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.

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.

To keep overflows of raw effluent to a minimum during wet weather conditions, we investigated the ability of a secondary clarifier of an activated sludge treatment plant to accept hydraulic overloads without being washed out. The experiments, which were conducted on a full scale 8000 p.e. treatment plant, showed the feasibility of the project, and suggested some features, which were included in a one-dimensional model designed to study the behavior of the sludge blanket in the clarifier. This model takes into account the effect of convection currents, suspected to play an important part in the rising of sludge blankets. The sensitivity of the model to sludge settleability prevented its use with long times-series, unless a continuous recalibration was performed. Nevertheless, this model appears very interesting for a better understanding of the dynamics of the clarifier, as described by measured data. It could be used in relation with sensors to improve the operation of the treatment plant.

No abstract