Simulation modelling of dissolved organic matter removal in a free water surface constructed wetland

• The model of SCOD removal by free water surface constructed wetland was developed.
• The simulated data showed moderate agreement with data from wetland operation.
• Desorption from sediment is the major cause affected on SCOD in treated water.
• To reduce the impact of discharge on water resource, HRT should be lower than 2 days.

A simulation model was developed to explain the reduction of dissolved organic matter (DOM) in a free water surface constructed wetland (FWS-CW). Laboratory-scale experiments were conducted in the FWS-CW system of the water reclamation plant of Hat Yai municipality in Southern Thailand. The FWS system was divided into different layers of FWS-CW components according to their mechanisms and the kinetic coefficient (k). The DOM, in terms of the soluble chemical oxygen demand (SCOD) concentrations simulated by the calibrated model, exhibited moderate agreement with the full-scale data. The model showed that the leaching of SCOD from the gravel bed is the most significant mechanism affecting the SCOD concentration in the wetland ponds. Although the organic loading rate (OLR) of the SCOD inflow of the FWS-CW was low, a high SCOD concentration in these ponds was still observed under the conditions of a long hydraulic retention time (HRT) and a slow hydraulic loading rate (HLR). Hence, FWS-CW management should focus on the relationship between HLR and HRT, as well as sludge management in the system, to prevent the release of SCOD. When this model was stimulated with the operational data of the FWS-CW under study, it revealed that the HRT should not exceed 2 days to prevent high SCOD discharge into the water reservoir.