Evaluation of the Influence of Loading Conditions on the Simultaneous Removal of Organic Matter and Nitrogen in SSHF Constructed Wetlands

Submerged constructed wetlands with sub-surface horizontal flow are suitable systems for wastewater treatment and reuse in small communities. Laboratory studies are a useful way to better understand the evolution of pollutant removal mechanisms throughout the system. Two Series of experiments were performed in a laboratory microcosms system colonized by Phragmites australis, at the hydraulic loading of 1.67 x 10^–3 m³ m^–2 h^–1 (approximately 4 cm d^–1), organic loading ranging from 10 to 23.4 g m^–2 d^–1 COD, ammonia nitrogen loading ranging from 1 to 2 g m^–2 d^–1 NH₄-N, C/NH₄-N ratio of 4 and hydraulic retention time of 4.6 d. The results show an improvement of organic matter and ammonia nitrogen removals rates over time with average values of 7.2 g m^–2 d^–1 COD and 0.5 g m^–2 d^–1 NH₄-N, respectively, a considerable development of the plants and a good adaptation of the biofilm to changes in incoming loading conditions. The higher values of both removal rates occurred in the initial bed section where the superior DO concentrations allowed the occurrence of aerobic pathways. In the remaining sections (approximately 1.7 m length) the system was DO limiting and the organic removal occurred at lower rates, which seemed to be associated with the prevalence of anaerobic pathways. Although nitrification appears as the most important mechanism of ammonia nitrogen removal, no significant concentration of nitrate nitrogen was detected along the bed, which may be explained by the quick removal of this compound by denitrification. For the range of applied organic and ammonia loadings there was no evidence of a significant variation of the global organic removal rate or the nitrification rate when the incoming characteristics where changed. However, the nitrification rate at the initial section was in fact affected by changing on incoming loading conditions.