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

Abstract

Orthophosphate removal from wastewater by planted vertical-flow wetlands (VFWs) occurs through three parallel paths, with reaction rates of: sorption to substratum>biofilm assimilation⪢macrophyte 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>substratum⪢biofilm, 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–PO 4 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.