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This paper presents the Constructed Wetland Model No1 (CWM1), a general model to describe biochemical transformation and degradation processes for organic matter, nitrogen and sulphur in subsurface flow constructed wetlands. The main objective of CWM1 is to predict effluent concentrations from constructed wetlands without predicting gaseous emissions. CWM1 describes aerobic, anoxic and anaerobic processes and is therefore applicable to both horizontal and vertical flow systems. 17 processes and 16 components (8 soluble and 8 particulate) are considered. CWM1 is based on the mathematical formulation as introduced by the IWA Activated Sludge Models (ASMs). It is important to note that besides the biokinetic model a number of other processes including porous media hydrodynamics, the influence of plants, the transport of particles/suspended matter to describe clogging processes, adsorption and desorption processes and physical re-aeration must be considered for the formulation of a full model for constructed wetlands.

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Constructed wetlands have become a good alternative to urban and industrial wastewater treatments. Because of the many interactions between the different parts that form the wetland, the design of these systems used to be very complex so a good design tool is required. Hybrid constructed wetlands mix the aerobic and anaerobic properties of vertical and horizontal subsurface flow constructed wetlands to improve COD, phosphorous and, especially nitrogen removal. A simulation of the concentration profiles of these compounds into a hybrid system constituted by four vertical flow constructed wetlands and a horizontal flow one with the K-C* model was achieved when the system treated an urban and a winery wastewater, respectively. Maximum limits of the flow rate to achieve the legislation limits of the different pollutants were calculated. Winery wastewater treatment required lower flow rates because of its higher organic loading. Because constructed wetlands had limited capacity to remove phosphorous, it was the most restrictive pollutant, requiring a lower flow rate.

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