Clear visibility in the process: measuring, interpreting and stabilising turbidity correctly

Clear visibility in the process: measuring, interpreting and stabilising turbidity correctly

In water and wastewater treatment, process stability, data quality and the early detection of deviations during ongoing operations are becoming increasingly important. One measurement parameter that is becoming ever more relevant in this context is turbidity.

In potable water and wastewater processes, turbidity is far more than just a quality parameter: when measured correctly, it serves as an early indicator of deviations and as a control signal for flocculation and filtration. Low turbidity is particularly challenging, for example after filtration or in clear water zones, as even the smallest changes must be reliably detected here. This requires a measurement signal with high reproducibility and sufficient stability. This article explores how turbidity values can be put to reliable use in operation and what this means for measurement and signal quality.

Turbidity as an early indicator in day-to-day process operations

Turbidity describes the scattering of light by dispersed particles in water. It therefore does not provide a specific substance signal, but rather an overall optical signal reflecting the cloudiness of the sample. It reacts quickly to changes in the process and is suitable for identifying trends before disturbances become clearly visible in other parameters.

In potable water treatment, increases in turbidity can indicate changes in raw water conditions, such as weather influences, changes in water source, or turbidity surges. In wastewater treatment, turbidity often indicates how stable solid removal, coagulation and flocculation are proceeding, and whether downstream processes such as filtration or discharge areas remain within the target range. The practical added value arises when the measured value is not viewed in isolation, but placed within the context of operating conditions and process events.

The measurement point determines whether the value is truly helpful

Many discussions about implausible turbidity readings have a common cause: the measurement point does not clearly answer the operational question it is intended to support. A simple guiding question helps with planning and operation: What decision should the turbidity reading enable?

In potable water treatment, raw water turbidity primarily serves as an event indicator, providing early warning of changes in raw water conditions and potentially necessitating adjustments in subsequent treatment stages. After coagulation and flocculation, turbidity can indicate whether process control is within the appropriate range and whether metering strategies should be adjusted. Measurement at the filter outlet is particularly relevant: it aids in detecting filter breakthroughs and adjusting backwash strategies accordingly. Provided turbidity values are not interpreted in isolation but are embedded within an overall measurement and control strategy.

In municipal and industrial wastewater treatment plants, measurement points after flocculation or at the outlet are particularly common. There, turbidity serves as a continuous stability indicator, making load changes or disturbances visible at an early stage.

The following applies to both potable water and wastewater applications: a measuring point must be operated under stable hydraulic conditions. Fluctuating pressure conditions, partial filling, dead spaces or air bubbles lead to measurement artefacts and complications in control processes.

The focus below is on low turbidity, as it is particularly frequently used as an early indicator after filtration and in clear water zones, and places high demands on the measuring point and signal quality.

Low turbidity: Why low values are more challenging to measure

Different requirements apply to measurement in the low turbidity range than to highly loaded media. The actual metrological challenge is not the measuring principle itself, but the signal quality at very low particle concentrations: the useful signal is close to the background noise, and the slightest disturbances – an air bubble, a deposit on the optics, a temperature jump – distort the result disproportionately.

Two characteristics are therefore crucial, yet are often underestimated in practice. Firstly, the long-term stability of the light source: an LED loses intensity over time, and without active compensation, the signal drifts even when there is no other influence from the sample or surrounding. Secondly, the signal-to-noise ratio: the fewer particles in the medium, the more important it is that the device can distinguish genuine turbidity changes from measurement noise.

The DULCOEYE LT from ProMinent is designed for this application. It measures the range of 0 to 100 NTU using the nephelometric measuring principle in accordance with ISO 7027, with an 860 nm infrared LED, and features automatic compensation for light source ageing. In practical operation, this means that the signal remains stable even over extended periods of operation.

Typical sources of error that distort measured values

Air bubbles are a common source of interference. Even very small bubbles scatter light strongly and produce seemingly high turbidity values. Typical causes include leaks, unfavourable installation situations, pressure fluctuations or cavitation. Two things help ensure robust continuous operation: a clean hydraulic design and sensor technology that detects interference. The DULCOEYE LT specifically addresses this problem: both the hydraulic design of the flow cell and the internal signal processing maintain measurement quality when air bubbles occur.

Contamination and deposits in the measuring zone are also relevant, for example biofilm, flocculation deposits or iron and manganese precipitates. Here, the combination of a suitable installation situation and a maintenance-friendly design determines the long-term stability. In the DULCOEYE LT, this principle has been incorporated into the design: the continuous flow generates gentle hydrodynamic self-cleaning, which does not require a separate cleaning module.

A third classic scenario involves backflushing and process cycles. Backflushing operations produce predictable turbidity profiles. If these patterns are not recognised as normal operating conditions, unnecessary alarms will be triggered and confidence in the measured value will be lost. In practice, it has proven effective to link alarms to operating states and evaluate trends, rather than relying solely on rigid limit values. The signal becomes particularly reliable when turbidity values are evaluated in conjunction with operating states and flow information.

From measured value to process control: turbidity as a controlling parameter

Turbidity delivers its greatest benefit when it is actively integrated into process control. In coagulation and flocculation, a turbidity signal can serve as feedback to adjust metering rates to changing water conditions. The key here is the interplay between measurement, evaluation and control parameter: the measured value must be consistent and reproducible, and the control strategy should take into account process conditions such as load changes, backwash phases or valve switching. The aim is rarely a single fixed value, but rather a stable process window with reproducible solid removal.

In filtration operation, turbidity at the filter outlet can serve as a signal for backwashing. Instead of purely time-controlled intervals, the actual filter condition is taken into account. This can increase operational reliability, as breakthroughs are detected earlier, whilst simultaneously saving resources because backwashing is triggered in a more targeted manner.

Trend analysis is also becoming increasingly important. It is not just the absolute value, but the rate of change or the occurrence of atypical patterns that provide indications of emerging disruptions. For such approaches to work, the turbidity signal must respond quickly enough whilst remaining stable enough to avoid false alarms. Key parameters used for this purpose include repeatability, resolution and response time. The DULCOEYE LT, for example, offers a resolution of 0.001 NTU, a repeatability of < 0.5% and a T90 response time of 5 seconds.

Requirements for reliable turbidity measurement in continuous operation

For turbidity to be suitable as a control variable, the measurement must remain stable over the long term. This requires a robust design, suitable sample routing and a clear maintenance concept. In practice, functions such as compensation for light source ageing, hydrodynamic self-cleaning of the measuring cell, and automatic air bubble detection and correction support signal stability in continuous operation.

Equally important are defined operating conditions for the measuring cell. For the DULCOEYE LT, flow rates of 20 to 30 liters per hour and a process pressure of 1 to 3 bar are specified, ensuring that the hydrodynamic conditions in the measuring cell remain stable and reproducible. The signal parameters of the DULCOEYE LT are tailored to the typical requirements of potable water and filtration processes. An accuracy of ± 2 % ± 0.02 NTU ensures reliable measurement results as a basis for process control. A repeatability of < 0.5% is relevant for applications where legal limits must be consistently and verifiably maintained below the threshold, as required by potable water regulations in many countries. The resolution of 0.001 NTU enables the observation of trend changes to be detected at an early stage. The T90 response time of 5 seconds is short enough to detect increases in turbidity due to filter breakthroughs in good time and trigger a backwash as required, whilst being sufficiently damped so that signal peaks caused by individual air bubbles or brief pressure surges are not interpreted as alarm events.

An RS-485 interface is implemented for integration into measurement and control systems. The sensor is connected to the higher-level control system via a ProMinent controller. The housing is made of 316L stainless steel and is IP68-rated. For commissioning and quality assurance, 1-point and 2-point calibration as well as calibration options using formazine are available, compatible with current potable water regulations. Solid-state verification also enables a quick functional check directly on-site without the need for calibration solutions.

Conclusion

Turbidity is one of the most frequently used parameters in water and wastewater technology. Its value depends less on the measured value itself than on its practical implementation. Operators who consistently align the measuring point, sample hydraulics and evaluation with the operational requirements obtain a rapid early indicator and a reliable signal for process control. Particularly in potable water supply, where compliance with legal limits is directly relevant to health, precise and stable turbidity measurement plays a central role.

Detailed information on integrating the DULCOEYE LT into existing measurement and control concepts, as well as on installation and calibration options, can be found at DULCOEYE LT – Optical Sensor for Fine Turbidity .

(Author: Nadja Lumme, Product Manager, Sensors, ProMinent GmbH)