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.