via co-digestion strategy. In this study, the effect of CaCl2 on the inhibitory behavior of linoleic acid in the anaerobic digestion of piggery wastewater was investigated. RESULTS: Linoleic acid exerted strong inhibitory effects on methanization of piggery wastewater with IC50 of 376 mg L-1 and 568 mg L-1 for ISR (inoculum substrate ratio) 0.38 and 2.0, respectively. Among tested cations (Na+, Mg2+, Ca2+, Al3+ and Fe3+), only Ca2+ effectively reversed linoleic acid inhibition. In addition, it was found that CaCl2 restricted linoleic acid inhibition only when it was added before or immediately after linoleic acid addition. Without CaCl2, linoleic acid was stoichiometrically transformed to oleic acid, which persisted for a prolonged period. In the presence of CaCl2, however, linoleic acid was completely converted to methane without accumulation of oleic acid. It was also found that oleic acid was self-inhibitory to its oxidation to acetate, and CaCl2 removed this inhibition. CONCLUSION: From these findings, it was concluded that calcium ion not only mitigated linoleic acid inhibition in the anaerobic digestion of piggery wastewater, but also enhanced its mineralization to methane. (C) 2010 Society of Chemical Industry