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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 wellfitted 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 6 4 L/(m 2 ?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 6 0.2 mg/L, during the trial period. This study presents the first long-term pilot-scale data for constructed wetlands using LWAs. Water Environ. Res., 82, 128 (2010).

The effect on gas-liquid mass transfer of dispersing an oil phase in water in an aerated stirred tank is not predictable, because the mechanisms involved are not well understood. To try to elucidate these mechanisms, a set of experiments was carried out that included: (i) measurement of the effect of oil addition on gas dispersion properties, (ii) quantification of mass transfer of two solutes (heptane and oxygen) with very different solubilities in the liquid phases, and (iii) variation of the spreading characteristics of the oil phase, which consisted of mixtures of dodecane and heptane in varying concentrations. It was found that in the case of heptane mass transfer, when the oil spreading coefficient S changes from negative to positive, the outlet gas becomes practically saturated, corresponding to a several-fold increase in mass transfer coefficient. In the case of oxygen mass transfer, the effect of S is not as dramatic, but it is also quite significant. For a spreading oil phase (S > 0), the mass transfer coefficient decreases upon trace oil addition, going through a minimum as oil holdup increases, and then increasing steadily. In the case of a non-spreading oil phase, mass transfer coefficient initially increases with oil holdup increase, going through a maximum and then decreasing. Comparison between mass transfer coefficients for the two solutes indicates that k L a for heptane is larger than that of oxygen by a margin not explainable through the difference in diffusivities, which is evidence for a difference in transfer paths=mechanisms. A physical interpretation compatible with these results is offered.

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