Softening vs pH dosing of cooling towers

[Emvir]

Hi there,

Does anyone know which option is best for the environment
1. Water softening prior to cooling tower through an ion exchange resin which is regenerated, discharging brine to foul sewer Or
2. Acid dosing of cooling towers which should give a reduced bleed off but will introduce acid loading.

Thanks in advance,
Chemicalenvironment

 

[saxon]

ChemEnv, The two process affect water quality differently and are used for different reasons. The first, softening, through an ion exchange resin, affects mineral content by reducing it. Hence less equipmnet fouling due to scaling. The second, pH adjustment, creates an acidic environment. Hence creating an environment unfavorable to the growth and support of such things as molds, slimes, and "snots" which also plug and foul heat transfer equipment. (I take it you have never been on a "Heat exchanger "Snots" detail" ) As with any pH adjustment, it must be held within the proper limits. If not, then corrosion issues become problematic.

As for your question about which is more environmentally benign, probably the sofetining system is. Provided the brine dump is not massive and overwhelms the treatment plant, or upsets the salinity index of the sewers or surface waters.

Hope this helps.
saxon

 

[gkandy]

Scale forming tendency of water can be reduced by the pH adjustment with acid. But this is not a prefered method as it can take water into the corrosive range to the detriment of the equipment and Tower MOC. I would not recommend PH ajustment with acid. Strict monitoring is required.

Make up with soft water is generally used where the emphasis is on zero discharge/ reduced blowdown frequency. Make up with softwater enables one to go higher on the cycles of concentration. But it can take water into the corrosive range. It should be noted that some of the commercially available corrosion inhibitors do not work well when the TDS is below 50ppm CaCO3. A proper selection of corrosion inhibitors is required. Moreover softwater CW tends to eat away Zinc from galvanised iron pipes.

The cost of softwater generation/ regeneration waste has to be viewed against the savings in blowdown and increased cycles of concentration (reduced make up.)

Unless one knows the specific water quality and quantities it will be difficult to say cost wise whether soft water make up or traditional treatment ( reduced cycles ) is economical from cost as well as waste water generation point of view.

 

[FrancisM]

Folks,

Most of the above comments are untrue.

When treating cooling water for scale reduction, the only items removed by softening are calcium and magnesium, reducing chances of mag silicate, calcium mag silcate, and calcium carbonate. If you remove all calcium, you will actually make the water MORE corrosive.

By feeding acid, you are attempting to keep the LSI (Langelier Saturation Index) at a level below the saturation point for the scales mentioned above. You can definitely keep the pH above the corrosive zone for steel and yellow metals (brass, copper, bronze) without going below 7. If you use acid at pH 7,2-7.6 along with scale inhibitors such as phospate, you can keep the exchangers very clean.

One company that can helpo is GE Betz, or email me and I will get you help.

 

[25362]

When speaking of the impact of blowdown water on the environment I tend to agree with gkandy in that one of the most important goals is to reduce the (fresh) water throughput with no detrimental effects.

The injection of chemicals (contaminants) is of utmost importance. As an example, biocides, used for cooling water treatment, may make the (concentrated) blowdown unsuitable for reuse. Any procedure that would minimize the need of fresh water make up would be indirectly beneficial to the environment.

Better integration by using, for example, pinch analysis, could reduce discharges where conventional water reduction programs by process changes had already made significant savings.

Comments are invited.

 

[abcmex]

Emvir,

I doubt seriously that you can compare both treatmentw, they may be used to solve different problems.

You may use softening to remove Mg/Ca ions but the total ionic load will not change because you will be adding Na ions. You substitute the permanent hardness with temporal hardness. As already mentioned in the other postings when you eliminate all ions you may get a higher corrosivity. You will get deposits only if the conditions are given (concentration, pH, etc.)

By adding Sulfuric acid you may lower the akalinity and also the possibilty of deposits but the amounts are really low (or you will get serious problems) so it probably will not affect the enviroment.

At the end only a comprehensive analysis and chemical treatment program can help you. This requires more than a posting and is where companies like Nalco, Betz, etc. make their business and where you will get qualified help.

Kind regards

 

[bwi]

Neither option is good for the environment. We have dealt with both soft water treated towers and acid feed as methods to boost cycles of concentration. Soft water fed towers causes a corrosion nightmare and also tend to get heavy salts of evaporation on the fill from the sodium in the soft water. We have had better results with acid feed to boost cycles, but a very good automated pH control and acid feed system, with constant monitoring, is needed to maintain pH in the 8.0-8.8 range.

Check out this link for a report on another alternative that was evaluated recently by our local utility:

http://www.smud.org/education/cat/cat_pdf/PulsePower.pdf

Dean