The effect of loading frequency and plants on the degradation of sulfamethoxazole and diclofenac in vertical-flow constructed wetlands

The aim of this study was to investigate the effect of loading frequency (1 pulse per day and 4 pulses per day) and vegetation (monoculture of Phalaris arundinacea ‘Picta’) on the removal and transformation of a mixture of pharmaceuticals diclofenac (DCF) and sulfamethoxazole (SMX) in lab-scale unsaturated vertical-flow constructed wetlands. Additionally, the system performance, the ecotoxicity of the wastewater (Microtox bioassay), and the microbial community structure and diversity in the top layer of the system were assessed. In the experiment, elevated concentrations of the pharmaceuticals (0.5 mg/L) were used to reflect the quality of wastewater from single houses or small settlements. The removal of DCF and SMX was deteriorated by the presence of Phalaris arundinacea ‘Picta’. The lower loading frequency enhanced the removal of DCF and SMX only in the planted columns, suggesting combined effect of the two factors. For the unplanted systems no significant effect of the loading frequency was observed. The observed removal efficiency of SMX and DCF in the experiment was in the range 52.8–91.2% and 47.3–74.2%, respectively. The pharmaceuticals did not affect markedly the removal of DOC, TKN and N-NH₄, but they reduced the bacterial diversity within the rhizosphere and in the adjacent substrate of the systems. The main transformations of SMX were (mono-, di-, tri-)hydroxylation, demethylation, deamination and formation of glutathione conjugates, whereas for DCF only trihydroxylation was observed. The toxicity of the raw and treated wastewater containing the pharmaceuticals was at a level comparable with the control samples. This study showed that vertical-flow constructed wetlands can be efficient in the removal of DCF and SMX. Given that the constructed wetlands are inherently planted systems, it is suggested that the loading frequency is the operational variable that could be adjusted to enhance the removal of these pharmaceuticals.