Cooling Towers: Treatment without chemicals?

Submitted by Fred Brindisi, BakerHydro Filtrations

Contents
Operational problems
Various treatment technologies
Chemical
Magnetic
Ozone
Coordinated electrochemical/physical alternative

Water related problems most commonly found in cooling towers include scale, fouling, corrosion and biological deposits. In an effort to overcome these operational nightmares, cooling tower operators have normally turned to traditional chemical solutions…but that may be changing!

As a result of aggressive laws relating to chemical use and handling which are now being enforced by local, State and Federal agencies, industrial companies, institutions and commercial operations are re-evaluating methods to treat their cooling tower waters. Additionally, increased chemical and utility costs, expenses incurred with the training of employees who must handle the chemicals, storage and disposal of chemicals, and the general maintenance involved with the use of chemicals has made this issue a priority with many organizations.

In recent years, a variety of "non-traditional" water treatment technologies have been developed and presented to users of cooling towers as an alternate method of treatment. Regardless of the technological basis of these new treatment systems, the issues to be addressed will, and have, remained the same.

Operational problems
Scale - Minerals such as calcium and magnesium are relatively insoluble in water and can form damaging scale deposits when exposed to conditions commonly found in cooling water systems. A layer of scale as thin as .015 inches can reduce heat exchanger efficiency by 15%.

Corrosion - Most metal materials used in the construction of cooling water systems are very susceptible to corrosion. Keeping surfaces clean is a most important aspect of preventing corrosion, including underdeposit corrosion, which can result in serious structural and equipment damage. If cooling waters are not treated and maintained properly, corrosion can be uniformly distributed throughout the system or it can be localized, causing severe pitting and rapid equipment failure.

Fouling - Solid materials such as airborne debris, corrosion formation and suspended solids accumulate in the system and contribute to considerable losses in performance, efficiency and equipment deterioration.

Microbiological deposits – More cooling water treatment programs fail due to an ineffective, or lack of, microbial control, than any of the other three causes mentioned above. Scale, corrosion, and fouling are often symptoms of an unsuccessful program, but the root cause is inadequate microbiological control. Makeup water and wind can carry microorganisms into a cooling water system and, if uncontrolled, microbiological fouling can develop and lead to problems at every point in the cooling water system. Corrosion can become evident beneath the bacterial slime layer causing inorganic foulants to become trapped, compounding the problem. (Return to top))

Various treatment technologies
In general, technologies encountered in the marketplace today may be broadly grouped as traditional chemical treatment, magnetic treatment, ozone treatment and combined technologies. (Return to top))

Chemical
Traditional Chemical Treatment Addresses corrosion by providing a protective chemical layer, generally within an alkaline environment; biological fouling and deposits through the use of oxidizing and non-oxidizing biocide programs; scale formation through a combination of polymers, polyphosphates and managed cycles of concentration within the tower and; suspended solids using dispersants and polymers.

Although coordinated in a systematic approach, a chemical program consists of a series of compromises. An alkaline program is certainly necessary to inhibit corrosion – yet the elevated pH severely impacts oxidizing biocides and their half-life. Dispersants that are needed to maintain suspended solids in solution so that they may be removed via the bleed, contribute to corrosion on pipe surfaces. Phosphates provide some measure of calcium sequestration, yet do not address pre-existing scale or scale formed during upsets. (Return to top))

Magnetic
Magnetic technology has been cited in literature and investigated since the turn of the 19th century, when lodestones and naturally occurring magnetic mineral formations were used to decrease the formation of scale in cooking and laundry applications. Currently, advances in magnetic and electrostatic scale control technologies have led to a proliferation of companies making claims with regard to their technology. For example, magnetic or electrostatic scale control technologies are promoted as a replacement for most water-softening equipment. Specifically, chemical softening (lime or lime-soda softening), ion exchange, and reverse osmosis are included. This would involve applications both to cooling water treatment and boiler water treatment using single pass and recirculating processes.

The primary savings factor achieved from this technology is generated from a decrease in energy consumption within heating or cooling applications. These cost reductions are associated with the prevention or removal of scale build-up on heat exchange surfaces, where even a thin film can increase energy consumption considerably. Secondary energy savings can be attributed to reducing the pump load, or system pressure required for the distribution of water through a scale-free, unrestricted plumbing system.

The availability of high-power, rare-earth element magnets has advanced the magnetic technology to the point where the technology has become more reliable. Similar advances in materials science, such as the availability of ceramic electrodes and other durable dielectric materials, have allowed the electrostatic technology to develop new designs and methods.

The general operating principle for the magnetic technology is a result of the physics of interaction between a magnetic field and a moving electric charge, in this case, in the form of an ion. When ions pass through the magnetic field, a force is exerted on each ion. The forces on ions of opposite charges are in opposite directions. The redirection of the particles tend to increase the frequency with which ions of opposite charge collide and combine to form a mineral precipitate, or insoluble compound. Since this reaction takes place in a low-temperature region of a heat exchange system, the scale formed is non-adherent. At the prevailing temperature conditions, this form is preferred over the adherent form, which attaches to heat exchange surfaces.

The operating principles for the electrostatic units are much different. Instead of causing the dissolved ions to come together and form a transient non-adherent calcium based particle, a surface charge is imposed on the ions so that they repel instead of attract each other. Thus the two ions, (positive and negative, or cations and anions, respectively), needed to form scale are never able to come close enough together to initiate the scale-forming reaction. The end result for a user is the same with either technology; scale formation on heat exchange surfaces is greatly reduced or eliminated when the technology is applied immediately in front of or within heat exchange surfaces.

Electrostatic and magnetic units, when installed, operated and maintained correctly, provide only for scale control immediately downstream of the actual installation. No biological or corrosion control is offered by these systems. (Return to top))

Ozone
Another technology widely promoted over the past several years is ozone based. As is often true with innovative technologies, the claims of ozone should be evaluated separately from the basis of the technology. Ozone, primarily an oxidizing biocide, provides several advantages over traditional oxidizing biocides. There is also a belief within the industry, (and some evidence), that under certain conditions, ozone performs as a descaling agent. The premise is that ozone oxidizes the biofilm that serves as a binding agent, adhering scale to heat exchange surfaces. When scale buildup on condenser tubes is reduced, higher heat transfer rates are achieved. Increasing the condenser heat transfer rate will reduce the chiller head pressure, which then allows the chiller to operate more efficiently and consume less energy. As the water in a tower evaporates, dissolved solid concentrations are increased within the recirculating water. In addition, biofilms also begin to form on the walls and other components of the tower. In essence, the biofilm may act as a bonding agent for mineral micro-crystals and over a period of time, deposition of organic and inorganic matter increases scale thickness. Ozone can loosen and remove scale in the event biofilm exists as an adherent, however, may be ineffective in removing the scale with the absence of a biofilm. It should also be noted that biofilm may not be the dominant contributing factor of scale development when the temperature of the heat exchanger is in excess of 135°F. Scale-forming minerals are less soluble at these higher temperatures and can deposit from solution directly onto pipe walls.

As a gas, ozone's ability to address large systems or biological demands at elevated temperatures is severely impacted. Further, although there are significant cases in which descaling has been noted, ozone does not prevent scaling or alter the scaling potential of cooling tower water. (Return to top))

Coordinated electrochemical/physical alternative
Recently, the integration of an electrochemical flow cell, (generating copper and silver ions), with an electromagnetic field and transient particle formation, (supporting a crystalline growth mechanism capable of suspended solids removal), has developed into a unique treatment alternative for cooling tower operators.

Biological Control: The bactericidal effects of electrolytically generated copper-silver ions on bacteria and algae are well documented. In comparison to chlorination, where a residual chlorine level must be maintained for several hours to effectively oxidize the biological material within a sump or cooling pond, copper and silver ions act continuously at low levels, (generally 0.2 - 0.4 PPM for Cu and 0.02 - 0.04 PPM Ag). The action of these ions is biocidal rather than bioinhibiting as in the case of chlorine. Temperature independent and pH tolerant to 9.3, copper and silver ions provide an aggressive biological control as experienced with ozone on a continuous basis without the associated oxidation of system components or ozone generating issues.

Scale and Hardness Control: Scale and hardness control within tower cooling waters may be accomplished via a non-chemical, ionization processes. Supporting features of an integrated ionization system are, electrocoagulation, mineral precipitation, crystallization and filtration. The main principles employed are associated with an electroflocculation of suspended and colloidal particles, which is achieved with small controlled quantities of metal ions being dissociated from the flow cell's anode, resulting in the generation of metal hydroxides to assist the flocculation of the suspended particles. Additionally, electrostriction causes suspended and dissolved particles to be stripped of their charges as they pass through the flow cell. These particles may then precipitate within the filtration media and be removed via the system's backwash cycle.

Crystallization of scale forming species within the engineered environment provides a reduction in water scaling potential with the physical removal of calcium and associated cations. To accomplish this, a vessel containing filtration media with a high affinity for those cations responsible for scale formation is placed in the feed water stream or within a side stream system.

With the excessive exposure to calcium, the media bed, (by way of equilibrium constant), maintains the vessels' aqueous environment which becomes saturated with scale forming cations. The micro environments immediately surrounding the individual media particles approach a supersaturated condition as additional cations are introduced and natural absorption and desorption occurs within the media. The media, selected for both its ion exchange capabilities and its unique surface structure, encourages precipitation of the scale forming species on its surface. Additional cations and seed crystals entering the vessel from the feed water serve to further induce crystalline formation on the surface of the media. As aragonite and calcite crystalline growth occurs upon the surface of the media, the scale forming species within the water are precipitated within the vessel and adhere to the media as a calcite and aragonite formation.

Solidification of the media bed and filtration vessel is prevented with a controlled, periodical water flow reversal and bed expansion. The internal vessel agitation created by the "backwash" cycle causes the crystals to become dislodged from the media and removed from the vessel via the backwash to waste process. The removal of the crystalline formation from the surface of the media is due to the difference in the Mohs hardness value of the media and the calcium based crystals. Calcite, with a hardness of between 2 - 3, and aragonite with a hardness of 3.5 - 4 are easily dislodged from the media during the backwash process, as the media with a hardness greater than 5, repeatedly strikes the crystals in a flurry of agitation.

The precipitation and crystalline formation within the vessel allows for a gross reduction of scale forming species within the water. In recirculating cooling tower water, both aragonite and calcite formation on the surface of the media results in the removal of calcium from the systems water. This action ultimately "softens" the water by reducing the concentration of "hard" cations, rather than the traditional methods of softening via the replacement of hard cations with soft cations. In that the calcite crystals will incorporate Fe, Mg, Mn, Zn and Co, a variety of cations may also be removed as a result of the integrated process. Basically, the overall effect is the removal of the cations, reduction of hardness, reduction of total dissolved and suspended solids to approximately 5 microns and the creation of a descaling circulating medium which reverses the scaling process within the contact surfaces of the system.

There have been a number of significant developments within the past few years pertaining to non-chemical treatment systems for cooling towers. Could it be that there is now an economical, meaningful solution available for the more than 500,0000 cooling towers in the country? A system that offers corrosion control, suspended solids control, biological control and scale control with water savings, certainly would be appealing to many facility engineers. (Return to top))

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