Phosphorus removal rates in bucket size planted wetlands with a vertical hydraulic flow

Rate studies of orthophosphate removal from wastewater in Schoenoplectus validus Vahl. (A Love and D Love) planted bucket-sized vertical upflow wetlands occurs simultaneously through three removal processes. In order of decreasing rate, these are reversible sorption to the gravel substratum, reversible conversion to complex phosphorus compounds, and irreversible uptake by plant roots. Mixing the rhizosphere liquid, using forced convection, accelerated orthophosphate removal; concentrations falling from ≈10 mg litre−1 to <0.2 mg litre−1 in <48 h. The rate determining step in systems without rhizosphere liquid mixing appears to be diffusive transport of phosphate. Over two years, initial removal rates, and minimum reactive phosphorus concentrations changed little, but aged or high BOD systems sometimes released reactive phosphorus or tannins holding large amounts of phosphorus. The relevance of these mechanisms to total wetland assimilative capacity, and effluent quality, are evaluated.