Henrique J. O. Pinho

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.

EFFECT OF SPREADING COEFFICIENT ON GAS-LIQUID MASS TRANSFER IN GAS-LIQUID-LIQUID DISPERSIONS IN A STIRRED TANK

Gas-liquid mass transfer in an aerated stirred tank containing two liquid phases (a heptane/dodecane solution dispersed in water) was investigated by measuring the saturation of heptane in the exit gas. Heptane saturation as a function of organic phase composition in a gas-liquid-liquid system supports the view that the organic phase spreading coefficient, S, is an extremely important factor in the mass transfer rate in gas-liquid-liquid systems. When the spreading coefficient of the organic solution changes from negative to positive, the outlet gas becomes saturated, corresponding to a severalfold increase in mass transfer coefficient. This clearly reinforces the hypothesis that the mass transfer pathways between organic and gas phase change depending on the sign of S, leading to a different structure of mass transfer resistances: when S > 0, there is direct contact between gas and organic phase; when S < 0, there is not. At S close to 0, the situation is intermediate, there being some evidence that turbulent kinetic energy may play a role in determining the amount of direct contact between gas and organic phase.

GAS ABSORPTION IN STIRRED GAS-LIQUID-LIQUID SYSTEMS: EFFECT OF TRANSFERRED SOLUTE SOLUBILITY AND OIL PHASE SPREADING CHARACTERISTICS

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 kLa 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.

Comparative evaluation of low cost materials as constructed wetland filling media

Three waste materials from civil construction activities were assessed as low cost alternative filling materials used in Constructed Wetlands (CW). CW are green processes for wastewater treatment, ...

Computation of bubble mean diameters using probability distribution functions

Bubble size is a relevant parameter of gas-liquid dispersions, which is the basis of several unit operations used in the chemical processes industries. For process and equipment design bubbles, size distributions are usually obtained from experimental data and used to compute mean diameters. This work evaluates the use of probability density functions to model experimental bubble size distributions and estimate bubble mean diameters from the raw moments of the adjusted distributions. Eight univariate probability density functions were tested. Among them the Gamma distribution is shown to fit better the experimental bubble size distributions, but the Rosin-Rammler distribution gives the best estimations of bubble mean diameters from the computed raw moments. The results may be valuable for computational fluid dynamics applications.

Valorisation of Phosphorus-Saturated Constructed Wetlands for the Production of Sugarcane

Constructed wetlands (CW) are a clean and environmentally friendly alternative to conventional wastewater treatment methods, namely in the removal of the nutrients responsible for the eutrophication of receiving water bodies, as is the case of phosphorus compounds. The materials used as CW filling can directly contribute to the removal of phosphorus compounds from wastewater, but with the operating time they tend to become saturated and treatment efficiency decreases. In order to evaluate the viability of producing an energy crop in phosphorus-saturated CW, sugarcane growth was monitored in two pilot-scale CW filled with two different expanded clay aggregates used for 10 years in wastewater treatment. This paper presents the results obtained during the first year of plant development in the plant-cane cycle. Morphologic aspects of sugarcane growth, such as height and average diameter of stems, average leaf area and number of new sprouts, have been monitored. The results obtained are comparable with those cited in the literature for traditional cultivation. Dry biomass productivity of 26.6 ton per hectare per year can be achieved. Estimated sucrose productivity can reach 13.5 ton per hectare per year, and related bioethanol production potential can be between 2.4 and 7.6 cubic meters per hectare per year, depending on the CW filter media used. It is concluded that the cultivation of sugarcane in CW allows to extend the life of these systems by reusing fillers, and simultaneously is an alternative to produce bioethanol raw-material without the use of scarce resources such as arable land, fresh water and plant nutrients.

Semi-Empirical Modeling of Gas–Liquid Mass Transfer in Gas–Liquid–Liquid Systems in Stirred Tanks

Two semi-empirical models of gas–liquid mass transfer in gas–liquid–liquid systems are presented, one for the case where the dispersed liquid phase has a positive spreading coefficient S > 0, the other for S < 0. The models are based on mathematical expressions inspired by assumed mass transfer mechanisms, which are different depending on the oil spreading characteristics. Model simulations compare well with experimental data (up to oil phase holdup ϕ ∼ 0.1), managing to simulate the shape of the experimental curves of oxygen mass transfer enhancement factor E vs. ϕ for the air–water–heptane (S > 0) and air–water–dodecane (S < 0) systems, better than other models available in the literature.