A Physical-Mathematical model is proposed to estimate the relative activity of anammox bacteria (RAA) for suspended enriched cultures (SEC). The dependence of the RAA on the concentrations of bacteria, substrates and other chemical species present in the medium that can enhance the activity or inhibit it is taken into account. The determinations of the constants and parameters of the model were found using data extracted from experimental works published by other authors. The RAA estimates for SEC match; at least in its tendencies, with previous experimental works. This model can be extended to other chemosynthetic bacteria.