pH Neutralization of Aqueous Waste
pH Neutralization of Aqueous Waste
(OP)
Hi all,
I'm working on an aqueous waste neutralization/discharge system. I have a certain volume of aqueous waste per batch (say 1000 L).
We have pH sensors and transmitters. And we need the pH of the aqueous waste to be within an acceptable range before being discharged into the city drain. (I'm in the process of figuring out what that range is - probably 6.5-7.5 or something).
The system has access to two injection lines, one connected to a barrel of strong acid w/ a meter pump, and the other connected to a barrel of strong base w/ a meter pump. So essentially, we will read the pH of the aqueous waste solution, and based on whether it is too high or too low to discharge to the city drain, we will inject a certain volume of strong acid or strong base.
So this is where my question arises: how do I calculate the required volume of strong base/acid to inject into this solution based on the initial pH reading? Would it be as simple as converting the pH of both solutions to a [H] concentration in mol/L and essentially using M_1*V_1 + M_2*V_2 = M_3*V_3 to solve for V_2? (V_2 being the required strong acid/strong base volume) Or is there more to it?
I understand that we all probably encountered a problem similar to this one back in school, but I can't find any resources that outline the method succinctly. If anyone can help me out (even if it means pointing me to a good resource) it would be very appreciated. I do not yet know the pH of the strong acid and strong base solutions but just use arbitrary values for example's sake if you can.
Thanks so much.
I'm working on an aqueous waste neutralization/discharge system. I have a certain volume of aqueous waste per batch (say 1000 L).
We have pH sensors and transmitters. And we need the pH of the aqueous waste to be within an acceptable range before being discharged into the city drain. (I'm in the process of figuring out what that range is - probably 6.5-7.5 or something).
The system has access to two injection lines, one connected to a barrel of strong acid w/ a meter pump, and the other connected to a barrel of strong base w/ a meter pump. So essentially, we will read the pH of the aqueous waste solution, and based on whether it is too high or too low to discharge to the city drain, we will inject a certain volume of strong acid or strong base.
So this is where my question arises: how do I calculate the required volume of strong base/acid to inject into this solution based on the initial pH reading? Would it be as simple as converting the pH of both solutions to a [H] concentration in mol/L and essentially using M_1*V_1 + M_2*V_2 = M_3*V_3 to solve for V_2? (V_2 being the required strong acid/strong base volume) Or is there more to it?
I understand that we all probably encountered a problem similar to this one back in school, but I can't find any resources that outline the method succinctly. If anyone can help me out (even if it means pointing me to a good resource) it would be very appreciated. I do not yet know the pH of the strong acid and strong base solutions but just use arbitrary values for example's sake if you can.
Thanks so much.
RE: pH Neutralization of Aqueous Waste
Start with this:
https://www.omega.co.uk/techref/phtour.html
RE: pH Neutralization of Aqueous Waste
RE: pH Neutralization of Aqueous Waste
I submit going the second route will open you up to a world of hurt, depending on the AHJ and what sort of teeth they have, and that's without even considering the environment itself...
I had the duty of neutralizing ~100,000 litre batches of acidic air preheater wash water with lime, and noticed that significant precipitation of solids and clarification of effluent took place prior to release; depending on your situation, you could encounter major headaches if solids settle out in your sewerage lines...
CR
"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]
RE: pH Neutralization of Aqueous Waste
RE: pH Neutralization of Aqueous Waste
RE: pH Neutralization of Aqueous Waste
CR
"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]
RE: pH Neutralization of Aqueous Waste
RE: pH Neutralization of Aqueous Waste
Using my personal definition of that phrase, if it isn't too far from neutral, it shouldn't need any pH adjustment to discharge. If I recall, 6.0 - 9.0 is considered ok to discharge as wastewater in most localities. That range by far envelopes "not too far from neutral" for me.
Andrew H.
www.mototribology.com
RE: pH Neutralization of Aqueous Waste
And when it is necessary, it will probably be a very minor pH adjustment, which is why I don't think a crude method of just pumping in acid/caustic solution till the solution is 'in spec' will be ideal; I think overshoot would be a significant issue if this method is used.
RE: pH Neutralization of Aqueous Waste
It might be more time consuming the way I described, but I think it reduces the chance for error and eliminates the need for the relative complexity of the automation which can be time consuming in its own way.
Andrew H.
www.mototribology.com
RE: pH Neutralization of Aqueous Waste
I should also note that, while infrequent, the neutralization process will probably still occur many times (system runs 24/7, neutralization may be needed once or twice a week).
RE: pH Neutralization of Aqueous Waste
1) As the OP suggested in one of his later posts, overshoot can definitely be an issue. You need to "creep up" on the target pH, as explained further below.
2) Unless your tank has very violent agitation, it will have a finite response time. You can't give the OK to discharge the moment pH hits target, you need to stop dosing, wait, and see what the pH lines out at.
3) Suggested control algorithm would be something like:
a) based on initial pH and quantity in tank, calculate rough amount of acid/base needed (this can easily be converted to time if you are using a metering pump of known capacity)
b) add not more than that amount - I'd suggest not more than 95% of it - and wait until pH lines out
c) repeat based on new pH if pH not in target range, else discharge
With experience, you will be able to modify the calculation to take into account any buffering and under/overshoot, and you should be able to hit target on the second addition if not the first. Undershoot and repeat is definitely preferable to overshoot and then having to change which reagent you need to add.
RE: pH Neutralization of Aqueous Waste
We have a class B wastewater facility on-site, and I set up pH neutralization system much as you described (except it is a 15,000 gallon semi-batch). The discharge limits for our waste are 6.0 - 9.5, so I had the controller set up to take no action between 6.5 and 9.0.
FWIW, we feed 2-3 gpm of 25% sulfuric or 50% caustic, and it takes anywhere from 5-20 minutes to raise pH from 6-7 or drop drop it 9-8. We rarely have any drift after stopping feed due to buffering effects in our target region.
You have a tiny volume to neutralize. As a rough start, make sure your acid/base feed is slow enough to let your tank turn over at least 10 times before setpoint is reached. For your application, that probably means using a 0-200 mL/min lab-scale pump or something similar. If you have to increase pH drastically (3+ units), consider taking the neutralization in two steps instead of one, and let the batch mix well after the first step. The 10 tank turnover is a rough starting point - you will have to run neutralization in the batch to determine the response time and any pH drift after neutralization is complete to get a better feel for mix time.