A model of factors controlling orthophosphate removal in planted vertical flow wetlands

La Trobe University Faculty of Science, Technology and Engineering Murray Darling Freshwater Research Centre

MDFRC item.

Orthophosphate removal from wastewater by planted vertical-flow wetlands (VFWs) occurs through three parallel paths, with reaction rates of: sorption to substratum>biofilm assimilationmacrophyte uptake. Short term (one or two loadings) plant removal of phosphorus (P) is small but irreversible, whereas P removed by substratum sorption, or non-reactive P (NRP) formation, can be returned as reactive phosphorus (RP). The quantity of P removed by the three paths is substratum>macrophyte biofilm, in the short term, but macrophyte>substratumbiofilm, over months. Rhizosphere hydrology restricts P removal, the rate is limited by mass transfer without liquid mixing, but trebled by mixing. Evapotranspirational mixing alone is small and erratic. In small, above-ground, systems environmental temperature changes cause daily mixing, but prevailing soil temperature gradients limit below-ground mixing. A planted wetland, conceptual model, explains: (1) retention times: determined by initial RP removal rates, and operationally dependent on RP concentration and mixing. Aqueous phase cycling reduces retention times several fold; (2) minimum outflow concentrations: controlled by the gravel–PO4 sorption equilibrium; (3) sustainable annual P removal: the quantity harvested in the macrophytes. Substratum Fe(III) oxide–hydroxide sorption provides additional assimilation for some years. Specific, model derived, VFW design and operation recommendations are made.