Removal of Phosphorus in a Laboratory Scale System of Artificial Wetlands

Abstract

Removal of phosphorus in four packed biological reactors simulating the root zone of an artificial wetlands was performed. Reactors packing was volcanic slag (tezontle in the Aztec language). The experimental design included the effect of the macrophytes (reactors RA1 with plant, and RA2 without plant), and its replicas (RB1 with plant, and RB2 without plant), as well as the initial concentration of pollutants measured as chemical oxygen demand (one with a C:N:P ratio of 30:1:0.04 equivalent to 450 mg COD/L, and the other one with a ratio 15:1:0.04 equivalent to 225 mg COD/L) in the phosphorus assimilation. Synthetic wastewater was made using sucrose as carbon source, C12H22O11; ammonium sulfate as nitrogen source, (NH4)2SO4, and dodecahydrated sodium phosphate as phosphorus source, Na3PO4.12H2O. Light was obtained from 8 lamps of 30 W fluorescent tubes automatically controlled to simulate light periods of 16 h and 8 h darkness. Three commercial electrodes (Ag/ AgCl-Pt) to measure oxidation-reduction potentials, pOR, at different depths were placed to evaluate the redox potential differences (2, 10, 30 cm from the upper reactor surface). Shoots of a Phragmites australis macrophyte were planted in the reactors RA1 and RB1. Once stability was reached total phosphorus, COD, redox potentials, pH, and temperature. Results show that all reactors had phosphorus removed, and the removal was significantly higher in the reactors with plant (RA1 and RB1, 93% in average for both reactors) (p<0.05, 0.01). No significant difference (p<0.05, 0.01) was found between the COD reduction and phosphorus for the two experiments with 450 and 225 mg COD/L. Removal of COD for the experimental design carried out (effect of the macrophyte and the initial COD concentration) in both cases was statistically different due to the presence of the plant, as well as the initial carbon concentration (p<0.05 for the plant presence, and p<0.05, 0.01 for the initial carbon concentration), being slightly higher for those reactors with plant, indicating that they do have an additional effect to the microbiological communities proliferating in the reactors.