Realising the potential of zeta

Bruce Jefferson and Simon Parsons from the School of Water Sciences at Cranfield University explore the physical processes involved in the treatment of water and wastewater flows

Physical processes such as sedimentation, flotation and filtration remain at the heart of most process trains for the treatment of water and wastewater flows. All physical processes work under the same collective principles related to size, density and charge of the particles to be removed.

The relative importance of the charge on the process efficiency is strongly affected by the size of the particles under investigation. Once the particles reach a certain size, their mass causes a rate of sedimentation due to gravity sufficiently large to outweigh any effects due to surface interactions. However, within the range of sizes normally encountered within water and wastewater processes (<1,000 mm) surface forces play a vital role in controlling the removal characteristics of the system. The measurement of surface charge, or more importantly zeta potential (z), is determined by measuring the induced velocity under a given voltage. Zeta potential is known to be a key determinant in understanding the performance of physical processes. Density differences between the solid and liquid phases cause sedimentation due to gravity.

The role of zeta potential in such unit operations relates to its effect on the size and density of flocs formed. Low zeta potentials reduce the electrostatic interactions between particles, allowing the particles to approach closely and hence produce more compact flocs. Figure 1 shows the residual turbidity after sedimentation of a coagulated, highly coloured, low turbidity, raw water. Low and stable effluent turbidity is observed across an operational zeta potential range between +3mV and -22mV (Zetasizer, Malvern Instruments). At zeta potentials more negative than -22mV, the effluent turbidity rises sharply as the suspended particles become effectively stabilised in the water due to mutual repulsion of the particles. The size of this operational window can be enhanced by changing the coagulants as seen in Figure 1, where a higher charge density material produces a much wider operational window at positive zeta potentials.

density differences

Flotation processes also function due to density differences but this time because of the reduced densities generated by attaching air bubbles to the solid phase. In such unit operations the importance of zeta potential relates to the ability of the bubbles and particle to adhere and remain attached. It was observed the process is driven by the combined zeta potentials of both the particles and the bubbles, although in many cases measurements of just the solid phase is sufficient. Figure 2 shows the impact of the product of both particle and bubble zeta potentials during the flotation of a coagulated highly turbid water. A clear relationship exists demonstrating lower zeta potentials result in higher removal efficiencies.

The efficiency reduces as the product of the zeta potentials increases, indicating if either surface is highly charged, then the process efficiency will be reduced. Examination of grade efficiency curves generated during the work reveals the loss in treatment performance occurs
initially at the smaller size ranges as expected due to the increasing dominance of charge effects as these smaller sizes. Filtration processes function by the attachment of particles on to media grains of materials such as sand and anthracite. The role of zeta potential here is in determining the ability of the particles to be captured. In principle, this is the same as in flotation except the media surfaces are less affected by changes in chemistry when compared with the bubbles.

zeta potential

Figure 3 demonstrates a similar relationship to Figure 1, where an operational zeta potential window exists, within which particle concentration in the effluent is both low and stable. A stable effluent concentration is dependant on standard filtration parameters such as media size and filtration rate whereas the operational zeta potential window is unaffected by operational variables and is determined by the chemistry of the system through parameters such as the coagulant type and pH.

The overall picture indicates the existence of operational windows of zeta potential, within which treatment efficiencies are high and particle concentrations in the effluent are low. Within these
windows, the process has effectively become independent of zeta potential as other factors become performance limiting. Interestingly, many process trains in the water industry appear to operate at zeta potentials (-15

The application of zeta potential for the diagnosis and control of water and wastewater treatment processes is not new. Research papers regularly cited on this subject date back decades and, in fact, most of the basic understanding of how zeta potential controls performance remains similar to those early investigations.
Early work was limited by the difficulty of the measurement itself and the reliability of the equipment. This tended to restrict work to small data sets that were unsuitable for the investigation of real treatment systems and
limited the experiments to idealised environments. The availability of modern methods and improvements in reliability and robustness of the measuring techniques has reinvigorated the application of zeta potential in the diagnosis and operation of physical processes. The regular use of zeta potential as a parameter has become more feasible, especially as improvements in the physical robustness of the technology mean a system can be taken on-site when necessary. Improvements in associated analytical areas have also been made, enabling much more extensive characterisation of the waters involved.

The challenge is to understand how to robustly control the zeta potential to keep it within the operational window of the system. This may require changes to the chemistry of the water or indeed manipulation of the physical processes themselves. This is an exciting time for the subject, enabling us to tackle real waters and real treatment plants with confidence and provide the necessary platform for the evolution of physical processes for water and wastewater treatment based on fundamental science

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