2nd pass R.O. as Open recirculating cooling tower make up

[ak83]

I would like to know is there a possibilty to use R.O. 2nd pass as make up for an open recirculating cooling tower. if yes, what would be the best treatment chemistry and if not why not?

Thanks for youe help.

 

[bimr]

Any high water quality water can be used a makeup to a cooling tower. If you use a higher quality water, then you can increase the concentration cycles in the cooling tower.

Generally the makeup water for a cooling tower is of lower quality and RO quality water is too expensive for use in a cooling tower.

 

[KiwiMace]

Are you talking about finding a use for RO reject water?

If the mineral concentration of the water you intend on using is substantially less than the tower blowdown concentration, then you will get some use out of it. If not, you are better just dumping it at the RO stage as it will be treated, then blown down at the tower anyway maybe with one pass through (on average).

Other than that, the biocide and pH control chemicals will be in proportion to the difference in chemical constitution between your RO water and the normal makeup.

 

[ak83]

@KiwiMace
No i am not talking about finding the use of RO reject water.

I want to know that if its possible to chnage our current make up of 1st pass RO to 2nd pass permeate which is very pure.
I have read that most chemistry used for corrosion protection wont work if 2nd pass RO is used. Because for the corrosion inhibitors to work they need certain level of carbonates and bicarbonates (alkalinity) to form a protection film.
I couldnt find much history and case study on using 2nd pass make up.
If you could explain more i will really appreciate it.

 

[bimr]

You will have the same concentration of alkalinity in your cooling tower no matter what raw water you use. Assume that you are controling your alkalinity to 200 mg/l

If your raw water has 50 mg/l of alkalinity, then you have 4 cycles of concentration. Blowdown is used to control the concentration.

If you treat that raw water with an RO unit, then you will have approximately 5 mg/l alkalinity in the treated water. With RO treated water, that would allow you to increase the cycles to 50 cycles of concentration and decrease the blowdown flow.

This is just a hypothetical discussion. Note that cooling tower alkalinity is usually controlled by adding acid.

I understand that hardness, alkalinity, and silica are the main control parameters for cooling towers.

Generally the makeup water for a cooling tower is of lower quality and RO quality water is too expensive for use in a cooling tower.

 

[KiwiMace]

Out of interest, what is your driver for this change? Increasingly strict discharge regulations?

I have designed an industrial cooling installation in rural Indiana discharging into a natural waterway. In that case, we were ok to mix 1st pass RO with the green filtered well water to meet discharge spec with a useful number of cycles to keep down the overall consumption at the well head. As bimr mentioned, it is a costly addition.

Biocides and possibly corrosion inhibitor are the other factors missing from bimr's list of main control parameters. Unfortunately, my good Betz water conditioning reference is silent on the effectiveness issue about which you are specifically inquiring.