Kinetic models in activated sludge, anaerobic digestion and other environmental modeling fields rely on the proper formulation of stoichiometric coefficients. Elemental balancing provides a simple and rigorous way to establish the stoichiometric coefficients of reactions represented in the Gujer matrix. The deduction of these coefficients is frequently trivial, from basic mass balancing considerations. In more complex cases, such as the Anammox growth reaction, rigorous elemental balancing is required to establish the proper formulation. This paper demonstrates the methodology based on a simple aerobic heterotrophic growth reaction where stoichiometry coefficients (such as the (1-YH)/YH term for oxygen) are well known. In the second step the methodology is applied for the Anammox growth reaction. The fraction of N2 gas in current models originates from the NH4+ and the NO2 electron donor/acceptor pair in equal proportion. This paper demonstrates that this stoichiometry is a simplification leading to elemental balance errors. The proper stoichiometric coefficients are derived.

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