Longitudinal dispersion in a horizontal subsurface flow constructed wetland: a numerical solution


  • Adérito Araújo
  • Ercília Sousa
  • António Albuquerque




We present a numerical solution for the dead zone model which describes the solute transport in a subsurface and horizontal flow constructed wetland. This model is a system of two mass balance equations for two conceptual areas: the main channel and the storage zone. We use finite difference schemes to determine the numerical solution of the system and we study its convergence by presenting properties related to the stability and accuracy of the schemes. Concerning the experimental results, the magnitude of the longitudinal dispersion and the extension of dead volumes is estimated for clean conditions and after a certain operating period under organic loading conditions. The results showed a considerable amount of longitudinal dispersion through the bed, which was very strong near the feeding point, indicating the occurrence of mixing and significant presence of dead zones and short-circuiting. This approach is expected to be useful to determine operating conditions, such as, the control of the incoming organic loading, and also to avoid the increase of dead zones as a means to improve treatment performance. References
  • J. M. Bahr and J. Rubin. Direct comparison of kinetic and local equilibrium formulations for solute transport affected by surface reactions. Water Resource Res. 23(3):438--452, 1987. doi:10.1029/WR023i003p00438
  • k: E. Bencala and R. A. Walters. Simulation of solute transport in a mountain pool-and-riffle stream: a transient storage model. Water Resour Res, 19(3):718--724, 1983. doi:10.1029/WR019i003p00718
  • K. Dekker and J. G. Verwer. Stability of Runge-Kutta Methods for Nonlinear Stiff Differential Equations. cwi Monograph 2, Elsevier, Amsterdam, 1984.
  • S. H. Keefe, L. B. Barber, R. L. Runkel, J. N. Ryan, D. M. McKnight and R. D. Wass. Conservative and reactive solute transport in constructed wetlands. Water Resource Res., 40: W01201, 2004. doi:10.1029/2003WR002130
  • C. J. Martinez and W. R. Wise. Analysis of constructed treatment wetland hydraulics with the transient storage model otis. Ecological Engineering, 20(3):211, 2003. doi:10.1016/S0925-8574(03)00029-6
  • C. F. Nordin and B. M. Troutman. Longitudinal dispersion in rivers: the persistence of skewness in observed data. Water Resour Res, 16(1):123--128, 1980. doi:10.1029/WR016i001p00123
  • R. L. Runkel and S. C. Chapra. An efficient numerical solution of the transient storage equations for solute transport in small streams. Water Resour Res, 29(1):211--215, 1993. doi:10.1029/92WR02217
  • J. G. Verwer and J. M. Sanz-Serna. Convergence of Method of Lines Approximations to Partial Differential Equations. Computing, 33(3-4):297--313, 1984. doi:10.1007/BF02242274
  • B. J. Wagner and J. W Harvey. Experimental design for estimating parameters of rate-limited mass transfer: analysis of stream tracer studies. Water Resource Res., 33(7): 1731--1741, 1997. doi:10.1029/97WR01067





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