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.

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