Increase Cooling Towers Concentration Cycles

[jisita]

Hi, I am a Mechanical Engineering from Mexico. I am responsible for water savings projects in the plant. One of the biggest opportunities areas that I saw is in the cooling towers. We have two towers with the following characteristics.

Tower # 1 Tower # 2
Water input (m3/day): 161 270
Evaporation (m3/day): 64 104
Blowdown (m3/day): 97 165
Concentration cycles: 1.66 1.63

Blowdown characteristics:
PH 7.57
Conductivity of rejected water 1800 microsiemens.
Halogen 1.33 ppm Cl2
Silica 130 ppm
Zinc 0.57 ppm Zn
Hardness 470 ppm CaCO3
Total alkalinity 61.43 ppm CaCO3
Iron 0.25 ppm

The make-up water, is deep well water with the following characteristics:

pH – 7.5
Conductivity of rejected water – 800 a 1100 µS
Sílica – 75 – 90 ppm
Zinc – < 0.03 mg/L
Total Hardness – 200 – 250 mg/L
Iron – 0.436 +- 0.010 mg/L

We treat the cooling towers with Multifunctional Chemicals, Sulfuric Acid, etc, supplied by Nalco. We have an automatic blow down system that purges when water reaches a conductivy of 1800 microsiemens.

I will appreciate your responses… I really need to save more water…

 

[Muggle]

Dear Jisita -

You should be able to accomplish two things here - first, treat the deep well water to remove the silcia, zinc, and iron - thus reducing scaling potential and reduce the acid chemical use. 2nd, you can clean up the blow-down water and re-use it, removing the silica, zinc, iron, and other heavy metals. Sodium can't be removed but by RO - but chlorides can be reduced 40-60%.

We have demonstrated electrolytic removal of these and other metals and mineral compounds for National Gypsum, Phosphate Mine Council, Tampa Bay Electric, and others - demonstrating potential "new" water reductions by 75% and saving on filtration. Suggest you contact Art Suma at AWS Consulting in Tampa, Florida - he knows the cost savings, capitol costs, etc. - as he consults to these and other large industry and municipal water/powr plans. He can be reached at 813-655-9406 (phone and fax) - He can also be reached by email at mrb.fla@verizon.net, but can be really slow at returning emails - best if call or fax to him.
Good luck -
DAve/Aquatic Technologies
(aquaticonsult@yahoo.com)

 

[GarySCWSVI]

Jisita,

Consider using ion exchange equipment to pretreat the feed water to the cooling tower. I suggest at least a Weak Acid Cation (WAC) system. Since your silica level is also high you should also consider a Strong Base Anion (SBA) system to reduce silica and TDS and anions like Sulfates and Chlorides. You should contact either your Nalco rep or a local water treatment equipment supplier for more help.

A WAC & SBA will very effectively remove problematic cations and anions and will result in a feed water that will allow you to drastically reduce blowdown water.

Whatever, you ultimately decide, make sure that the system you select uses NSF Certified components.

Gary Schreiber, CWS VI
The Purolite Co.

 

[bimr]

Why are you so concerned with water savings? The answer to that would go a long way to providing you a recommendation. Is water expensive? Is wastewater discharge expensive? Do you have the money and time to run a water treatment system?

 

[jisita]

Thanks very much for your responses...

In addition we have an unused Reverso Osmosis unit (We used it before replacing it by a newer one). Do you think that it could be cost effective to use it for treating the blowdown of the cooling towers?

Also, if you can gave me an approximation of the costs of the systems that you are talking about would be really nice.

garyscwsvi:

Thanks very much for your response, I have contacted a local water treatment equipment supplier that will give my an estimate of the costs of doing that.

bimr:

I’m concerned about water savings because we have a limited supply of water in the plant (1 Deep Well) and there are many restrictions in water extraction in the zone where the plant is. So we need to optimize the use of water.

The discharge is free (We irrigate all the discharges in our gardens) and the cost of the well water is nearly 1 US$/M3.

 

[bimr]

From the information that you have provided, you are limited by the silica in your raw water to the ~1.6 concentration cycles.

However, this is a good application for RO. I would go back to the RO supplier and ask them to give a recommendation on the use of the RO. Have them provide you with the cost to operate the RO system and then you can calculate the cost benefit for use of the RO.

After you get the recommendation from the RO supplier, go back to Nalco and have them determine the recommended cycles of concentration. With RO usage, you should be able to greatly increase the number of cycles.

The reject from the RO would probably have to be discharged. The salinity of the RO reject water is probably going to be too high for usage in a garden.

You probably do not want to get involved with ion exchange. Most people would not want to mess with the chemicals with such a small flow application. However, if you do decide to go with ion exchange, consider the use of service ion exchange equipment. Service equipment is when people deliver (and pickup) the ion exchange bottles (with resin) and regenerate the ion exchange resin offsite. With service equipment, you do not have to mess with the chemicals or regenerant wastes.

One additional note: You do not need to treat the entire stream. You can treat a portion of the stream and then blend it back in. For example, you can treat half the flow and blend it back in. With ion exchange treating half the flow, that would reduce the TDS of the blended water by half.

 

[BobPE]

I would avoid equipmet manufacturers like the plague. I worked on some power plants in Mexico and it seems to be a free for all when the vendors try to perform engineering. My take on it was that the laws for engineering are much weaker down there.

RO/Nano can be a very cost effective solution given you water use constraints. The projects I worked on also had severe water use constraints and the seasonal variation in water availability was also a problem. You may be able to treat RO reject with RO a second time, this would get your treatment waste stream down to only a couple of percent.

Like bimr said, ion exchange or other physical/chemical treatment can chemical intensive and that is what the chemical companies prefer, only they are shipping you chemicals at outrageous prices from the states most of the time. The system I worked on treated about 20 MGD of a total 80 MGD flow and was a side stream and was a combination RO/Nano filtration. Power was cheap since it was made right at the plant...I cannot recal the cycles of concentration off the top of my head, but thee values were over 3 I know.

Let us know how you make out....

BobPE

 

[LHA]

The steam released is just under 40% of your total water usage. Its condensate would be almost pure water, and would therefore dilute your other concentrations at the same time, greatly reducing pretreatment.

Is it feasable to capture any of the evaporated water and re-condense it? Perhaps a pre-heating system for the primary steam is possible. I don't know what your process is, and a new heat exchanger will be a very high initial cost. But it could pay for itself, unless the well water is free (unmetered), then it would probably not be cost effective. It is worth a quick investigation, and will make your Feasability Report more complete.

 

[cub3bead]

The RO would be just as prone to silica scaling as the cooling tower so I can't see and advantage there.

Lime softening with magnesium chloride feed in a reactor clarifier would address the metallic ion problems and precipitate the silica.

My guess is that your raw water is fairly alkaline and that you are using tremendous amounts of sulfuric acid. This would be reduced significantly by the lime softening process.

As an alternative, have you considered using one of the alkaline treatment programs?

have you tried one of the silica scaling inhibitors?

 

[jisita]

Well, thanks very much for your responses.

I have contacted an engineering firm, and they suggested me and Ion Exchange System for treating the rejects from:
- Cooling Towers
- Reverse Osmosis
- Boilers
. Softeners regeneration

They are talking about removing 95-98% of the hardness of the water, and 95-98% of the Silica. The system is a Strong Base Anion and a Strong Acid Cation. For treating 260 m3/day they are talking about a system of US$ 300,000, and a cost for treating the water of US$ 0.20 including chemicals.

How do this numbers look?

 

[bimr]

If the engineering company suggested treating the rejects, then you should run away and find yourself another engineering company.

It is much easier to treat a clean stream then a wastewater stream laden with dirt, crud, and organics that will be aborbed from the air. It can be done, but it is a pain in the neck. And ion exchange is not the best method for treating the rejects. If you used the RO on the makeup side, then you would want to go with an evaporation process on the waste side. I was down in Monterey and saw evaporation ponds in use there for that purpose.

Previously, you said that you had a free RO unit. Why would you be interested in buying more equipment?