hey there,
I operate a 20gpm wastewater pretreatment system that treats wastewater from two parts washers on two powder coat lines in the plant that process cold rolled steel, galvaneal, and aluminum substrates.  They both use iron phosphate for the conversion coating and one is a 5-stage and the other is a 3-stage.  Currently we don't have any limits on our phosphorus discharge (but we know they are coming soon) from our wastewater treatment system but I usually can treat it down to a level of <1 mg/L.  We are looking to switch away from iron phosphate (green initiative) to a zirconium conversion coating but this would introduce fluoride into the wastewater stream.  Our current limit on our fluoride discharge is 2.4 mg/L (shockingly low I know) so in the past we have never used any washer products that contain fluoride so we can keep it out of the stream.  I have been tasked in finding out if our current pretreatment system can handle fluoride and remove enough of it to keep it below our discharge limit.  My pretreatment system is as follows:

In the first tank the pH is adjusted to 7.25 and calcium chloride and aluminum sulfate are added as coagulants.  Retention time is 90 min.

In the second tank the pH is adjusted to 9.25 to precipitate zinc out because our limit is 1.48 mg/L (shockingly low again I know) Retention time in this tank is also 90 min.

Then it enters the inclined plate clarifier where polymer is added and solids are settled out.  Retention time in the clarifier is 115 min.

The clarified water then goes to a final tank where pH is adjusted below our high limit of pH 9 (another shocker, our limits are not industry friendly) and then discharged.

Back to the question: does this sound like an efficient way to remove fluoride to a level consistently below 2.4 mg/L?  From what I have read calcium chloride and aluminum sulfate are the preferred coagulants for removing fluoride but I don’t know how low of a concentration it could get the fluoride down to.  From the calculations I have been doing the pretreatment influent will contain anywhere from 10 to 50 ppm of fluoride.  And I will be doing jar tests soon to determine the fluoride removal efficiency of this current treatment recipe.  Is there anything else I should be trying out with these jar tests or should expect with this current treatment strategy?  Thanks for the help in advance.
Adam

Hi... May i know what is your estimated concentration of incoming fluoride?

yinyu83 wrote on Nov 3^rd, 2011, 8:48pm:

It could be anywhere from 10ppm to 50ppm.  Do you think this is a efficient treatment recipe or is there something else I could add to the recipe?  
I will post jar test results when I have them.  Usually take a few weeks for the results.
Anyone have any better ideas or is this a good path to start on?

The solubility of CaF₂ from Wikipedia:

Solubility in water 0.0015 g/100 mL (18 °C);
0.0016 g/100 mL (20 °C)

If you supply enough Ca⁺⁺ ion, CaF₂ should precipitate and Al(OH)₃ should help settle any colloids formed (perhaps even at 7.25 pH.

What are your other pretreatment limits? My other concern is that you'll be trading a zinc and potential phosphorus limit for a possible zirconium limit in the future; so be aware that this is a relatively new surface treatment which doesn't have discharge limits yet.

grrun

What's the feasibility of separating out the fluoride-containing portion of the waste for separate treatment by RO?

Thanks for the replies...
Unfortunately the fluoride containing streams cannot be separated because of the parts washer setup we will be using.  The washer uses counter flows as makeup water so the whole washer will contain some fluoride from the stage with Hexafluorozirconic acid in it.

Adaman

Stage 1: Alkaline cleaner
Stage 2: overflowing RO rinse
Stage 3: RO rinse
Stage 4: zirconium conversion coating
Stage 5: RO rinse
Counter flows are as follows:
5 to 4
4 to 3
3 to 2(overflowing to treatment system)
2 to 1

Because of the carryover of stage 4 into stage 5 and then stage 5 couterflowing into all the other previous stages, all stages will contain fluoride.

adaman

I need some advice, my company is a fiber optics co. our waste water comes from a wet scrubber that scrubs the Machinery’s Exhaust air, of chemicals. Our water usually has with in it, Boron Trichloride, silicon tetrachloride, silicon tetrafluoride, with high fluoride level (100-550ppm)

I need to get my fluoride down below 20 ppm. I have a 2400 gal tank, when the waste water reaches 1900 we test and treat the water. The automation system automatically keeps the PH of the water at 9.30 (with sodium hydroxide). We have a metering pump on a 50 gal tank to send calcium chloride up the to the main tank to treat the fluorides (in the 50 gal tank we mix a 50 lb. bag of calcium chloride flakes with 30 gal of DI water, let mix for 5 min and send to main tank.) and we also add 1/10 gal of ultra-bright, 250 ml of alum (Aluminum Sulfate), and 1/3 bag of fiber (chem formula: C6H10O5)N). let recirculate for 30 min (to let chemicals react) then filter the water through a filter press.
The outcome: the fluorides drop to about 50ppm after treatment/before filtering, after we have treated the tank and our fluorides reach 50 ppm, we have to usually add 2 more 50 lb. bags directly to the 2400 gal tank (known as slug feeding) to get the fluorides under 20ppm.
Questions:
1. Is there any way to better the automation system?
2. Is there any other chemicals that can be added to treat the tank more efficiently?
3. Should we change the ratio of DI Water to calcium chloride flakes?
Companies Goal:
           To find an inexpensive solution.