The following chemical parameters were determined: total chemical oxygen demand (COD t ), soluble chemical oxygen demand (COD s ), total volatile solids (TVS), volatile suspended solids (VSS), alkalinity, sulfate, sulfide, pH, total Kjeldahl nitrogen (TKN), and ammonia. These parameters were analyzed according to APHA Standard Methods (APHA et al., 2012) . All analytical determinations were made at least in duplicate with a frequency of two or three times per week, except methane volume, which was measured daily. The determination of metal concentrations was performed using a Perkin Elmer Optima 3000 ICP according to standard protocols based on official methods of the US-EPA (Cortés and Montalvo, 2016) .
The Minitab 8 software was used for the statistical processing and analysis of the data. A Fisher distribution (Test F) analysis was performed and a comparison of 95% confidence intervals for mean values was made for all cases studied.
The Minitab 8 software was used for the statistical processing and analysis of the data. A Fisher distribution (Test F) analysis was performed and a comparison of 95% confidence intervals for mean values was made for all cases studied.