By M. Michael Pitts Jr., Ph.D.
Zeta Corporation, Tucson, AZ 85718
April, 1995

ABSTRACT

Electrochemical dispersion of colloidal organic and inorganic particles as well as microorganisms has been practiced for many years to treat the water in small commercial and industrial water systems. To date the source of the anti-foulant treatment effect has been a mystery or at best is incompletely explained. This paper attempts to reconcile the theoretical foundation of the treatment effect as a linkage of well understood functions of physics, surface chemistry, and electrochemistry. The interactions of the capacitor and dielectric particles within a framework of electrochemical principles leads to a rational explanation of the enhanced cleanliness of wetted surfaces obtainable in aqueous heat exchange systems. This discussion is illustrated by citing applications of a new design ceramic field generation electrode with the trade name 'Zeta Rod'.

Fig. 1: Particle Interaction at high and low zeta potential relative to van der Waals attraction.

THE DISPERSION/FLOCCULATION EFFECT - interaction of zeta potential and van der Waals forces

The installation of an insulated and sealed electrode into a metal pipe or vessel forms a capacitor within which the surface charge is elevated on wetted surfaces of the containment as well as any particles in suspension. The boost in surface charge is readable as an elevation in zeta potential (Pitts,1992). Zeta potential thus provides a measure of the level of the surface charge and serves as an indicator of the relative magnitude of the repulsion force between colloidal particles in aqueous suspension. Inter-particle repulsion counters the van der Waals attraction between colloidal particles and between particles and the walls of the pipes. Van der Waals attraction is generally considered to be a function of the material composition and the resultant molecular dipole. A balance between the two forces, zeta potential and van der Waals, results in a stable dispersion.

When colloidal particles are sufficiently low in surface charge or the electrolyte content of the water is elevated, colloidal particles may be pulled together by van der Waals attraction in what we recognize as flocculation (Benefield, 1982). This bonding together of particles is the action commonly sought in achieving clarification of water or wastewater processes through sedimentation of suspended solids. The desired flocculation in most cases is promoted by the addition of electrolytes such as iron or aluminum salts to the aqueous suspension to reduce zeta potential (Heimenz, 1977).

Fig. 2: Cross sections of cylindrical capacitor. Capacitance is defined by the equation shown in the figure, where: C is capacitance, q is charge, V is voltage, I is length, and b/a is the ratio of radii.

The natural zeta potential of inorganic particles is usually determined by crystal structure and by the location and intensity of charge sites on crystal edges. Zeta potential of organic materials is commonly governed by the degree of hydrolysis to which the organic substance has been subjected or by the composition of the organic substance. Thus hydrolysed organic matter often exhibits a net positive charge, living bacteria exhibit a low negative charge (Marshall, 1991), and the plate-like crystalline structure of clay minerals carries a net negative charge. In the presence of turbulence, heat, or change of electrolyte concentration potential foulants such as mineral salt precipitates, silt and clay particles, hydrolysed vegetable matter and various microorganisms may flocculate forming troublesome masses recognized as biofouling, sludge, or scale.

THE CAPACITOR SYSTEM - as a means of altering charge on particle surfaces

The Zeta Rod capacitor system functions by inducing an alteration of the natural surface charge density of dielectric colloidal particles irrespective of the particle composition. The installation of the Zeta Rod forms a cylindrical capacitor, much like those pictured in physics texts (Halliday,1962).

The conducting surfaces are the metallic lining of the Zeta Rod and the metal of the pipes or vessels. The ceramic dielectric of the Zeta Rod establishes a static electric field within the piping system or the vessel. A direct current power supply charges the capacitor system. The field strength across the water is a function of charge voltage, system dimensions, and the dielectric constant of the water.

Fig. 3: The nature of electrical forces around a particle in solution (after Shamlou, 1993).

At sufficiently elevated voltage the field strength across the water influences the capacitive charge of the particle. The result is a sharp increase in the surface charge of all wetted surfaces. The particle set thus influenced includes newly formed precipitates, soil or mineral based substances and objects of biological origin such as bacteria or planktonic organisms.

THE DOUBLE LAYER - density of ions determines zeta potential.

The surface charge of the dielectric particles attracts ions of opposite polarity. The distribution of ions upon the particle surface is determined by charge density and the geometry imposed by various sizes of ions. The resultant layer of ions is tightly bound and is referenced as the inner layer of the double layer. The charge density of the inner layer attracts a layer of counter ions of opposite polarity. This outer layer exists as a diffuse zone of ions sufficient in collective charge to balance the charge density of the inner layer. When the population of particles in suspension possess similar diffuse layers flocculation is retarded and a stable dispersion prevails.

It follows that the surface charge of a dielectric particle can be manipulated by exposing that particle to a high intensity electrostatic field. The alteration of the zeta potential thus obtained is determined by the dielectric constant of the particle and the field strength imposed across the system.

The polarity of the charge imposed upon the conductor within the electrode determines the polarity to be imposed upon the suspended particles. The inner layer ions may thus be manipulated to be cations or anions. The geometry of the ions that compose the inner layer is important to the success of the dispersion depending on the isoelectric properties of the compounds involved.

Various measuring techniques can approximate the potential between the two layers. One of the most common and one for which industrial instrumentation exists is zeta potential.

The success commonly experienced in controlling the populations of micro-organisms may be directly attributed to the elevation of the cation concentration near or against the cell wall of the organism. For several reasons anions do not pack as tightly in the inner layer as do cations possibly explaining the greater popularity of positive polarity installations in controlling biofouling and silt deposition on heat transfer surfaces.

Fig. 4: Combination of attractive and repulsive forces between two charged particles showing interaction curves at (1) low salt concentration; (2) high salt concentration.

THE IONIC STRENGTH OF THE WATER - the determinant of operating voltage level

The third factor and a strong participant related to achieving the desired antifouling effect is the dissolved electrolyte content or ionic strength of the water (Shamlou,1993). At relatively low concentrations of dissolved solids common to most municipal water supplies, ionic strength of the water is not particularly important in determining success or failure of the electrostatic application. However as the ionic strength of the water increases, especially with the increase of polyvalent ions, the capacitance of the water declines. The level of charge induced on the particle surface is lessened and the outer or disperse layer of the double layer is reduced in radius. The distance between particles maintained by diffuse layer charge declines allowing closer inter-particle proximity. The resultant reduced dimensions of the individual particle diffuse layer may not be sufficient to maintain the inter-particle spacing that offsets van der Waals attraction. All other factors remaining constant a high level of dissolved solids content can nullify the dispersion effect that results in reduced accumulation of particles on heat transfer and other surfaces of the piping system.

Fig. 5: Static dielectric constant of NaCl solution vs. Ion Concentration.

The shift in dielectric constant was first recognized by Hasted (Hasted,1948)who described the effect as being caused by the distortion of the lattice of pure water by emplacement of ions especially the larger polyvalent ions. The presence of high levels of dissolved solids and especially the abundance of polyvalent ions such as calcium and iron is thus seen as contributing to the loss of efficiency in electrochemical water treatment applications. This factor which has been intuitively accommodated in early equipment designs can now be included in the designs for major installations of electrochemical anti-fouling equipment.

Hydrometals Research has demonstrated that the efficiency reduction from elevated electrolyte content can be offset by elevating the voltage level with which the capacitor system is charged. In field studies, advancing from 10,000 volts dc to 30,000 volts dc has been shown to restore the antifouling effect in waters with high concentrations of polyvalent ions (Hydrometals, 1994). High ionic strength waters are typical of process streams and the waste waters of mining and industrial processes.

APPLICATIONS - illustrating the versatility of electrochemical installations

Operating at elevated voltages has allowed the successful control of massive iron oxide deposition in a four mile pipe line carrying acid mine drainage away from a major environmental remediation project. This water has a TDS of 3500 ppm, pH 4.4, and iron content in excess of 500 ppm. The installation treats a flow of 1000 gpm and was put into place for a complete first year cost amortization of 1.5 cents (U.S.) per 1000 gallons of water treated. Life expectancy of the installation exceeds twenty years. The alternative chemical treatment program for this high iron content was too expensive to warrant serious consideration.

Other applications involve large air conditioning systems such as a central air conditioning plant for a community college and another for a major hospital. Both these are operating in water of heavy scale forming potential. Circulation rates exceed 6000 gallons per minute of a high hardness high alkalinity water. Operating results in both systems are superior to the well administered chemical programs previously used. Installation costs of these systems were well under the annual cost of chemical treatment.

Biofouling in all applications is extremely well controlled. Bacterial slime accumulation is non-existent, algae does not accumulate, and foul water odor disappeared within hours.

A PROMISING RESEARCH AREA - Zebra mussel control

The electrochemical surface charge manipulation that disperse particles, controls bacterial development, and softens and disperse calcium deposits; may provide a control mechanism for Zebra mussel infestations. In the life cycle of the zebra mussel are stages ranging from egg and sperm through planktonic veliger and into the mature adult with its calcium shell structure. Within the zebra mussel life cycle there may be windows of opportunity to reduce the success of infestations.

CONCLUSION

Electrochemical water treatment offers a viable alternative to many conventional chemical water treatment techniques. Environmental concerns or problems are non-existent since there is no chemical residual in the discharge form the system. There is no possibility of chemical burn to workers, accidental spills, or damage to components of the heat exchange system from misapplication of chemical agents. In all cases the present value of capital and operating expenses is dramatically less than with conventional chemical applications. The unique electrochemical reactions make many applications possible that have not been feasible with chemical technologies.

Monitoring of the status of the electrochemical system is easily assigned to computerized control centers. Available instrumentation allows testing of the electrochemical treatment effect in the water as an operating parameter. Electrochemical water treatment can now be viewed as a viable and economic alternative in well engineered fouling mitigation programs.

REFERENCES

Benefield, L.D., Judkins, J. F., Weand, B. L. (1982). Process Chemistry for Water and Wastewater Treatment, Englewood Cliffs, N.J Prentice_Hall Inc.

Halliday, David and Resnick, Robert (1962). Physics, Part 2, New York, John Wiley and Sons, Inc.

Hasted, J. B., Ritson, D. M. and Collie, C. H. (1948). "Dielectric Properties of Aqueous Ionic Solutions. Parts I and II," The Journal of Chemical Physics, vol 16, No.1.

Hiemenz, Paul C. (1977). Principals of Colloid and Surface Chemistry, New York, Marcel Dekker, Inc.

Hunter, R. J.(1980).Comprehensive Treatise of Electrochemistry, Volume 1: The Double Layer, eds, Bockris, J.O., Conway, B.E., Yeager, E., New York, Plenum Press.

Hydrometals Research Inc., research project 1994, Tucson, Arizona 85718

Marshall, K.C. and Blainey,B.L..(1991),"Role of Bacterial Adhesion in Biofilm Formation and Biocorrosion," in Biofouling and Biocorrosion in Industrial Water Systems, eds, Fleming H. and Geesey, G.G., Berlin, Springer-Verlag.

Pitts, M. M.,(1992). "Solids Control in Solvent Extraction Circuits Using Electrostatic Dispersion," paper presented at the Society for Mining, Metallurgy, and Exploration 1992 Annual Meeting, Phoenix, Arizona, Preprint No. 92-147.