Effect of alternative substrates on treatment performance and microbial community structure in constructed wetlands

This study aimed to evaluate the treatment performance of constructed wetland (CW) systems using alternative sustainable substrates derived from cork as filter media. Four lab-scale CW units were operated using raw cork, expanded cork, gravel enriched with biochar-based cork, and gravel as substrate materials. The effects of substrate type on organic matter removal, nitrogen transformation, micropollutant attenuation, plant development, and microbial community composition were investigated. Results showed significant differences among substrates for chemical oxygen demand (COD) and nitrate nitrogen (NO₃⁻–N) removal (ANOVA test, p < 0.05). Expanded cork showed more consistent COD removal performance, with lower variability, while cork-based substrates showed enhanced nitrate removal compared with conventional gravel-based systems. These results suggest that cork substrates may provide favorable physicochemical conditions for microbial processes involved in nitrogen transformation. For bisphenol A (BPA), raw and expanded cork achieved high removal rates >90% under the applied experimental loading conditions. Cork based-substrates also provided favorable conditions for plant growth, especially in horizontal flow constructed wetlands (HFCWs). Shotgun metagenomic sequencing revealed that microbial communities were dominated by bacterial taxa across all configurations, with Pseudomonadota being the most abundant (60–80%), followed by Actinomycetota, Bacteroidota, and Bacillota.Fungal taxa were detected only in the HFCW with raw cork (HFCW2). Overall, the findings highlight the potential of cork-based substrates as sustainable filter media for CW applications.