Phosphorus removal by expanded clay--six years of pilot-scale constructed wetlands experience.

Constructed wetlands, which facilitate phosphorus removal via precipitation, adsorption, and biological assimilation, offer a promising appropriate technology for advanced treatment in wastewater treatment plants. Because adsorption and precipitation are pointed out as the major phosphorus-removal mechanisms, the selection of a medium with high phosphorus-sorption capacity is important to obtain a sustained phosphorus removal. The objective of this study was to evaluate two kinds of lightweight expanded clay aggregates (LWAs)--Filtralite NR and Filtralite MR (Maxit Group, Avelar, Portugal)--as substrates in constructed wetlands to improve phosphorus-removal performance. Laboratory experiments were performed to test the potential of the LWAs to remove phosphorus from a phosphate solution. The experimental data were well-fitted by both the Langmuir and Freundlich isotherm models. Pilot-scale investigations were carried out to evaluate the phosphorus removal under field conditions. Four subsurface constructed wetlands were operated since June 2002; two of them were planted with Phragmites australis, and the other two were unplanted. The beds were filled with the two kinds of LWAs. Total phosphorous and pH were monitored since 2003, at a mean hydraulic load of 50 +/- 4 L/(m2 x d), during 6 years. The inflow phosphorus concentration was in the range 4 to 13 mg/L. Under the conditions used, beds with Filtralite MR had better efficiency, and the bed with Filtralite MR planted with Phragmites australis provided a phosphorus effluent mean concentration of 0.7 +/- 0.2 mg/L, during the trial period. This study presents the first long-term pilot-scale data for constructed wetlands using LWAs.