The effects of microelements (zinc, copper, magnesium) supplementation (provided as ZnSO₄ and ZnCl₂ at 50, 70, and 100 mg/L Zn²⁺; as CuSO₄ and CuCl₂ at 10, 30, and 50 mg/L Cu²⁺; as MgSO₄ and MgCl₂ at 50, 150, and 250 mg/L Mg²⁺) on biogas (methane) production were evaluated while co-digesting a mixture of food waste(water) and domestic wastewater (at 0.185 ratio, v/v). Regardless of microelements supplementations, all three microelements increased the cumulative methane yield compared to the control without supplementation (p<0.05) and were considered involved in providing the synthesis of enzymes required for both acetoclastic and hydrogenotrophic methane production. The supplementation of 50-100 mg/L Zn²⁺, 10 mg/L Cu²⁺, and 150 mg/L Mg²⁺ stimulated the conversion of organic matter to methane, increased the chemical oxygen demand (COD) removal efficiency, and accumulated less ammonia-N. The improvements on the cumulative methane yield obtained could be associated with not only the intermittent feed strategy but also the micronutrient supplementation. Microelements may improve the long chain fatty acids (LCFAs) removal by two different mechanisms, biological and physical (precipitation). The exact role of each mechanism warrants further studies, but both definitely contribute to alleviate the toxic effects of LCFAs. The upflow anaerobic sludge blanket (UASB) reactor used was operated under the stable and steady condition for 260 days.