Reproducibility and uncertainty of wastewater turbidity measurements

Abstract

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.

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

“…The dry weather ratios are less variable and display lower overall values than their wet weather counterparts; this result can be explained by the type of particles involved Water Science a Technology | 64. during wet weather (i.e. less homogeneous, more mineral in nature, finer-grained, see Joannis et al (2008)). …”

“…Recent studies have demonstrated how it can be used to control and evaluate effiuent quality (Langeveld et al 2005;Lawler et al 2006), but it must be emphasized that the use of suitable methods is mandatory to obtain reliable and accurate continuous turbidity measurements within the sewer system over short time steps (Joannis et al 2008).…”

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

“…Turbidity data were recorded every minute during one year, according to a protocol that has led to reliable and accurate measurements (Joannis et al 2007). Table 1 lists the main characteristics of the two catchments associated with these measurement points.…”

“…The development potential of quality-based RTC strategies bowever is constrained by a lack of available quality data in sewer systems. Recent studies bave doi: lü,2166/wst,20U,545 demonstrated the use of continuous turbidity measurements in monitoring and assessing sewer system effluent quality (Langeveld et al 2005;Joannis et al 2007). Turbidity is well correlated witb suspended solids (SS) in sewer systems (Ruban et al 2001;Mels et al 2004;Fletcber & Delefic 2007;Lacour et al 2009), and SS are recognised as tbe primary source of pollution transfer witbin sewer systems (Chebbo et al 1995;Ashley et al 2005).…”

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

“…Recent studies have highlighted the potential for using turbidity sensors to continuously monitor pollutant discharges (Langeveld et al 2005;Lawler 2005;Joannis et al 2007). When strict protocols are applied, turbidity sensors are able to provide accurate and reproducible data, both in the laboratory and in situ (Joannis et al 2007).…”