Removal of Organic Contaminants in On-Site Wastewater Treatment Systems: The Role of Sorption and Transformation

Nitrogen-removing biofilters (NRBs) are alternative on-site wastewater treatment systems that can remove some trace organic contaminants (TOrCs) from domestic wastewater, though the dominant removal mechanisms are uncertain. We conducted column experiments representative of the nitrifying sand layer of an NRB to evaluate the contribution of sorption to removal of 16 wastewater-relevant TOrCs. The contribution of sorption was >25% for eight of the 16 TOrCs in at least one experimental treatment and >50% for five TOrCs. Transformation appeared to account for 51-93% of TOrC removal in columns. Transformation product screening resulted in the tentative identification of three TOrC transformation products in column effluent. To compare the bench-scale experiment to realistic field conditions, we analyzed solid samples from a recently excavated full-scale NRB. Median concentrations of sorbed TOrCs ranged from 0.02 to 5.09 ng/g in column studies and 0.05-7.14 ng/g in the full-scale NRB. Overall, the majority of TOrC removal in our laboratory study was by transformation, though some hydrophobic TOrCs exhibited significant removal by sorption. The concentration of sorbed hydrophobic TOrCs in aged NRBs and release of transformation products of frequently detected TOrCs should be taken into consideration during future system design and optimization.