Modelling pollutants transport and degradation through wetlands

Abstract

Wetlands play a number of roles in the water environment, principally water purification, flood control, and groundwater replenishment. In addition to these benefits, United Nations Millennium Ecosystem Assessment and Ramsar Convention defined wetlands to be of biosphere significance and societal importance in the areas of shoreline stabilisation, storm protection, cultural values, recreation and tourism, and climate change mitigation and adaptation. Wetlands are also considered the most biologically diverse of all ecosystems, serving as home to a wide range of plant and animal life. The function of most natural wetland systems is not to treat wastewater, however, their high potential for the filtering and the treatment of pollutants has been recognized by environmental scientists who specialize in the area of wastewater treatment. In the past wetlands used to be in place naturally and used to provide ecological benefits to the mankind and environment. Through recognizing their immense benefits, human being started to construct artificial wetlands. Recently, constructed wetlands are recommended as one of the salient features of water sensitive urban design, which play an important role in water management and ecologically sustainable development. These constructed artificial wetland systems are highly controlled environments that intend to mimic the occurrences of soil, flora, and microorganisms in natural wetlands to help in treating wastewater effluent. Artificial wetlands provide the ability to experiment with flow regimes, micro-biotic composition, and flora in order to produce the most efficient treatment process. Constructed wetlands are increasingly being designed and used to treat wastewaters. Majority of constructed wetlands are designed based on steady-state releases of pollutants loading. However, in some cases (i.e. aquaculture ponds) pollutant loadings are not steady-state, rather are intermittent. Pollutants transport analysis based on steady-state release (inflow) will be quite different from an analysis based on intermittent loading/inflow. In the past several studies were conducted on pollutants transport and degradation through wetlands using steady-state inflow of pollutants. In this paper, a simple numerical model is proposed and developed based on conservation of mass principle for the pollutants and transport through a wetland, considering a series of tanks. Tank-in-series approach assumes that the wetland is comprised of several interconnected tanks, each of which can be modeled as a continuous flow stirred tank reactor. As for pollutants, in this study organic matters are considered. Same numerical model can be used for different organic matters, considering different values of degradation rate. Using first-order kinetic equations of pollutants transport and degradation and applying Euler's method of difference equations a numerical model was developed. Developed numerical model can simulate pollutant transport and degradations for steady- state, continuous and/or irregular/intermittent pollutant loadings. Numerical model results were verified with earlier developed analytical solutions for intermittent pollutant loadings, which were applied for aquaculture ponds in Texas (USA). Numerical model results are close to the results derived from analytical solutions for the same condition. Reasons of some primary discrepancies are discussed. Developed numerical model was used to present different scenario using different flow rates, pond volumes and different masses of intermittent pollutants. It is found that all of these parameters have significant impact on outflow pollutants' concentrations.