RESUMO
Methanogenesis involves several enzymes with trace metal components that catalyze major metabolic pathways and, therefore, requires a sufficient supply of micronutrients such as iron, nickel or cobalt. The statistically-based Plackett-Burman experimental design was adopted in this study to identify which trace metal have a statistically significant effect on the maximum methane production from domestic sewage. The addition of Barium (Ba), Cobalt (Co), Copper (Cu), Iron (Fe), Manganese (Mn), Nickel (Ni) and Selenium (Se) was tested in batch reactors using domestic sewage as the substrate and sewage sludge as the inoculum. The results showed that the addition of Fe was statistically significant, positively affecting the maximum CH4 production (p-value 0.05). The results are expressed in L of CH4 per g of Chemical Oxygen Demand added, increasing it from 0.10 to 0.13 LCH4 gCODadd-1. In L of CH4 per g of Volatile Solids, increased it from 0.031 to 0.040 LCH4 gVS-1. The addition of Se was statistically significant, but with a negative effect on the maximum methane production (p-value 0.002), which decreased from 0.13 to 0.10 LCH4.gCODadd-1. Moreover, six different Fe concentrations (0, 40, 80, 120, 160 and 200 mgL-1) were tested, showing that the addition of 120 mgL-1 had the greatest effect for statistically improving the maximum methane production, with 33% improvement (0.12 ± 0.003 to 0.16 ± 0.012 LCH4 gCODadd-1) compared to no addition of Fe and the specific CH4 production to 0.040 ± 0.001 LCH4 gVS-1.