We describe a useful and simple model for studies of simultaneous methanogenic-denitrification (M-D) systems. One equation predicts an inverse relationship between the percentage of electron donor channeled into dissimilatory denitrification and the loading ratio X given by grams degradable COD per gram nitrate-N in the influent. It allows the determination of the extent of use of organic matter by the denitrification and the methanogenic pathways for a given concentration of nitrate salts in the influent. This equation was validated with experimental data from the literature and good agreement between predicted and experimental percentages was found. The other equation allows the calculation of the N nutrient requirements in the ammonia nitrogen form (that is, with oxidation number -3). (c) 2005 Society of Chemical Industry.