The relative roles of sorption and biodegradation in the removal of contaminants of emerging concern (CECs) in GAC-sand biofilters

The removal and fate of contaminants of emerging concern (CECs) in water treatment systems is of interest given the widespread occurrence of CECs in water supplies and increase in direct potable reuse of wastewater. In this study, CEC removal was investigated in pilot-scale biologically-active granular activated carbon (GAC)-sand and anthracite-sand filters under different hydraulic loading rates and influent CEC concentrations over a 15-month period. Eight of the most commonly detected compounds in a survey of CEC occurrence in drinking water were selected for this study: atenolol, atrazine, carbamazepine, fluoxetine, gemfibrozil, metolachlor, sulfamethoxazole and tris(2-chloroethyl) phosphate (TCEP). GAC-sand biofilters provided superior CEC removal for all compounds (mean removal efficiencies: 49.1–94.4%) compared to anthracite-sand biofilters (mean removal efficiencies: 0–66.1%) due to a combination of adsorption and biodegradation. Adsorption was determined to be the dominant removal mechanism for most selected CECs, except fluoxetine, which had the greatest biodegradation rate constant (0.93 ± 0.15 min⁻¹ at 20–28 °C). The mean removal efficiency decreased by 16.5% when the loading rate increased from 2 to 4 gpm/ft² (4.88–9.76 m/h). A significant reduction in CEC removal was observed after 100,000 bed volumes when the influent CEC concentration was low (100–200 ng/L), whereas no significant reduction was observed during spike dosing (1000–3000 ng/L). A regression analysis suggested that biodegradation rate, hydraulic loading rate, influent CEC concentration, throughput, influent dissolved organic carbon (DOC) concentration, and CEC charge are important parameters for predicting CEC removal performance in GAC-sand biofilters.