| 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{dollar}sb2){dollar} into the rhizosphere of aquatic plants and periodic draining of the wetland. Field studies were conducted to determine: (i) the influence of plants and draining of batch-load vegetated submerged beds (VSBs) on the oxidation of C and N from wastewater, (ii) the effect of hydraulic retention time (HRT) in VSBs on first-order CBOD removal rate coefficients and, (iii) the efficiency of C removal in VSBs and floating aquatic macrophyte systems (FAMs). Plants had significant effects on C and N oxidation, however, after an 18 hour HRT there was no difference between VSBs with and without plants. Draining VSBs had no effect on oxidation of C and N. Greater than 90% CBOD removal occurred within 18 hours. The first-order CBOD removal rate model was not appropriate after 24 hours. At a hydraulic loading rate of 20 cm/day, VSBs removed more CBOD than FAMs. Oxygen transport in VSBs estimated from C oxidation was 28.6 g/m{dollar}sp2{dollar} day, and for N oxidation 2.4 g/m{dollar}sp2{dollar} day. These rates agree with published O{dollar}sb2{dollar} transport rates.; Laboratory studies evaluated C and N oxidation in the rhizosphere of plants grown in wetland microcosms. Nitrate- and sulfate-reducing biofilms were established on plastic matrix in anaerobic continuous-flow stirred tank reactors with and without Scirpus validus. Steady state pH and Eh were maintained for at least 40 days in reactors. Redox potential was not affected by the presence of plants. An acetate mass balance accounted for reduction of NO{dollar}sb3sp-,{dollar} SO{dollar}sb4sp{lcub}-2{rcub},{dollar} and Fe{dollar}sp{lcub}+3{rcub},{dollar} and methanogenesis. Differences in acetate mass balance between reactors with and without plants, were used to estimate oxygen transport through the plants. Oxygen transport rate estimated from acetate mass balance was about 10 g O{dollar}sb2/rm msp2{dollar} day; and the estimated rate from {dollar}rmsp{lcub}15{rcub}NHsb4{dollar} oxidation was 0.5 g O{dollar}rmsb2/msp2{dollar} day. This reactor design for simulating anaerobic wetland environments may be a useful tool for studying plant/microbial interactions, and for studying treatment of wastewater in wetlands. |
