Abstract
Constructed wetlands (CWs) offer a sustainable approach for industrial wastewater treatment; however, their performance significantly deteriorates under cold-climate operating conditions due to suppressed microbial kinetics and plant dormancy. This study evaluated the performance of horizontal subsurface flow constructed wetlands (HSSF-CWs) amended with 10% (v/v) softwood biochar for the tertiary treatment of textile dye effluents at low temperatures (5 ± 1°C) compared to ambient conditions (20 ± 1°C). Synthetic textile wastewater containing Reactive Black 5 (RB5), high chemical oxygen demand (COD), and total nitrogen (TN) was fed to biochar-amended CWs (BC-CW) and unamended control CWs (C-CW) across varying hydraulic retention times (HRTs of 2, 4, and 6 days). At 5°C, BC-CW achieved impressive removal efficiencies of 91.4% for color (RB5), 83.2% for COD, and 68.5% for TN at a 4-day HRT, whereas the control CW exhibited marked performance drops to 58.7%, 52.1%, and 41.3%, respectively. High-performance liquid chromatography and enzymatic assays revealed that biochar facilitated enhanced extracellular electron transfer and maintained elevated azoreductase and laccase activities under thermal stress. Microbial community profiling demonstrated a distinct enrichment of psychrotolerant, dye-degrading taxa, including Pseudomonas and Acinetobacter species, within the biochar biofilm. These findings demonstrate that biochar amendment effectively buffers cold-climate performance penalties, offering a resilient, eco-friendly solution for year-round textile effluent polishing in temperate and subarctic regions.
