Validation of krypton as a new tracer gas for the standardization tests of collective and individual protection systems.

Sulfur hexafluoride (SF6) is the reference tracer gas in many international standards for characterizing respiratory protective devices (RPD), fume cupboards, building ventilations, and other installations. However, due to its significant impact on global warming, its use is becoming increasingly restrictive. Krypton 84 (Kr) was chosen to be a possible replacement based on theoretical and practical criteria for the properties that a substitute gas should possess. While compliance with these criteria is generally sufficient to guarantee the reliability of the choice, it is essential in the case of widespread use such as a standard to validate experimentally that this tracer has the same behavior as SF6. In this regard, numerous tests have been carried out to characterize the face leakage of RPD and the rupture of containment of fume cupboards performance tests under different operating conditions. The results obtained are identical with both tracers and lead us to propose the use of Kr as a new reference gas in standards for which SF6 was used.

Rare earth elements (REE) in the urban wastewater of Cotonou (Benin, West Africa).

The rare earth element (REE) contamination of urban wastewater, which was collected from open sewers and the inlet of a wastewater treatment plant in Cotonou (Benin), was assessed. The drinking water distributed to the inhabitants of Cotonou and water samples from private wells were also analyzed. The sampling occurred between October and December 2016 and the samples were analyzed by ICP-MS. Although the only magnetic resonance imaging facility in Cotonou opened in November 2016, pollution by anthropogenic gadolinium (Gd), which is included in phase contrast agents, was observed: there was 30-620 times more Gd in wastewater samples than in drinking and well water samples. Europium was another REE presenting positive anomalies. It is hypothetized than the europium came from the leachates of solid waste piles in the street. In the absence of any wastewater treatment, the REEs found in the wastewater are spread to the aquatic environment. It would be interesting to monitor the wastewater REEs over the long term. So far, the aquifers used for water provision have not been polluted by the anthropogenic REEs.

Population mobility and urban wastewater dynamics.

Dynamic influent models, which have been proposed to test control strategies using virtual wastewater treatment plants, should be as realistic as possible. The number of inhabitants in the catchment at any given time and their ways of life are among the parameters affecting the quality of these models. Census data related to work and school commutes were used to evaluate the number of people present in a given urban area. Based on the example of a large urban catchment (Grand Nancy, France), the results show that a population increase of 30% could occur during working hours resulting from the imbalance between workers leaving and coming into the catchment. Combined with information related to the local way of life, variation in the population helps to explain changes in wastewater flow rate and pollution (carbon, nitrogen, phosphorus and heavy metals), which present several maxima reflecting daily activities, such as bladder voiding, meals, the use of washrooms, etc. However, no well-defined variation patterns for pH and conductivity, which are linked to the concentrations of anions and cations in the wastewater, were observed. Slight reductions (up to 10% on Sundays) in the flow and pollution load were observed on weekends as the commuter flow decreased. Census data proved to be efficient in helping to understand the daily pattern of urban wastewater characteristics.

Removal of Cr(III) from chrome tanning wastewater by adsorption using two natural carbonaceous materials: Eggshell and powdered marble.

In the present paper, eggshell and powdered marble, two carbonaceous materials, were used to remove Cr(III) ions from a real chrome tanning wastewater. The effects of initial effluent pH, adsorbent dose, contact time and temperature were studied. The maximum uptake of chromium ions was obtained at pH 5.0 with the dose 20 g L(-1) and 12 g L(-1) for eggshell and powdered marble respectively. Adsorption equilibrium was reached after 14 h contact time for eggshell and only after 30 min for powdered marble. Under these conditions, almost 99% Cr(III) was removed from chrome tanning wastewater having an initial concentration of chromium of 3.21 g L(-1). Kinetic data were satisfactorily described by a pseudo-second order chemical sorption model. The equilibrium rate constant was notably greater for powdered marble than for eggshell with 1.142·10(-3) (g mg(-1) min(-1)) and 0.041·10(-3) (g mg(-1) min(-1)) respectively. The adsorption isotherm were well described by a Langmuir model and showed that the interaction of chromium with the two adsorbents surface is a localized monolayer adsorption with a smaller energy constant for the powdered marble than for eggshell (0.020 (L mg(-1)) and 0.083 (L mg(-1)) respectively). The powdered marble was able to adsorb faster a large amount of Cr (III) in comparison to eggshell. The use of a standardized lettuce seed bioassay allowed evaluating a better effectiveness of the Cr adsorption on the powdered marble, removing up to 40% of the treated effluent toxicity than by eggshell 25%. The powdered marble could be considered as an effective, low cost carbonaceous material to be used for chromium removal from tanning wastewater.

Aeration efficiency over stepped cascades: better predictions from flow regimes.

Stepped cascades are recognized as high potential air-water gas exchangers. In natural rivers, these structures enhance oxygen transfer to water by creating turbulence at interface with increasing air entrainment in water and air-water surface exchange. Stepped cascades could be really useful to improve the natural self-purification process by providing oxygen to aerobic micro-organisms. The aeration performance of these structures depends on several operating and geometrical parameters. In the literature, several empirical correlations for aeration efficiency prediction on stepped cascades exist. Most of these correlations are only applicable for operating and geometrical parameters in the range of which they have been developed. In this paper, 398 experimental sets of data (from our experiments and collected from literature) were used to develop a correlation for aeration prediction over stepped cascades derived from dimensional analysis and parameterized for each individual flow regime in order to consider change in flow regime effect on oxygen transfer. This new correlation allowed calculating the whole set of data obtained for cascades with steps heights between 0.05 m and 0.254 m, cascade total height between 0.25 m and 2.5 m, for discharges per unit of width ranging from 0.28 10(-3) m(2)/s to 600 10(-3) m(2)/s and for cascade steps number between 3 and 25. In these ranges of parameters, standard deviation for aeration efficiency estimation was found to be less than 17%. Finally, advices were proposed to help and improve the structure design in order to improve aeration.

Electrocoagulation for the treatment of textile wastewaters with Al or Fe electrodes: compared variations of COD levels, turbidity and absorbance.

Electrocoagulation technique has been used for the treatment of two wastewaters issued by textile industry. Treatment was carried out in a discontinuous system provided with aluminium or iron electrodes, and with recirculation of the liquid. The efficiency of the technique was followed depending on the electrode material in terms of water treatment, current efficiency of the dissolution, cell voltage, energy consumption to reach the same COD or turbidity abatement: regardless of the quality of the phase separation in the flocculation section downstream of the electrocoagulation cell, the two metals were found to be of comparable efficiency. Besides COD and absorbance were shown to follow similar, regular variations along the treatment; experimental data could be interpreted by a simple model involving the overall equilibrium between the metal dissolved--in the form of hydroxides--and the polluting substance. Abatement of the waste turbidity was observed to obey another law, with a sharp reduction of turbidity after a preliminary phase, where accumulation of metal hydroxide has no effect on this variable.

Characterization of two-phase flow regimes in horizontal tubes using 81mKr tracer experiments.

The diagnosis of heat exchangers on duty with respect to flow mal-distributions needs the development of non-intrusive inlet-outlet experimental techniques in order to perform an online fault diagnosis. Tracer experiments are an example of such techniques. They can be applied to mono-phase heat exchangers but also to multi-phase ones. In this case, the tracer experiments are more difficult to perform. In order to check for the capabilities of tracer experiments to be used for the flow mal-distribution diagnosis in the case of multi-phase heat exchangers, we present here a preliminary study on the simplest possible system: two-phase flows in a horizontal tube. (81m)Kr is used as gas tracer and properly collimated NaI (TI) crystal scintillators as detectors. The specific shape of the tracer response allows two-phase flow regimes to be characterized. Signal analysis allows the estimation of the gas phase real average velocity and consequently of the liquid phase real average velocity as well as of the volumetric void fraction. These results are compared successfully to those obtained with liquid phase tracer experiments previously presented by Oriol et al. 2007. Characterization of the two-phase flow regimes and liquid dispersion in horizontal and vertical tubes using coloured tracer and no intrusive optical detector. Chem. Eng. Sci. 63(1), 24-34, as well as to those given by correlations from literature.

A general correlation to predict axial dispersion coefficients in aerated channel reactors.

Tracer studies have been widely applied to characterize the flow behavior in activated sludge reactors. The channel reactor with bottom aerators is one of the widespread designs in large wastewater treatment plants. Its flow behavior is well modeled either as a plug flow reactor with axial dispersion or as the perfect mixing cells in series. Several correlations have been developed to estimate the dispersion coefficients as a function of operating and geometrical parameters. These correlations fit properly the data derived from a given reactor in the range of operating and geometrical parameters for which they have been determined. Unfortunately they cannot be applied straightforwardly with a sufficient level of confidence to scale-up industrial units or scale-down laboratory pilots. Recently, two papers have proposed more general correlations [Makinia, J., Wells, S.A., 2005. Evaluation of empirical formulae for estimation of the longitudinal dispersion in activated sludge reactors. Water Res. 39, 1533-1542; Potier, O., Leclerc, J.-P., Pons, M.-N., 2005. Influence of geometrical and operating parameters on the axial dispersion in an aerated channel reactor. Water Res. 39, 4454-4462] but they are still not able to represent the whole set of data from the literature. This paper proposes a general correlation, which can represent all the available and usable sets of data from the literature and more than 170 experimental results obtained in our laboratory with a precision of 18%.

Interpretation of radiotracer experiments in an industrial battery of desanders with simultaneous stochastic and non-stochastic flows.

Radiotracer experiments have been conducted in an industrial battery of twelve desanders located at the water treatment plant of Lima. The flow behavior is complex because it includes the superposition of a stochastic flow, which is intrinsic to the process itself, and of a non-stochastic flow induced by local strong currents at the surface. These currents are non-stochastic because they are different from one desander to another one even if all the desanders are identical. A general model has been proposed to simulate the whole behavior of the process. The model is composed of two parts: the first one, describes the stochastic flow behavior, and is the same for each desander; the second one is adapted to represent the turbulent shortcuts at the surface of each desander. The parameters of stochastic part have been determined to be consistent with the physical description of the process. The value of the Peclet number of the shortcut is a monotonic function of the flow rate. Despite remaining uncertainties due both to the complexity of the flow and to specific problems of tracer measurements in large industrial water treatment plant, this paper proposes an extension of the applications of radiotracer experiments and interpretation.

Influence of inlet positions on the flow behavior inside a photoreactor using radiotracers and colored tracer investigations.

The influence of the inlet positions on the flow behavior inside a photoreactor has been studied using colored tracer and radiotracer investigations. The photoreactor for water disinfection is a tubular reactor inside which UV emitting lamps are placed. The key to its good performance is a combination of optimum irradiation dose and fluid residence time distributions (RTD). The inlet system is one of the better possibilities of controling the fluid RTD. Three configurations have been tested. In the first step, colored tracer experiments help us to raise the mean trends of the flow behavior for the different configurations. When the inlet is located at the central bottom of the reactor, the flow is closed to a perfect mixing whereas lateral inlets lead to a plug flow with axial dispersion behavior. RTD measurements using radiotracer was carried out in a wide range of flowrates. The axial dispersion coefficient increases linearly with the Reynolds number. It is lower when the lateral inlet is divided in three inlets uniformly distributed around the bottom of the reactor. This paper shows the usefulness of radioactive isotopes as tracers in the field of wastewater treatment.

Influence of geometrical and operational parameters on the axial dispersion in an aerated channel reactor.

Residence time distribution experiments have been performed on an activated sludge 3000 m3 channel reactor aerated by gas diffusion (for different liquid flowrates under constant aeration rate and constant water depth) and on a bench-scale channel reactor aerated from the bottom (for different liquid and gas flowrates and water depths) in order to characterize their hydrodynamics. Both units can be modeled as plug flow reactors with axial dispersion. A general correlation has been obtained to predict the axial dispersion coefficient as a function of the gas and liquid velocities and the geometrical parameters of the full-scale and bench-scale reactors. Finally, to facilitate the simulation of biological reactions in transient state, an equivalent model based on tanks-in-series with variable back-mixing flowrate is proposed.