Long-term monitoring of constructed wetlands in distilleries in Scotland - Evaluating treatment performance and seasonal microbial dynamics

Constructed wetlands (CWs) have been used to treat industrial wastewater in Scotland, including within the whisky industry to treat the second distillation byproduct known as “spent lees”. However, despite the notable advantages of CWs (including simplicity, cost-effectiveness, and energy efficiency) their adoption by distilleries still remains relatively limited. Concerns regarding unstable performance, clogging, and sensitivity to cold temperatures have tended to limit their widespread utilisation. This paper considers these concerns through a comprehensive analysis of water treatment performance and microbial community variation across different seasons in three horizontal subsurface flow constructed wetlands (HSF CWs) situated within several whisky distilleries in Scotland. One of these wetlands operates year-round and the other two are generally only active in autumn and spring. Across these CWs, consistent success was observed in the removal of dissolved copper (dissCu) throughout the seasons. However, significant differences in performance were observed both within the same CW (across different seasons) and between different CWs (during the same season) for various water quality parameters (including suspended solids, turbidity, dissolved organic carbon, chemical oxygen demand, nutrients, and pH). Microbial diversity within each CW showed minimal variation between seasons at the class and order levels. However, substantial differences in bacterial community were detected between different CWs during the same season. Gammaproteobacteria, Alphaproteobacteria, and Bacteroidia consistently ranked among the top 5 most abundant classes in each CW, though their specific abundance rankings varied. At the bacterial class level, redundancy analysis revealed strong correlations between treatment performance metrics and specific taxa, including Planctomycetacia with TON removal, Acidobacteriia with COD and DOC removal and pH buffering, Bacilli with SRP removal, and Alphaproteobacteria with NH₄⁺ removal. At the fungal class level, Rozellomycota_cls_Incertae_sedis demonstrated significant associations with multi-pollutant removal and pH buffering, highlighting its ecological role in treatment processes. This research provides insights regarding the seasonal dynamics of CWs, offering recommendations that could be used to optimise their application within the whisky industry and beyond.