Abstract: The subsurface flow wetland reactor model for sewage treatment was studied by approximating the Michaelis-Menten equation into a nonlinear power law function of two orders. The analytical solutions were obtained in terms of a rapid convergent power series by utilizing the Adomain decomposition technique. The mathematical technique employed in this paper is also of significance in studying the problems when the Michaelis-Menten equation is approximated by other nonlinear functions.