Anaerobic digestion is a complex biochemical process that is represented by the stoichiometric Buswell's equation. Buswell's equation is the study of the interface among biochemistry, mathematics, and chemistry. Students often have difficulty in understanding the nature of biochemical stoichiometry. Buswell's equation is significant for the calculation of the quantity of biomethane. Although the general formula CxHyOz of organic pure substance or mixture has been well studied, the Buswell's equation for biomolecules containing phosphorous has rarely been explored. The aim of this article is to study the nature of Buswell's equation using the H-atom method. The new general Buswell's equation, which contains phosphorous, is deduced from the H-atom and algebraic methods.