The effect of plant density on the organic matter removal from municipal wastewater by vertical flow constructed wetlands (Phragmites australis case study)

Vertical flow constructed wetlands with varying plant densities of Phragmites australis (0, 3, 6, and 9 plants /m²) were evaluated for domestic wastewater treatment over a 12-month period. The experimental system consisted of four independent circular units (0.125 m²) filled with three substrate layers: coarse gravel (20–25 mm), medium gravel (10–15 mm), and fine sand (3 mm). Results demonstrated that increased plant density significantly enhanced treatment performance. The high-density configuration (9 plants/m²) achieved superior removal efficiencies for biochemical oxygen demand (93.86 ± 2.49%), chemical oxygen demand (81.26 ± 7.19%), and TSS (85.40 ± 5.66%) compared to medium-density, low-density, and unplanted controls. Statistical analysis confirmed significant differences between planted and unplanted systems (p < 0.05), with strong positive correlations between plant density and contaminant removal (r = 0.99 for biochemical oxygen demand, r = 0.94 for chemical oxygen demand). high-density also improved dissolved oxygen levels (from 0.46 ± 0.14 mg/L in influent to 4.75 ± 0.73 mg/L in high-density effluent) and stabilized effluent quality, with high-density treatments showing 19.12% and 57.71% lower standard deviations than low-density treatments for biochemical oxygen demand and chemical oxygen demand, respectively. Treatment mechanisms included enhanced root surface area for biofilm development, increased oxygen transfer to the rhizosphere, and greater production of exudates supporting microbial activity. All configurations achieved effluent quality compliant with Algerian regulatory standards, demonstrating Vertical flow constructed wetlands as effective, low-cost wastewater treatment solutions.