PERFORMANCE OF SUBSURFACE FLOW WETLANDS WITH BATCH-LOAD AND CONTINUOUS-FLOW CONDITIONS

Use of constructed wetlands designed for wastewater treatment is becoming common throughout the world. Oxidation of carbon (C) and nitrogen (N) may be increased by transport of oxygen (O-2) into the rhizosphere of aquatic plants and periodic draining of the wetland. Outdoor microcosms of subsurface flow wetlands (SFs) were operated receiving continuous flow or batch loads of primary and secondary wastewater. Six-square meter microcosms planted with Scirpus pungens, received primary or secondary wastewater in batch load or continuous flow. Plants in SFs had significant effects on C and N oxidation, whereas, periodic draining of SFs had no significant effect. Greater than 90% carbonaceous biochemical oxygen demand (CBOD) was removed within 18 hours in SFs with and without plants, after 6 and 12 hours plants had caused significant decreases in CBOD in the batch-load SFs. The first-order model for CBOD removal was not appropriate after 24 hours; removal rate coefficients were similar for batch-load and continuous-flow SFs with hydraulic retention times of 12 and 24 hours. Methanogenesis is the major respiratory pathway for CBOD removal. Oxygen transport in SFs estimated from C oxidation was 28.6 and 2.4 g/m(2) . d estimated from N oxidation.