The Dichlorpropane is ...

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The Dichlorpropane is probably not being formed in your system. More likely it is just being concentrated to above the detection limit.  

1,2-DCP has a vapor pressure much lower (40 mm Hg) than water (760 mm Hg). This means that less of it is going to evaporate into the air in the cooling tower relative to the evaporating water.

How many cycles of concentration are you running?  If you are running 6 cycles and assuming that none of the 1,2-DCP is transferring to the air, then the incoming source water may have up to 0.000016 mg/L. If some is transferring to the air, then the source water could be even higher.

I would start and try and find a lab that can give you a lower detection limit on the source water. I would look for something in the range of .000001 mg/L as if the 1,2-DCP is not transferring to the air it may be at much higher cycles of concentration.

For the short term, I would drain and refill the cooling tower and/or decrease your cycles of concentration by about 66% to reduce the expected value to less than the detection limit.

Long term after you determine if you actually have 1,2-DCP in your source water and at what concentration, I would look at the feasibility of using activated carbon or another adsorbent to remove it before feeding to the tower.

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Could you run a small tower beside the big one using the same feed water and see if the DCP is concentrated there also? Otherwise, Activated Carbon or other adsorbent to remove it is your best bet. I would look at renting a relatively large carbon bed and running is directly off the basin to pull the main concentrations down. Then a smaller bed run on a side stream should be adequate.  You currently have about 37 grams of DCP dissolved in the water. At 7 cycles you will have less than 100 mg/day of incoming DCP.

 

From Henry's Law, the very low mole fraction of the DCP and the low vapor pressure mean that relative to the evaporating water far less of the DCP is evaporating. That means that your 7-10 cycles for the water could be 100-1000 cycles of DCP.  Since you are only at about two times the detection limit, you could increase your blowdown rate and you should see a reduction. But adsorbent is probably the better solution.

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