Performance of two macrophyte species in experimental wetlands receiving variable loads of anaerobically treated municipal wastewater

Two highly productive emergent macrophytes, Zizaniopsis bonariensis and Typha subulata were established in experimental subsurface flow, sand-based wetlands receiving anaerobically treated municipal wastewater. The hydraulic loading rate was tested in two levels, sequentially, 6.8 cm.d^-1 and 13.6 cm. d^-1, for 70 days each. In the 13.6 cm. d^-1-loading treatment, among all monitored variables only COD, PO₄-Total-P, and Turbidity were removed more efficiently by the planted beds in comparison to unplanted sand beds (P<0.001). When the 6.7 cm. d^-1-loading rate was applied no significant improvement in removal was found comparing macrophyte beds to unplanted sand beds, except for PO₄-P. T. subulata beds were significantly more efficient than Z.bonariensis beds for most of the variables. The highest significant differences (P<0.001) were related to the main effect of the factor hydraulic loading rate, with decreased removal for increased load when considering the variables Total Coliforms (99.4% to 87.7%), Fecal Coliforms (100% to 89.7%), NH₃-N (95.8% to 55.2%), NO₃^--N (-54% to -396%), Total-N (90.4% to 59.6%), and TSS (86.1% to 46.1%). The performance similarity of planted and unplanted wetland beds in the lower loading condition, except for PO₄-P, indicates that plants may not be needed under low loading. However aquatic macrophytes improved wetland efficiency under high loading.