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pH Neutralization of Aqueous Waste 2

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yene

yene

Chemical
Sep 17, 2019
12
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.
 
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You need more information than just the initial pH, unless the water is absolutely pure except for the extra acid or base. Real waters have weak acids or bases in them which produce buffering capacity. The only way to know how much acid or base to use is to titrate the waste to the desired pH.
 
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Do you plan on neutralizing by the batch, iow in a holding tank or tanks, or do you hope to neutralize on the fly?

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]
 
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@moltenmetal The water coming in is not absolutely pure but it is expected to be relatively neutral every time (i.e. these acid/caustic injections will not be necessary at all for the vast majority of batches). So do you think we need to go with the more crude approach of just pumping in acid or caustic solution, depending on if the water is slightly acidic or slightly basic, until the water is neutralized? Instead of calculating the exact volume that needs injecting based on the pH of the solution?
 
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@crshears Neutralizing by batch.
 
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We went with the "crude" empirical approach; agitation plus lime injection until desired pH was achieved seemed to work quite well, and avoided all those annoying calculations...plus in our application the pH only changed slowly, so overshoots rarely occurred.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]
 
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Interesting, thanks. I fear that the empirical approach might not work for our application though since we are dealing with roughly 10,000L of water max. It sounds like overshoot might be an issue, given that the pH of the initial solution is not expected to be too far from neutral.
 
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What do you consider "not too far from neutral"?

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.
 
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@SuperSalad I don't have exact numbers or ranges, however I was told that in most cases, the water will not need any neutralizing. This acid/caustic injection system is anticipated to be necessary only some of the time.

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.
 
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You know your process better than I do, but from the information you've provided, to install an automated process for this infrequent possibility seems overly complicated to me. If it were me and overshoot was a serious concern, I would take a sample, neutralize the sample, scale-up that neutralization for the tank, reduce the initial scaled-up addition to remain conservative, and repeat as necessary.

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.
 
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Thanks, I like that idea a lot. If I can't figure out the math then we might have to go that route. The rest of the system is fully automated, though, so we are going to try hard to automate this if it's possible.

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).
 
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I have done similar systems for a few clients over the years, so I'll offer some thoughts based on my experience.

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.
 
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If you are going to automate this, and have both acid and base fed off of a split-range controller, I suggest you incorporate a healthy deadband of no acid or base feed as long as you are permitted to do so.

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.
 
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