Hello everyone,
We're actually considering switching from acid to carbon dioxyde since we're rejecting too much sulfate according to the environmental regulations. The only information we're getting are from commercial selling those methods so there is no information about the actual disadvantages of using carbon dioxyde.
Disadvantages such as the cost of maintaining the facilities or the impact on the environment of carbon dioxyde emission are what we expect but cannot really be sure of.
If you have experienced any of those downside or have come across publication that may confirm or be relevant about the subject I'd be glad if you could provide me those information.
Thank you all.
The exact nature of "downside" would depend upon the composition of the alkaline wastewater and the requirements placed upon the effluent from your system, but almost any carbonation process is going to result in the formation of salts and precipitates. CO2 forms carbonic acid (H2CO3) when dissolved in water; and it is carbonic acid that leads to the neutralization of alkalinity in solution. Essentially by carbonating you are using a weak (non-sulfur) acid instead of strong acid (sulfuric?) which could strongly effect your sulfate problem. https://www.jim.or.jp/journal/e/pdf3/52/10/1983.pdf, http://aaqr.org/VOL12_No5_October2012/8_AAQR-12-06-IR-0149_770-791.pdf, and http://pubs.acs.org/doi/abs/10.1021/ef900615h?journalCode=enfuem may be of value. The last two are more focused on using the process for sequestration of CO2 than neutralization of alkalinity but it's the same issue from two different perspectives. This presentation (slide deck) may also be of value http://cpe.njit.edu/dlnotes/CHE685/Cls06-1.pdf
It is very important to realize that adding CO2 doesn't change the alkalinity of the water as adding sulfuric acid does. It just lowers the pH.
The consequence is that effect on the sewage treatment plant of adding the acid will disappear as the water will get to equilibrium with atmospheric CO2 during the aeration. So by adding CO2 you can prevent high pH in your effluent, but the sewage treatment plant might experience an increased pH during treatment. If this is a problem depends on the alkalinity and amount of other wastewater that the sewage treatment plant receives.
Another concern is precipation of most polyvalent kations as carbonates. This can be managed (add the CO2 in a settling tank to avoid precipations in pipes). The process can remove some heavy metals from the water which might be better for the sewage treatment plant.
Dear friend First of all what is the unit consumption of CO2 for neutralizing a unit of alkalinity. Secondly what would you think if you mix the alkaline waste with the acidic waste (from pickling, anodizing and polishing) in neutralizing tank before disposal. Taking into consideration the ppt of aluminum hydroxide.
CO2 doesn't neutralize alkalinity.
If pH is very high CO2 can give two protons per mole to lower the pH.
Another mineral acid you could add might be HCl to lower the pH. This would reduce the use of sulfuric acid. One of the heavy metals which would require lowering the pH is the ammoniacal complexation of copper metal in the wastewater. My experience in the wastewater treatment in to lower the pH to about 3 using a combined reagent of aluminum chlorohydrate with a small amount of sulfuric acid. Then after breaking the bonds of the ammoniacal metal complex, we woul bring the pH back up to about 8 or 8.5 before addition of polymers to sink the heavy metal into a sludge. All these steps are first done in a large beaker in the lab to make sure of the success in precipitating the heavy metals. Removal of the sludge and sending it to the sludge press is usually a good practice. The clear liquid over the top at pH 8 or above which contains no more heavy metal ions can be discharged safely with limited amount of sulfate.
Good luck to you!
Eugene Lo Wei (Hands-on industrial wastewater treatment for more than 20 years.) [email protected]
Several notes from experience.
1. Cost and good availability is always an issue.
2. If they go from a sulfate to TDS limit, you have a problem again.
3. This applies more to organic streams, but you can create organic carbonates that can cause wastewater problems.
4. Also, in wastewater, you can get entrained CO2 which can cause floating sludge.
5. The reaction is slower and it can lead to people outgassing CO2. That is a waste.
One thought to consider is to use a dual system that would hit the stream with H2SO4 for a bulk reduction in pH and then polishing with CO2. Would that get you below the sulfate limits?
First of all, I'd like to thank you all for your fast and precise answers,
as you deduced what we're using for now is pure sulfuric acid to reduce the pH of the wastewaters (from all the rincing bath: anodization, etching...) which when blended together have a pH of about 11.5 .
We first tried to reduce this pH by controlling the flow of water in the rincing bath of etching which have a pH of 12.5. But the added water wasn't enough to reduce the pH in this rincing bath.
We also experimented in the lab the use of lime (first we reduced the pH at a value of 5 using sulfuric acid, then we increased it using liquid lime to a pH of 8 which is favorable to floculation) but the experiment still wasn't succesful since we were still beyond the sulfate limitation.
In terms of availability and cost our partner thinks that using CO2 is cheaper than controlling the water flow, or using another type of acid and since we don't use heavy metals they aren't an issue for us.
I understand now the reaction that will occur when using CO2, the pH will be lowered thus basicity will be reduce but the alkalinity will remain. Which in itself doesn't sound like a problem to me as long we avoid a sudden increase in pH when the equilibrium is reached (As Mr. Andersen said).
However to avoid this occurrence what should be done? Should the alkalinity be reduced as well using a dual system like Mr. Boyter suggested?
I came to learn as well that a sulfuric acid backup system would be necessary anyway.
The main disadvantages I have seen for CO2 pH control in are that it is difficult to get below pH 7 and the equipment costs are higher. If you are still interested in CO2 for pH control, I recommend that you get a price quote for equipment and CO2. Please let me know if you are interested in a referral-- the original post is over 1 year old.
The CO2 comes out again and your water will be as alkaline as before, so that is a waste of energy. look for something acidic that you have around, or use hydrochloric acid (the chloride is irrelevant when it goes finally into the sea).
As mentioned before CO2 doesn't change alkalinity, but it does decrease pH.
Say an industrial wastewater has pH 12 and the sewage authority usually have a upper limit for pH of 9. In such case CO2 can be used and it doesn't come out again but remain carbonate/bicarbonate.
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