Looking beyond Sorption in GAC Filters: How Extended Contact Times and Functionally Distinct Microbial Biomass Enable Enhanced Micropollutant Biodegradation

Micropollutant abatement in granular activated carbon (GAC) filters is governed by both sorption and biodegradation processes. Yet a comprehensive understanding of which micropollutants are biodegraded, how much biodegradation contributes to GAC's performance, and how GAC's properties affect biodegradation, remains limited. This study addresses these knowledge gaps by combining reactive transport modeling with pilot-scale experiments, investigating the fate of 45 micropollutants in parallel GAC and sand filters operated at contact times of up to 100 min. Tracer tests and modeling showed that GAC's high intragrain porosity (40%) prolongs solute-biomass contact times by a factor of 1.7 compared to sand. Higher respiration activity in GAC was attributed to prolonged contact times for biodegradation of both micropollutants and other dissolved organics. Significantly enhanced abatement at contact times exceeding 25 min suggested biodegradation for 25 of 45 micropollutants. The formation of biotransformation products further confirmed biodegradation of diclofenac and the highly sorptive compounds venlafaxine and hydrochlorothiazide exclusively in GAC. Overall, these findings demonstrate that (i) GAC enables biodegradation of more micropollutants than sand (24 vs. 16), regardless of sorption affinity; (ii) intragrain porosity prolongs solute-biomass contact times, being relevant for degradable compounds featuring little sorption; and (iii) GAC hosts functionally distinct microbial biomass capable of unique biotransformation compared to sand.