High-resolution monitoring reveals treatment wetland resilience across temperature and loading conditions: Factorial analysis of operational parameters in domestic wastewater treatment

Treatment wetlands (TW) offer promising decentralized solutions for domestic wastewater treatment, yet their performance under varying operational conditions remains incompletely understood. This study implemented a 2x2x2 factorial experiment examining wastewater strength (500 and 2000 mgCODt/L), temperature (6 and 20 °C), and vegetation (planted and unplanted) in lab-scale horizontal subsurface wetlands. High-resolution monitoring of oxidation-reduction potential (ORP), dissolved oxygen (DO), pH, and temperature revealed previously unobserved treatment dynamics. Results demonstrated remarkable system resilience, achieving 91–95 % COD removal across all loading rates. Temperature emerged as the dominant factor (F = 554, p < 0.001), with warm systems (20 °C) showing 94 % COD removal versus 63 % in cold conditions (6 °C). Planted systems achieved peak nutrient removal (96 % ammonia, 87 % phosphate) under optimal conditions. Systems exhibiting ORP increases from -300 mV to +30 mV achieved highest COD removal efficiencies, with ORP providing more reliable process control than DO measurements. The pH increased from 6.6 to 7.6, correlating with reduced phosphate retention particularly above pH 7.3. These findings establish the viability of treatment wetlands for high-strength wastewater treatment in cold climates while highlighting ORP and pH as critical real-time performance indicators. The study demonstrates the value of high-resolution monitoring in optimizing treatment wetland operation and design across diverse environmental conditions. • Treatment wetlands consistently remove COD (91–95 %), even at high organic loads. • Temperature drives removal efficiency (94 % at 20 °C vs. 63 % at 6 °C). • Optimal retention: 18 h (low-strength), 4 days (high-strength). • ORP (-300 mV to +30 mV) outperforms DO for monitoring wetland performance. • Planted systems boost nutrient removal, but a pH increase decreases phosphate uptake.