Monitoring and modelling of free water surface constructed wetlands

Integrated water management has encouraged the construction of small scale wastewater treatment plants to treat residual discharges. For this study, six free-water-surface constructed wetlands have been monitored from September 1998 till April 1999 to determine their operational efficiency. These natural systems have been constructed by the Flemish Land Authority (VLM) in Wontergem. Design loads vary between 100 and 500 PE. The constructed wetlands seemed to function properly, even in winter conditions with low temperatures and high precipitation rates. COD, BOD and SS effluent standards for small scale wastewater treatment plants were rarely exceeded. N-removal seemed limited during the coldest period but clearly functioned better at higher temperatures. The monitored wetlands showed large differences in P-removal and removal of bacteria. In order to obtain a better insight in the operation of free-water-surface constructed wetlands, a new model was implemented in the WEST simulator (Hemmis NV, Kortrijk, Belgium), based on the biofilm model of Rauch et al. (1999). The model confirmed the role of biofilms in these natural systems and seemed capable to simulate both microbial processes and meteorological influences.