A Mixed Zone Transport Model Of Flow Through A Constructed Wetland

Abstract

Free water surface wetlands are widely used for the treatment of both wastewater and urban stormwater. The treatment of contaminants in these wetland systems rely on physical, chemical and biological processes that take place as the water flows through the system. The efficacy of these treatment processes is dependant upon the residence time of water within the system. Flow through these treatment reactors is rarely equivalent to plug flow, and generally a distribution of residence times is observed. This paper describes a numerical modelling approach that has been developed to describe the flow of water through a constructed wetland treatment reactor. The model combines a mixed zone tank reactor, with a conceptual flow reactor described by a Muskingum transport model. The modelling approach has been used to model the tracer response curves produced from a simple laboratory flow reactor and from a two-dimensional numerical flow model of a hypothetical rectangular wetland. It is shown that the modelling approach replicates the distribution of residence times observed in these systems, which are characterised by a steep rise, followed by a long receding tail. Furthermore, the approach is shown to be superior to the use of continuously stirred tank reactors in the modelling of these flow processes. The model parameters are also shown to be functions of the length to width ratio of the treatment reactor.