Nature-based solution for treating pesticide-contaminated groundwater: performance of a combined two-stage biofilm system.

Biological treatment represents an alternative to conventional adsorption technologies for the removal of pesticides from contaminated groundwater and landfill leachates. A pilot-scale treatment combining moving bed biofilm reactors (MBBRs) and downstream biofilters was operated for two years to evaluate the removal of the chlorophenoxy herbicides mecoprop (MCPP) and dichlorprop (DCPP) under realistic field conditions. After establishing famine operation and aeration, removals were 78 ± 3% (MCPP) and 73 ± 4% (DCPP) in one treatment line, while the other one achieved lower but consistent removal (47-51%). Mass balance analyses demonstrated that decreases in parent compounds were not due to production of known transformation products, indicating effective sequential biodegradation (mineralisation). The upstream MBBR primarily improved iron and particle management to protect the downstream biofilters from clogging, whereas biofilters accounted for most of the removal of both parents and metabolites. Removal was largely independent of hydraulic retention time when varying this between 10 and 25 h but strongly linked to dissolved oxygen concentrations, highlighting aerobic biofilm activity as the key controlling factor. Microbial community analysis revealed biofilms dominated by Pseudomonadota, with higher abundance of putative phenoxyacid degraders and fewer anaerobic taxa in the better-performing line, supporting an oxygen-driven biodegradation mechanism. An in silico full-scale life-cycle cost analysis indicated substantially lower costs and environmental impacts compared to granular activated carbon treatment.