Laboratory scale combined constructed wetland system composed of a subsurface vertical flow unit connected to a floating macrophyte unit was used to treat domestic wastewater. Hydraulic loading rates (HLR) of 5 arid 8 cm/day were operated to evaluate the efficiencies of the system. Canna(Canna indica Linn.) was planted in a subsurface vertical flow unit and Lettuce (Pistia stratiotes L.) was planted in a floating macrophyte unit. Domestic wastewater from Chiang Mai University campus was fed to each unit with every 7 hours for 1 hour. The removal efficiencies for COD, TKN, NH4-N, SS and TP were dependent on the hydraulic loading rates and the removal efficiency was higher at lower HLR due to longer retention time. Mass removal rate for COD,TKN, NH4tN ,TP and SS were 5.0, 0.38, 0.32, 0.02 and 9.7 g/m2.day, respectively at HLR of 8 cm/d. For HLR of 5 cm/day the mass removal rate for COD,TKN, NH4-N ,TP and SS were 3.7, 0.33, 0.21, 0.015 and 5.3 g/m2.day, respectively.
