Pursuing Purer Water

Microelectrophoresis helps optimize U.K. coagulation treatment.

Tightening regulations are presenting new challenges for the United Kingdom’s water treatment and distribution companies. In recent years, there has been a considerable reinforcement of water quality regulations with new World Health Organization guidelines being published in 1993 and the recent revision of the European Union Drinking Water Directive. These regulations include very stringent guideline values or maximum concentration limits for organic pollutants. Although the water companies are confident of meeting emerging regulations using current technology, they are also exploring new ways to meet future controls without raising the cost of water to domestic and industrial users.

The Challenge

Natural water generally contains 1-10 mg/L of dissolved organic carbon (DOC) arising from the microbial and photolytic degradation of natural organic matter (NOM). Current water treatment processes, relying primarily on coagulation/separation technology, remove approximately 50% of the NOM. As well, subsequent chlorination results in the formation of disinfectant byproducts (DBPs) for which increasingly stricter limits are being proposed. For example, current U.K. regulations of <100 µg/L of trihalomethanes (THM) as a rolling average over a 3-month period will be replaced by a single analysis limit of <100 µg/L by December 2003. There are already moves in the United States to lower this limit still further to 80 µg/L, coupled with more specific limits on selected halogenated organics. This places an increasing challenge on the current water treatment processes. Until the 1970s, research into water treatment was focused primarily on the need to remove color and turbidity from public supplies. Much of this research centered on coagulation, an effective and affordable technology by which coprecipitated aluminum or ferric hydroxide and NOM are removed by sedimentation, flotation, or filtration. More recently, an emphasis on removing all NOM, rather than simply color and turbidity, has led to the development of “enhanced coagulation”. Most utilities expect to comply with newly revised regulations using their existing coagulation facilities. But coagulation is not necessarily the most effective way of removing NOM. It is, however, already firmly established in the water treatment protocols of most plants and represents a significant capital investment. Much attention, therefore, is being focused on optimizing this long-established technology.

New Input

To help the water companies meet post-2003 targets, the U.K. Engineering and Physical Sciences Research Council, under the Water Infrastructure and Treatment Engineering Programme, wishes to attract the input of academics who would not normally have any involvement with the water industry. For Mike Garvey, a scientist with 25 years of industrial experience in colloid and surface science (the study of submicrometer particles, macromolecules, and the physical chemistry of interfaces), this offers new research opportunities.

Garvey, principal scientist at the University of Liverpool’s Surface Science Research Centre, said, “The companies and government hope that the fruits of this interaction will be novel, creative, and cost-effective methods for improved water treatment.”

At present, there is considerable effort in the United States, France, and Australia targeted at identifying NOM constituents. This effort is focused toward understanding the structure of the refractory (difficult to remove) components of NOM responsible for THM formation and attempts to rationalize the aluminum or ferric hydroxide processes for enhanced NOM removal.

The more refractory NOM components”dominated by phenols, quinones, amides, and esters”are notable for their absence of carboxylate groups and their relatively low molecular weight. The lack of carboxylate groups to complex with aluminum explains why current coagulation processes do not readily remove the refractory NOM. Improved water treatment processes should consider the characteristics of these functional groups. The effectiveness of the improved treatments will probably not depend on how much of the DOC can be removed, but rather, how well the process can remove particular components.

Garvey believes that as regulations tighten over the next 10 years, new and improved technologies will be necessary. Coagulation will remain the primary means of NOM removal, as it is effective in removing the higher molecular weight and more hydrophobic components. Garvey says that colloid and surface science can be used to further enhance NOM removal, but it should be targeted toward the refractory NOM fraction. This would require the identification of the refractory species and the development of colloid and surface chemistries to either complex with or adsorb these molecules. Additional constraints include recognizing the restrictions imposed by the high capital investment in current treatment programs and ensuring that any technological improvements are compatible with existing plants.

The Technology

One of the techniques being used by Garvey and collaborator Paul Stevenson is microelectrophoresis. This involves the movement of particles under the influence of a carefully controlled electrical field and measures a colloid’s zeta potential (see “What is Zeta Potential?”), a property related to the surface charge of a particle in a conducting solution. The zeta potential is a sensitive indicator of both adsorption and subsequent flocculation, and it is frequently used as a control parameter to optimize coagulant dosage. As an analytical tool, however, it is able to detect the increase in adsorption of soluble species resulting in a monotonic increase or decrease in zeta potential. With macromolecular species, a more complex pattern arises due to a shift in the electrophoretic shear plane as a function of the quantity of adsorbed material. To facilitate this work, Garvey contacted Peter McFadyen, managing director of Brookhaven Instruments, Ltd. (Worcestershire, U.K.) to set up his microelectrophoresis system, opting for the ZetaPlus analyzer.

Describing the microelectrophoresis system, McFadyen says that the sample cell is designed to eliminate electro-osmosis, a significant source of errors. In conventional capillary-type electrophoresis cells, sedimenting particles that deposit on the inner surface of the cell change its characteristics and therefore the apparent electrophoretic mobility. Thus, frequently cleaning the cell and checking the optical alignment of the instrument become essential for accurate results.

“As [Garvey] was actually going to be working on sedimentation of model colloids, this was clearly an important aspect of the unit’s design,” said McFadyen. “Moreover, any particles which do sediment in the cell, fall out of the measurement zone and therefore cannot affect the result in any other way.”

“The facility to measure particle-size distribution as well as zeta potential on the same sample and in the same instrument can be very useful in flocculation or colloid stability studies,” he adds. “As flocculation proceeds, particle size obviously increases, and a knowledge of its relationship to the zeta potential can be invaluable in process optimization.”

Another option is to determine zeta potential by the technique of phase analysis light scattering (PALS). This is an extremely sensitive development of the microelectrophoresis method and is useful for measurements of very low potentials”exactly the situation likely to occur when flocculation is at its most efficient. Should Garvey’s work take him in this direction, a PALS upgrade can be added to his ZetaPlus instrument without its leaving the laboratory.

Garvey is using the microelectrophoresis data to devise new water treatment methods. After investigations using a range of model colloidal dispersions, colloidal alumina was selected as a model to represent the coagulant. Under appropriate pH conditions, the colloid is positively charged, mimicking conventional coagulants. The sequential addition of increasing concentrations of both untreated and unchlorinated clarified water results in changes in electrophoretic mobility reflecting adsorption of the NOM. The response of alumina to the addition of refractory NOM is a strong indicator for the potential for the refractory NOM to submit to further adsorption. This is to say that they achieved “enhanced coagulation” using conventional coagulants. This analysis, coupled with chemical analyses, is facilitating both the elucidation of the adsorption mechanisms and the identification of alternative adsorbents.

Future Uses

But the potential for microelectrophoresis does not stop in Garvey’s lab. “We are now starting to foster links with other departments of Liverpool University who are expressing an interest in the [microelectrophoresis] facility,” he explains. “For example, one group is researching nanotechnology, the manipulation of nanometer-sized particles to give high-tech structures. Another department has a particular interest in the interaction of colloidal particles with the surface of biomaterials, such as prosthetic implants, in order to achieve improved biocompatibility. Future collaboration in these fields would make a great deal of use of this equipment.”

Garvey anticipates that his research could help water companies meet stricter regulations that are expected in the near future. Water quality regulations in the United States are already tighter than in Europe.

Further Reading:

Industrial Water Soluble Polymers; Finch, C. A., Ed.; Royal Society of Chemistry: London, 1996.

Proceedings of Natural Organic Matter in Drinking Water”Origin, Characterization, and Removal, Chamonix, France, Septenber 1993.

Water Quality and Treatment, 4th ed.; Pontius, F. W., Ed.; McGraw-Hill: New York, 1989.

Clare Butterfield is associate director of kdm communications limited, Milton Keynes, U.K. Comments and questions for the author can be addressed by fax at 202-776-8166 or by post at 1155 16th Street, NW; Washington, DC 20036.

Applications: HealthWater Treatment
Instruments: NanoBrook Series