Claude Joannis

Relationship between turbidity and total suspended solids concentration within a combined sewer system.

This article confirms the existence of a strong linear relationship between turbidity and total suspended solids (TSS) concentration. However, the slope of this relation varies between dry and wet weather conditions, as well as between sites. The effect of this variability on estimating the instantaneous wet weather TSS concentration is assessed on the basis of the size of the calibration dataset used to establish the turbidity - TSS relationship. Results obtained indicate limited variability both between sites and during dry weather, along with a significant inter-event variability. Moreover, turbidity allows an evaluation of TSS concentrations with an acceptable level of accuracy for a reasonable rainfall event sampling campaign effort.

Turbulent velocity profile in fully-developed open channel flows

The determination of velocity profile in turbulent narrow open channels is a difficult task due to the significant effects of the anisotropic turbulence that involve the Prandtl’s second type of secondary flow occurring in the cross section. With these currents the maximum velocity appears below the free surface that is called dip phenomenon. The well-known logarithmic law describes the velocity distribution in the inner region of the turbulent boundary layer but it is not adapted to define the velocity profile in the outer region of narrow channels. This paper relies on an analysis of the Navier–Stokes equations and yields a new formulation of the vertical velocity profile in the center region of steady, fully developed turbulent flows in open channels. This formulation is able to predict time averaged primary velocity in the outer region of the turbulent boundary layer for both narrow and wide open channels. The proposed law is based on the knowledge of the aspect ratio and involves a parameter CAr depending on the position of the maximum velocity (ξdip). ξdip may be derived, either from measurements or from an empirical equation given in this paper. A wide range of longitudinal velocity profile data for narrow open channels has been used for validating the model. The agreement between the measured and the computed velocities is rather good, despite the simplification used.

Assessment of annual pollutant loads in combined sewers from continuous turbidity measurements: sensitivity to calibration data.

This article presents a methodology for assessing annual wet weather Suspended Solids (SS) and Chemical Oxygen Demand (COD) loads in combined sewers, along with the associated uncertainties from continuous turbidity measurements. The proposed method is applied to data from various urban catchments in the cities of Paris and Nantes. The focus here concerns the impact of the number of rain events sampled for calibration (i.e. through establishing linear SS/turbidity or COD/turbidity relationships) on the uncertainty of annual pollutant load assessments. Two calculation methods are investigated, both of which rely on Monte Carlo simulations: random assignment of event-specific calibration relationships to each individual rain event, and the use of an overall relationship built from the entire available data set. Since results indicate a fairly low inter-event variability for calibration relationship parameters, an accurate assessment of pollutant loads can be derived, even when fewer than 10 events are sampled for calibration purposes. For operational applications, these results suggest that turbidity could provide a more precise evaluation of pollutant loads at lower cost than typical sampling methods.

Reproducibility and uncertainty of wastewater turbidity measurements.

Turbidity monitoring is a valuable tool for operating sewer systems, but it is often considered as a somewhat tricky parameter for assessing water quality, because measured values depend on the model of sensor, and even on the operator. This paper details the main components of the uncertainty in turbidity measurements with a special focus on reproducibility, and provides guidelines for improving the reproducibility of measurements in wastewater relying on proper calibration procedures. Calibration appears to be the main source of uncertainties, and proper procedures must account for uncertainties in standard solutions as well as non linearity of the calibration curve. With such procedures, uncertainty and reproducibility of field measurement can be kept lower than 5% or 25 FAU. On the other hand, reproducibility has no meaning if different measuring principles (attenuation vs. nephelometry) or very different wavelengths are used.

Potential of turbidity monitoring for real time control of pollutant discharge in sewers during rainfall events

Turbidity sensors can be used to continuously monitor the evolution of pollutant mass discharge. For two sites within the Paris combined sewer system, continuous turbidity, conductivity and flow data were recorded at one-minute time intervals over a one-year period. This paper is intended to highlight the variability in turbidity dynamics during wet weather. For each storm event, turbidity response aspects were analysed through different classifications. The correlation between classification and common parameters, such as the antecedent dry weather period, total event volume per impervious hectare and both the mean and maximum hydraulic flow for each event, was also studied. Moreover, the dynamics of flow and turbidity signals were compared at the event scale. No simple relation between turbidity responses, hydraulic flow dynamics and the chosen parameters was derived from this effort. Knowledge of turbidity dynamics could therefore potentially improve wet weather management, especially when using pollution-based real-time control (P-RTC) since turbidity contains information not included in hydraulic flow dynamics and not readily predictable from such dynamics.

Study of the shear stress in narrow channels : application to sewers

The management of deposits in combined sewers is of vital importance from a technical and management point of view. In combined sewers, there are a wide range of shape of cross-sections that have not been characterised from a hydraulic aspect and where it is difficult to measure accurately the near wall velocity and shear stress. A research programme, therefore, is in progress to study how flows behave in such channels and the numerical simulations can be used systematically to investigate some parameters viz. velocity field, shear stress. In the present study, a numerical method was employed to compute distribution of shear stress in whole of the cross sections. The effect of turbulence modelling in simple and combined cross section was investigated. A comparison between one-dimensional and three-dimensional flow equations was carried out to estimate distribution of the shear stress. The three-dimensional model results were in close agreement with experimental data, whereas one-dimensional equations could not fit with experimental values.

Velocity Distribution in Open Channel Flows: Analytical Approach for the Outer Region

AbstractThis paper presents an integration procedure for the Reynolds-averaged Navier-Stokes equations for the determination of the distribution of the streamwise velocity using the vertical component. This procedure is dedicated to the outer region and central part of channels. The proposed model is applicable to both rough and smooth flow regimes, provided the velocity at the inner-outer boundary has been properly defined. To generate a simplified expansion, a number of hypotheses are proposed, focusing in particular on the analytical modeling of the vertical component by adopting a negligible viscosity. The proposed hypotheses are validated by the experimental data existing in the literature. The proposed simplified expansion is studied through a sensitivity analysis and proved consistent in regards to model experimental data. The proposed model seems capable of demonstrating different kinds of flows, including dip phenomenon flow patterns.

An assessment of the respective contributions of flow-rate and concentration variations to mass discharge variations at the outlets of two combined catchments during rain events

This paper presents a method for assessing the respective contributions of the variations of flow-rate and concentration of any pollution parameter to the variations of the mass discharge of the same parameter. This method uses a specially designed decomposition of variance, which gives a priority to the information provided by flow-rates, either directly, or through the correlation between flow-rates and concentrations. To demonstrate the potential interest of this method, data monitored on two catchments in the city of Paris (France) are processed according this method, with turbidity being used as a surrogate for suspended solids concentration. Results show that volumes provide a fairly good evaluation of masses at the scale of whole events. Inside any particular event, concentrations are major contributors to the variations of mass discharge, despite the correlations between flow rate and concentration which may be observed for many rain events.

Précision et reproductibilité du mesurage de la turbidité des eaux résiduaires urbaines sur échantillons

Measuring the turbidity of urban sewage is interesting because in raw waste water, pollutants are mainly particulates and/or bound to particulates. Moreover it is a real time and continuous measurement. This paper deals with knowledge elements and methods for the determination of the components of the uncertainty on turbidity measurement on water samples (principle of measurement, calibration, drift,...). These methods contribute to the use of this measurement technique by operators of urban drainage systems. They were applied for comparing experimental results obtained with various devices and various calibration and sampling strategies. The level of overall short term accuracy reached for turbidity measurement in urban waste water is satisfactory, provided that the non-linearity of some devices to corrected if needed.

Efficiency of a turbidity-based, real-time control strategy applied to a retention tank: a simulation study.

This paper compares several real-time control (RTC) strategies for a generic configuration consisting of a storage tank with two overflow facilities. Two of the strategies only make use of flow rate data, while the third also introduces turbidity data in order to exercise dynamic control between two overflow locations. The efficiency of each strategy is compared over a wide range of system setups, described by two parameters. This assessment is performed by simulating the application of control strategies to actual measurements time series recorded on two sites. Adding turbidity measurements into an RTC strategy leads to a significant reduction in the annual overflow pollutant load. The pollutant spills spared by such a control strategy strongly depend on the site and on the flow rate based strategy considered as a reference. With the datasets used in this study, values ranging from 5 to 50% were obtained.