Reuse of RO water in the Cooling Tower

[LIKALA7]

Has anyone reused the rejected water from a RO plant as make-up for the cooling tower in an air-conditioning system? How does this affect the chemical treatment of the condenser water system?

 

[SAK9]

RO water is devoid of any chemicals and as such very corrosive.Cooling water pipework of carbon steel may not withstand it

 

[LIKALA7]

Thanks SAK9. I am not sure if I was clear in my question.

A hospital client of mine uses the RO plant to generate pure water for use in dialysis.

It is my intention to reuse the water rejected from the RO plant. I understand this rejected water has approximately twice the concentration of dissolved solids as scompared to the potable water fed to the RO plant. At present this rejected water is drained away. I am hoping to use this rejected water in the cooling tower substituting for the potable make-up water currently used, thereby saving water. I would appreciate your help.Thanks

 

[quark]

That depends upon your reject water chemistry. You should check for pH, Total Alkalinity, Silicates, Calcium, Magnesium, Iron, TDS and Chlorides etc.

Generally high pH, silicates, Mg and Ca(which slip from the softener) in the RO reject characterize the scale forming tendency of RO water. Either you have to reduce COC of cooling water or increase chemical dosage.

Regards,

 

[MedicineEng]

I believe that you can go ahead. I had the same idea in my plant. Just didn't finish because meanwhile the PW plant was stopped so no meaning to extend the rejected pipe til the cooling water tank. If one day we restart with the plant I will finish the connection.
Since as you well note RO rejected water has much more dissolved solids, you can expect a little bit more scaling. Furthermore, since RO water is also chlorine free, you should also expect a little increase on the chlorine consumption for bio control.

 

[SAK9]

I do not see any harm in reusing waste water from the RO generation skid as it wont be corrosive.The high temperature inside the condenser tend to precipitate the chemicals as Quark highlighted.This could be overcome if you have a make up water tank for the cooling tower.The RO waste water could be led into the make up tank to mix with normal water and bring down its concentration of chemicals.

Youc could also try and recover condesate from the air handlers and use it as make up water for the towers.

 

[LIKALA7]

Thanks all of you for your responses. They have pointed me in the right direction. I will test a sample of the rejected water and at the first instance try to restore the concentration of dissolved solids by mixing with fresh potable water if that will still enable me to save all the water rejected by the RO plant. If not, I will account for the extra chemical required to neutralise the excess concentration. Thanks once again.

 

[Whoops]

The last DI water plant I dealt with had a softener, a RO machine and then a de-ionization plant. If it's like this, the RO reject stream would contain more TDS than the feed, but these would be mainly sodium salts. The dissolved sodium salts should remain in solution, despite the cooling tower evaporating some of the water. I don't think they'd cause a problem. I'm vague on the water requirements for cooling towers beacuse they're fairly out-of-fashion in the UK, due to the legionella hazards; I've never dealt with one.

If the RO inlet isn't softened, then I wouldn't re-use it in the cooling tower. There would also be a reject water stream from the DI plant. This has low TDS and should be re-usable if mixed with some mains water.