The Role of Zeta Potential in Controlling and Optimizing Water Treatment Processes Application note

Process efficiency is heavily reliant on the size of the particles of interest, which can be controlled by the particle charge. The range of particle sizes normally encountered within water is smaller than 1,000 μm. Sedimentation due to gravity will occur once the particles achieve a certain size owing to their large mass. Therefore, zeta potential measurements play a vital role in controlling the removal of these particles from the water system.

Surface charge or more commonly known, zeta potential (ζ), is a scientific measurement measured in the Malvern Panalytical Zetasizer. This is done by measuring the particle velocity induced when a potential difference is applied across a capillary cell that contains the sample of interest. As turbidity of raw water varies due to source, season and run-off events, control of coagulant dosage, pH or polymer addition can impact the overall plant performance. Furthermore, the determination of zeta potential allows the performance of physical processes such as flocculation and sedimentation to be easier understood.

Figure 1. Various types of suspension and flocculation events correspond to the various zeta potentials.

The zeta potential of raw water is typically negative, in the region of -25 mV to -15 mV. Adding coagulants neutralizes this negative charge, moving zeta potential towards zero and ultimately into the positive range. Flocculation is typically seen at zeta potentials from -8 mV to +3 mV with particle re-stabilization occurring above +5 mV. Controlling coagulant dosing, either manually or automatically, by directly referencing zeta potential to a set point, therefore optimizes the charge neutralization process.

Figure 2. Coagulant dosage to achieve optimum dose/zeta potential.

Figure 3. Zeta potential (mV) against coagulant concentration (mg/L) of raw water with different weather conditions – no rain vs rain.

Regularly measuring the zeta potential of water following coagulant dosing:

• Indicates whether more or less coagulant is required – predictive, rather than reactive control
• Optimization of coagulant dosage – reduction in coagulant cost expenditure
• Permits a timely response to rapid changes in raw water quality
• Provides the data for automated process control
• Detects a shift in plant operation before it becomes a major problem
• Time-saving and efficient – reduction in analysis time compared to traditional jar-test