Scaling Back the Mineral Problem
Physical water treatment (PWT) is a general term that refers to non-chemical methods of controlling or preventing fouling, especially mineral fouling or scale. PWT technologies use the laws of physics to impact water chemistries and mitigate scale without the use of chemical additives. Such technologies target lime scale, an extremely adhesive crystalline precipitate of calcium carbonate, which is responsible for the majority of scaling problems. Calcium carbonate also traps other minerals, such as magnesium, to form combined scales, just as it traps soap in residential sinks and bathtubs to form so-called "soap scum."
Despite fast-growing use and success with these non-polluting, maintenance-free PWT technologies, their types and operating principles are still not widely nor well understood. The purpose of this article is to begin the process of overcoming this conspicuous lag between application and understanding. We do so by reviewing PWT technologies, and by comparing and contrasting these with respect to hardware and operating principles. Based on a growing body of rigorous scientific research conducted over the past 10 years, these non-chemical methods of descaling are divided into three broad categories: mechanical precipitators, catalytic devices, and electrical field generators.
We will provide an overview of the first two categories, and then focus our discussion mainly on the third. Filtration and separation are very important and highly effective non-chemical water treatment tools that are not covered in this article.
The Scale of the Problem
While the use of permanent magnets for descaling goes back more than a century, over the past decade rapid advances in this and newer PWT technologies, along with burgeoning demand, have come largely as a market response to the expense and pollution associated with conventional chemical descaling. Removing and/or preventing scale by chemical methods costs an estimated $25 to $30 billion per year in the United States alone.
A study by the University of Surrey, United Kingdom (UK), found that, in 1992, the costs associated with scale totaled $14.2 billion in the United States, which equaled 0.25 percent of the country's 1992 gross national product (GNP).1 Scale costs for other advanced industrial economies -- including Japan, Germany, and the UK -- also equaled approximately 0.25 percent of those nations' GNP. Assuming that a similar proportional relationship exists today between scale costs and the total market value of all goods and services produced -- now termed Gross national income (GNI) -- as a rough measure, we would estimate the total cost of scale in the United States at $27.5 billion in 2003. (Calculation: 0.25 x $11 trillion (2003 GNI) = $27.5 billion). These scale-related costs include:
- Chemicals: acids, antiscalants, etc.
- Maintenance: acid washes, repairing or replacing acid- and scale-corroded equipment, handling chemicals, etc.
- Replacement equipment: pipelines, valves, pumps, heat exchangers, condensers, boiler tube bundles, cooling tower components, etc.
- Energy losses: A scale layer 1/1000-inch thick reduces heat transfer by approximately 10 percent.
- Production losses due to downtime
- Waste disposal
- Pollution prevention and/or remediation
The conventional use of toxic chemicals in scale control requires large expenditures on waste disposal and pollution prevention. Even so, chemicals, such as hydrochloric acid, are routinely and legally discharged, after neutralization, into municipal sewer systems, along with the heavy metals that they strip from equipment in the scale-cleaning process.
A growing body of scientific evidence suggests that the three different categories of PWT technologies described herein -- mechanical precipitators, catalytic methods, and electrical field generators -- all use the same underlying mechanism of action. By forcing mineral ions to precipitate in the bulk water instead of on the surfaces of pipes and equipment, each type protects equipment from scale. Carried along in suspension, the sludgy particles formed by PWT are removed from the system either by the sheer force of the water flow or by other means, such as cooling tower blowdown.
Mechanical Precipitators: Mechanical precipitators commonly use turbulent flow, sudden drops in pressure, or the application of kinetic force to precipitate hardness-causing mineral ions into soft particulate matter. Vortex geometries, injection methods, or pumps may be used to generate turbulence, pressure drops, or kinetic force. In all such cases, energy is somehow being added or transferred to the water. This added mechanical energy enables mineral ions to precipitate in the bulk solution -- not on the pipes or equipment.
Catalytic Systems: Catalytic water-treatment systems are based on the same principles as an automobile catalytic converter, which may use nickel, palladium, and/or platinum to change carbon monoxide into carbon dioxide. In PWT applications, many of the same catalytic metals or their alloys are used to catalyze scale-forming reactions, converting hardness-causing mineral ions into soft particles in bulk solution. Catalytic systems can take on a variety of geometries, such as the insertion of rods into tanks or cooling tower sumps, or a flow chamber filled with nickel-coated spheres, etc. In general, catalytic methods are known to be effective for preventing scaling, and catalytic surface reactions are considered an important field of water treatment research. One disadvantage, however, is the need for regular cleaning and maintenance.
Electrical Field Generators: Electrical field generators, such as permanent magnets, solenoid induction coils, and inline electrodes, have been the most successful among the categories of PWT technologies in recent years and will be the focus of the remainder of this article. The two most common types of permanent magnets are clamp-on magnets and magnets positioned at the center of a pipe such that water flows through an annulus gap between the pipe and magnets.
There are a variety of different geometries that can be used by permanent magnet PWT manufacturers. What is held in common in all of these cases, however, is the alternating magnetic fields -- the north, south, north, etc. variation -- which generate alternating electrical fields by virtue of Faraday's Law of Induction. These electrical fields generated by magnets can ultimately prevent scale formation on pipes and capital equipment. Disadvantages of permanent magnets can include high equipment costs and the high water flow velocity requirements necessary to generate electrical fields and the PWT effect.
The solenoid coil type of PWT uses Faraday's Law in a manner similar to the mechanism of permanent magnets. However, rather than applying magnets directly to the pipe or water, a solenoid is used to generate the magnetic field. Wire is wrapped around the pipe, and an electrical current is passed through the coil to generate a magnetic field.
The magnetic field inside a solenoid coil has identical characteristics to the one generated by a bar magnet. By using a control unit connected to the coil to alternate the direction of the current flowing through the wires of the coil, the directions of the magnetic field vectors can reverse themselves several hundred times per second or more. This generates electrical fields inside the pipe oriented in a direction perpendicular to the magnetic field.
In a typical solenoid coil-type PWT application, the induced electrical fields are oriented with the pipe cross-section, i.e., normal to the magnetic fields that are oriented along the length of the pipe. Because the induced electrical fields are changing direction at the frequency of the alternating current flowing through the coil wire, the positively and negatively charged ions experience a "clothes washer effect" of being pushed one way, and then the next. Because positively charged calcium ions and negatively charged carbonate ions are compelled to move in opposite directions with every pulse of the AC signal, the probability of collision and precipitation is said to increase significantly. Like in the case of permanent magnets, this molecular agitation by induced electrical fields is at the heart of the PWT effect.
To effect PWT by means of a solenoid, the two ends of the coil wire are simply connected to an electronic control unit. A sine wave signal, square wave signal, or other alternating current signal must be generated by the control unit and driven through the cylindrically wrapped coil wire. The coil can be wrapped over any non-ferrous metal or plastic pipe, and the electrical field induced by the solenoid coil is present inside the pipe, without necessarily cutting the water-bearing pipe. The solenoid-coil type of PWT generally maintains efficacy in all non-ferrous pipe materials.
A third type of electrical field generator produces induced electrical fields based on a capacitor method, usually applied directly to the water. A leading example of an inline capacitor system is Zeta Rod, produced by Zeta Corp. Others include ESP, Hydro-Tron, and Scalemaster. Using a high-voltage electrode positioned at the center of a feed pipe and coated in a dielectrical material, an inline capacitor adds a charge and makes the water itself into a capacitor, wherein the capacitance is determined based on the geometry of the electrode. The electrical field can be estimated for a given charge in water as the voltage varies from zero to the maximum value. The magnitude of the electrical field is in the order of 1 volt per meter (V/m), which is on the same order of the magnitude of the induced electrical fields for permanent magnets and solenoid coil device.
Water Chemistry and Activation Energy Calculation
In 2004, Won T. Kim, PhD, Director of Research at the PWT Center, published a benchmark technical paper together with researchers from Drexel University in Philadelphia, PA, titled "Physical Water Treatment for Fouling Prevention in Heat Exchangers."2 Some researchers criticized the mechanisms of actions we use, as above, to describe the operation of PWT devices as overly simplistic from a water-chemistry perspective. In response, this paper was published to communicate the water-chemistry of scaling and scale prevention more comprehensively than any study in the past. Some of its key findings are included here.
Water Chemistry of Calcium Carbonate Scale Formation: The chemical reaction at the heart of the scaling problem is clearly the precipitation of calcium carbonate. Under saturated conditions, CaCO3 does not easily precipitate out of solution because both calcium and bicarbonate ions are surrounded (i.e., hydrated) by water molecules. When the energy level of hard water is raised above a critical point -- such as by an increase in temperature in heat-process conditions or by kinetic turbulent flow through a pipe elbow or valve -- the ions are agitated and freed from their water molecule captors, resulting in the precipitation of CaCO3.
Three reactions govern the rate at which dissolved calcium and carbonate ions can recombine and crystallize -- and therefore the rate at which scaling will occur. Reaction 1 below relates to the dissociation of bicarbonate ions into hydroxide ions and carbon dioxide in aqueous solution.
HCO3-(aq) >>> OH-(aq) + CO2(aq) (Reaction 1)
OH- (aq) + HCO3 -(aq) >>> CO32-(aq) + H2 O (Reaction 2)
Ca2+(aq) + CO32-(aq) >>> CaCO3 (s) (Reaction 3)
This dissociation of bicarbonate ions is critical to the water chemistry of scaling and scale mitigation. Note: whereas the bicarbonate ions do not cause scaling (as long as they remain bicarbonate ions), carbonate ions are a dangerous species because they can spontaneously transform into scale on the surface of heat-transfer equipment and pipes. Reaction 1 shows the first step in this process. The presence of hydroxide ions is usually indicated by a localized increase in pH, and over time carbon dioxide gas is typically released from the water.
Activation Energy: In Reaction 2, hydroxide ions produced from Reaction 1 react with existing bicarbonate ions, producing carbonate ions and water. Reaction 3 occurs between calcium and carbonate ions, resulting in the precipitation and crystallization of calcium carbonate. Based on the Gibbs free energy value for this precipitation reaction (-47.7 kilojoules per mole), calcium carbonate will form spontaneously once the initial activation energy is overcome.
Without PWT, the activation energy required in Reaction 1 is supplied by an increase in temperature or turbulence. In such cases, inside heat exchangers or in pipe elbows or valves, scaling will occur and cause damage to capital equipment. The small amount of energy added to water by PWT technologies, on the other hand, delivers the threshold activation energy because it is amplified by the surface roughness of the pipe. The precipitation of non-adhesive, sub-micron particles of CaCO3 is thereby induced within the bulk water, pre-empting its precipitation on surfaces as lime scale.
When an electrical field is applied to water by PWT technology, the local electrical field around minute bumps (E) and roughness on the pipe surface is many orders of magnitude larger than the space-averaged field within the pipe (E0) in general. Such small bumps and surface roughness can range in size from 0.1 mm for a galvanized iron pipe to 0.0015 mm for drawn tubing (e.g., ductile copper).
This suggests that surface roughness of pipes amplifies the PWT effect so that the activation energy necessary to precipitate scale is met. Due to these irregularities, the local electrical field at the surface of the pipe is 106 times greater than that in the bulk solution. Thus, despite the fact that the energy typically generated in the water by a PWT device (E0, space-averaged electrical field) is much too small (~1 V/m) to dissociate bicarbonate, the strength of the field local to the surface irregularities is on the order of 1,000,000 V/m, a value large enough to cause the dissociation of the bicarbonate ions in water. Therefore, without PWT, Reaction 3 tends to occur on the surfaces of pipes and equipment. With PWT, however, this nucleation reaction takes place in the bulk water.
Perspectives and Areas for Further Research
PWT is an area of research that lies at the intersection of two fields: physics and chemistry. Getting physicists and chemists to talk can be, in itself, a remarkable achievement. In the case of PWT, however, it has been a necessity. Water treatment has historically been the domain of chemists and microbiologists, but with the growing use of membranes, various forms of mechanical filtration, and numerous physical water treatment devices, water treatment has become a fully multidisciplinary industry that includes physics as well. That multidisciplinary approach will be, we believe, critical for organizations to succeed in the business of water treatment.
The PWT Center sees numerous areas for further research, particularly in combining mechanical filtration and membrane technologies together with PWT. Pre-treatment of reverse osmosis and deionization are also areas of focus, in addition to treatment of aerobic wastewater effluent with some combination of filtration and PWT.
- University of Surrey School of Engineering -- www.surrey.ac.uk.
- Y.I. Cho, A.F. Fridman, S.H. Lee, and W.T. Kim. "Physical Water Treatment for Fouling Prevention in Heat Exchangers." Advances in Heat Transfer, Vol. 38. Elsevier, 2004.
This article originally appeared in the November/December 2005 issue of Water & Wastewater Products, Vol. 5, No. 6.