Exploring the usage of artificial root exudates to enhance the removal of contaminants of emerging concern in slow sand filters: Synthetic vs. real wastewater conditions

In previous studies we observed that laboratory-scale constructed wetlands exposed to contaminants of emerging concern (CECs) increased the release of certain rootmetabolites which were probably linked with the enhancement of CEC biodegradation. Based on this, the current study tested if the addition of artificial root exudates in slow sand-filtration systems could also enhance the removal of CECs fromwastewater. First, in a preliminary study, twelve laboratory-scale slow sand-filtration columns were operated under synthetic and unrealistic wastewater conditions. Thus, by using synthetic wastewater, high concentration of CECs (100 mu g L-1 of benzotriazole, sulfamethoxazole, carbamazepine and diclofenac) and artificial root exudates (2-9 g L-1 of glucose, salicylic acid or arginine) we observed that attenuation was enhanced for most of the studied CECs (up to 400%). This enhancement was attributed to cometabolism because the effects on CEC removal ceased when the supply of root exudates was stopped. A follow-up study was conducted to approach real-wastewater treatment conditions. Sand columns were operated with real wastewater, lower concentrations of the selected CECs (20 mu g L-1) and of root exudates (0.2 mg L-1 of salicylic acid and 1.14 mg L-1 TOC of Cyperus alternifolius' root exudates). Under these conditions, the addition of root exudates on CEC removal had no effects. Thus, we conducted another test with three different concentrations of salicylic acid. When the concentration of salicylic acid increased to 200 mg L-1, CEC removal modestly increased (up to 40%). Divergence between synthetic and real wastewater studies might be explained, mainly, by the presence of organic sources of nutrients in wastewater, which probably masked the effect of root exudates addition at lower concentrations. This study demonstrates that the effectiveness of root exudates application on the attenuation of CECs from wastewater should be explored under real wastewater conditions.

Automated summary

This study investigated the effect of artificial root exudates on the removal of contaminants of emerging concern (CECs) from wastewater in laboratory and real wastewater treatment conditions. The results showed that adding root exudates enhanced CEC removal in synthetic wastewater but had no effect in real wastewater. This suggests that the application of root exudates for CEC removal should be further explored under real wastewater conditions.