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ADVANCED COOLING TOWER WATER TREATMENT

 

Recent advancements in cooling tower water treatment are revolutionizing the management of cooling towers.  Innovative techniques developed over the last few years around the world will reduce by about one third the water that is used in cooling towers today.  This will save the country a significant amount of water that is being wasted today.

 

This revolution is the result of the serendipitous marriage of the following technologies:

  1. The application of precipitation technology,

  2. The use of electrical scale control, and,

  3.  Ionization water treatment technology to control microbiological growth. 

 

Precipitation Technology

 

Precipitation technology is well utilized by the wastewater treatment equipment manufacturers such as makers of industrial evaporators and companies growing crystals.  The technology is well established.  It involves removing solids from supersaturated solutions by precipitating them out in crystal form.  For more information see “Theoretical Introduction to Precipitation Phenomenon”.

 

Recent work by H. Elfil and H. Roques has made a significant contribution to the understanding of the scaling processes observed in cooling towers today.  Their work shows that when solutions with calcium carbonate are allowed to reach the Solubility Product of amorphous calcium carbonate (ACC), approximately 1,200 times that of calcite, that the precipitation will be almost immediate, with predominately homogeneous germination.  These results are often reproducible.  However, if the concentration of calcium carbonate only reaches a level between the solubility products of ACC and monohydrate calcium carbonate (MCC)), approximately 20 times that of calcite, then the results are much different and often not reproducible.  Germination is slower, the quartz crystal microbalance (QCM) is time to displaced, there is barrier wall material dependence, and the germination is heterogeneous (increasing as the ionic activity product approaches the solubility product of MCC.  Unfortunately these are the conditions under which most of the cooling towers using chemical control are now operated.

 

Clearly, in order to have control over the scaling phenomena, the system must be allowed to operate at conditions that will allow the calcium carbonate to precipitate out as amorphous calcium carbonate.  This means to allow the water to reach its saturation equilibrium for all of the dissolved components. 

 

Amorphous calcium carbonate is a hydrated form of calcium carbonate.  This unstable compound is spherical with a sub-micron diameter.  Within a few minutes of formation, it transforms into one of the more stable anhydrous forms of calcium carbonate (calcite, aragonite and/or  vaterite) at temperatures around 15oC to 30oC.  The type of calcite that develops depends upon the chemistry of the water.  Other metal ions compete with calcium for the carbonate ion.  Pure calcium carbonate is rhombohedral and is the hardest form of calcium.  Fortunately in cooling tower water, normally there are many other metal ions that act as contaminates for the calcite.  The form of calcite crystal will depend on which other metal ions are available in the water.  Each crystal has unique shapes and properties.  K. Pitt, et al of the Crystal Science Group at Keele University have done an excellent job of showing and describing many of the various calcite forms. 

 

Magnesium is usually the contaminate with the highest concentration(after sodium and calcium) in the water.  When crystallized at a ratio of 5:1 the calcite crystal is truncated prismatic.  At crystallization ratio of 1:1, the calcite crystal is negative rhombs.  At a ratio of 1:10, it is called aragonite and the crystal is acicular.  The initial form of crystal is not that important.  Other studies have shown that many contaminants, such as Zinc, iron, and silicon, have a similar affect as magnesium on the formation of calcite.  What is important is that the water contains contaminates that do not allow the calcium to precipitate as pure calcium carbonate.  Of the many different crystals that can form, because of the prevalence of magnesium in most water systems, aragonite is usually the most common compound.

 

Which crystals form is governed by the principles of thermodynamics.  Gibb’s free energy of formation and of reaction can be used to predict the behavior of the calcite based upon pressure, temperature and time.  A compound can change from aragonite to calcite (and the reverse is also true).  There is an equilibrium boundary between the two based upon temperature and pressure.  One side of the boundary aragonite is stable and the other side calcite is stable.  On the boundary, the compound is in equilibrium.  At higher temperatures calcite is more stable.  The standard Gibbs energy of formation of calcite is -1128.79.  The standard Gibbs energy of formation of aragonite is -1127.75.  Thus at standard temperature and pressure calcite is more stable.  This means that even with the other ion contaminants, energy forces will favor the formation of calcite in cooling towers.

 

Another consideration in precipitation chemistry is Ionic strength effect.  Debye Huckel theory states that solubility product of a compound is equal to the activity coefficients of the components times their concentrations.  When total ionic strengths increase (by other compounds) the activity coefficients will decrease.  Therefore, molar concentrations must increase.  Alakendra N. Roychoudhury’s paper LEC06 gives an excellent overview of the water chemistry involved.

 

In summary, in precipitation chemistry mode, the higher saturation levels of all of the metal ions will tend to discourage the formation of calcite, but, thermodynamic forces will tend to encourage its formation.

 

R. J. Ferguson and A. J. Freedman (in a paper on Ozone) noted in their conclusions: “Analysis of the water chemistry data for a number of recirculating cooling systems treated solely with ozone show that calcium carbonate (Calcite) scale forms most readily on heat transfer surfaces in systems operating in a calcite saturation ratio in the range of about 20 to 150.  This range is typical of conventional alkaline cooling water operations.  At much higher saturation rations, above 1,000, calcite precipitates in the bulk water.  Because of the very high surface area of the precipitating crystals compared to the metal surface in the system continuing precipitation lead to crystal growth in the bulk water rather than scale formation on heat transfer surfaces". These results agree completely with the results of H. Elfil and H. Roque in the above mentioned study. Although not conclusively proven, it seems that the main role of ozone may be to keep the system clean and free of befouling that can encourage scale formation.”

 

Electrical Water Conditioning

 

The water chemistry will determine which tendency is predominate.  However, with the addition of electrical water treatment, we can control this variable.  Adding electrical energy (work) to the system now changes the energy equation.  When the electrical energy is properly applied, it will make the water more reactive by breaking some of the hydrogen bonds.  This increases the solvency properties of the water.  Calcite that has already formed in the system now has the tendency to dissolve.  This can be seen by the sloughing off of the existing scale in a cooling tower when electrical scale control equipment is first applied.

 

Electrical water conditioning also appears to have the affect of moving the aragonite/calcite equilibrium boundary towards the higher temperature direction.  This means that aragonite stability is achieved at higher temperatures thus eliminating the tendency to form calcite at the typical temperatures at which cooling towers are operated.  This means that now the tendency of the calcium carbonate scale is in the aragonite form.

 

Ionization Water Treatment

 

From the above, it should also be clear that the chemical reactions that take place in the cooling tower precipitation process will be highly dependent on the chemistry of the makeup water.  Until recently this has been a major factor in the failure of many attempts at using non-chemical methods for scale control.  However, with the addition of ionization, we gain control over one more of the important ingredients for successful water treatment.  With ionization with electrodes of zinc, copper, or other metal, we do not leave to chance the type of scale that will form in the cooling water system.

 

Marriage Of Technologies

 

By marrying these three technologies, we now have Advanced Water Treatment for Cooling Towers.   By using precipitation chemistry we now have control over the scaling phenomena in the cooling water system.  By using electrical water conditioning we can control the kind of scale formed.  By using ionization water treatment we can ensure adequate water chemistry for successful water treatment.

 

Scale Removal

 

The scale will continue to accumulate in the cooling tower as water is evaporated and makeup water is added.  The calcium carbonate will precipitate out increasing the size of the crystals until if allowed to grow big enough, they fall out of the water as soft scale.  It is important to ensure that the scale crystals are removed from the system on a continuous basis so that they do not have a chance to change their form again and compact into hard scale.

The easiest manner of removing the crystals is by mechanical filtration.  This can be done by sediment filters, most commonly multimedia bed filters.   These filters are considered one of the most efficient ways to remove sediment as the only water they use is to periodically backwash the filter for sediment removal.

 

Benefits

The advantages of electrical anti-scaling water conditioning are significant:

  1. Elimination of anti-scaling chemicals that pollute the environment

  2. Reduction of water usage helping to conserve a precious resource

  3. Reduction of electrical energy usage

  4. Significant operational cost reductions

The advantages of electrical ionizing water treatment are significant:

  1. Elimination of algaecide and bio-growth control chemicals that pollute the environment

  2. More effective bacteria (such as legionella) control as bacteria develop resistance to biocides and thus the biocides must be changed on a regular basis.

  3. Less system damage due to algae and bacteria attachment

Traditionally, cooling towers had to be bled off in order to maintain a certain level of cycles of concentration.  With chemical treatment, the cycles of concentration had a maximum of about three as the chemicals lost their effectiveness above that.  This requirement necessitated monitoring the total dissolved solids (TDS) in the water and bleed off the water when they reached a critical level.

 

With Advanced Water Treatment of cooling tower water, this restriction has been removed.  This means that now the solids in the cooling tower water can reach their natural saturation levels and precipitate out.  As the water is being evaporated and the mineral salts concentrations start to increase, they precipitate out at the saturation equilibrium point for each compound.  Once stabilized, the cooling water reaches equilibrium with respect to total dissolved solids and pH.  They do not need to be closely monitored as they are not being controlled.  This also eliminates the possibility of problems due to faulty control of chemical systems.  Companies such as Superior Water Conditioners have been using this technology for many years successfully all over the world.

 

Removing the precipitate by a sediment filter allows the water to be used over and over again thus eliminating the costly waste of water to the drain.  This means that in terms of water usage, the cooling tower will approach 100% efficiency with respect to water usage. For a water cooling system operated at 3 cycles of concentration, the bleed-off volume of water for each dynamic ton of cooling is 21.6 gallons per day.  A 100-ton system will waste 2,160 gallons per day to drain.  Yearly water down the drain would be 788,400 gallons.  The only water “lost” in this system (other than the useful water that is evaporated) is the water used to backwash the sediment filter.  Thus, this system eliminates the last main draw back in cooling tower systems.

 

Typical Advanced Water Treatment System

 

The advanced water treatment system typically consists of:

  • a draw pipe at the bottom of the sump, a solids canister to capture any debris before the pump

  • a recirculating pump

  • a sediment filter (back washable)

  • a flow control/monitor/switch

  • the non-chemical water conditioning device

  • the return pipe to the sump

  • the system control box

The water pick-up at the bottom of the sump and the return lines must be properly positioned to ensure the maximum removal of sediment in the sump.  Sump cleanings may need be more frequent if sediment build up occurs in the sump.  Over time, the sediment can turn into hard scale.  It is very important that good circulation occur in the sump.

 

A significant advantage is that these systems can be retrofitted onto existing cooling towers without significantly altering the cooling towers themselves.  The current chemical system just has to be turned off.  The new equipment can be added as basically a non-intrusive system to the cooling tower sump.

 

Scalability And Corrosion Concerns

Since this technology is significantly superior to chemical treatment technology, the chemical suppliers are naturally very concerned about loosing their sales.  Some, less than ethical, companies have tried to distill a fear into their customers that if the customer uses this technology, that it will destroy their cooling tower systems as silicates will plate out in the heat exchangers.  This is a desperate attempt to try to convince their customers not to even try the new technology.  The problem with this argument, as they are well aware (or should be), is that the solubility level of silicates in solution (unlike calcium and some other metals) decreases with increasing temperature.  Thus, the higher temperature in the heat exchangers actually helps to keep the silica in solution.  That means that the last place that the silica would precipitate out would be in the heat exchanger.  Since it would precipitate out at other places, it is not available to precipitate out in the heat exchanger.  In addition, as Stuart Michael of NoAb Biodiscoveries pointed out recently in a French Creek Software Discussion Group, the silica will form insoluble mineral salts with the other metals such as calcium, iron, etc. even at high pH.  The high TDS water in the cooling tower provides the favorable conditions for this.

 

However, scaling and corrosion are valid concerns for cooling tower operators.  For this reason, corrosion and scaling coupons are added to the system to measure these phenomena.  Also, indices exist for the purpose of predetermining the potential for both scale and corrosion.  Some of these include the Langelier Saturation Index (LSI), the Ryznar Saturation Index (RSI), Puckorius Scaling Index, Stiff-Davis Index, and the Larson-Skold Index

In a recent internet discussion Orin Hollander pointed out, “The LSI parameter is related to scaling only, not corrosivity. The factors of scaling and corrosivity are not inversely related.”  Scaling tendency is often calculated using the Langelier Saturation Index (LSI) and the Ryznar Stability Index (RSI) because of their ease of use.  However, the most reliable index is the saturation level (SL)(ratio of the Ion activity product (IAP) to solubility product) of the compound in the water.  Solutions over 1 are supersaturated and over time will precipitate out.  Solutions under 1 will not scale and will dissolve scale over time.  Dr. Paul Dillon pointed out in the same discussion, “There is no correlation between the LSI (or RSI) and corrosion of steel in waters.  They are simply indications as to the deposition or dissolution of calcareous deposits.”  He recommended the Larson-Skold Index for indicating the corrosivity of the water.

Pilot Study For Advanced Cooling Tower

 

A pilot study has been underway since June 23, 2003 at McAllen Texas to show the efficacy of this technology.  Its results confirm the advantages of using such a program for cooling tower water treatment.

 

Technical Papers

  1. Non-Chemical Water Treatment Techniques for Cooling Towers

  2. Non-Chemical Conditioning of Water

 

Advanced Water Conditioning Reference Documents

 

  1. AWWA
  2. DOE/EE-0162, 1998 Federal Technology Alert
  3. Martin Chaplin, South Bank University
  4. S. A. Parsons, Cranfield University
  5. Garratt-Callahan’s Reference Desk
  6. http://www.chem1.com/acad/sci/aboutwater.html
 

 

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