pH drift in wastewater pond 2

[Veta]

As we are transferring wastewater from one impound to the other, we treat it with caustic to a pre-set pH. Setpoint is between 8-9. Incoming water pH stays constant at 3.8. Sporadically, water pH going to the creek will fall out to 5-6.

Investigating carbon dioxide absorption, mixing, but are these enough to make such a difference? We normally get a large dip: with a target of 8-9 ph, we try to achieve 6-9 final pH, but at times it goes lower than 6.

Does anybody have any ideas?

THANKS

 

[saxon]

Veta, A few of questions: 1: Does the waste water inflow rates change at any time to the impoundments? This will affect residence time req'd. for the neut. reaction to take place, not too mention the amount of caustic added to the system. 2. Does the influent water temp. change? This affects the neut. reaction rate. At lower temperatures the rate is significantly lower. 3. Are there any types of batch dumps into the waste water stream that alters its chemical composition. 4. Simply because the pH is reading constant, this does not mean that the concentration of the compound causing the low pH is the same. There may well be greater amounts of acidic material requiring more caustic addition than is allowable given the residence time of the system. 5. Is the system well agitated or mixed? Reactant stratification can and will occur in non-turbulent flow regimes. 6. Has the pH and control system been cleaned and calibrated and repaired on a regular basis?

These are just a few things to consider when trouble shooting a pH control system.

Hope this helps.
saxon

 

[Veta]

Saxon,

Thank you so much for your reply. All of the questions you've asked had given me a bette understanding of the problem. However, is there a cure? Would you recommend installing a treatment plant to ensure that neutralization is controlled prior to dumping the material into the final pond?

Here are some answers to your questions:

1. Water inflow rates do change. I am trying to establish the range. This will not only answer my questions about the residence time, but also help determine whether there is ground water infiltration.
2. Influent water temperature is that of the atmosphere. I suppose, during the colder months, we will be experiencing more problems.
3. The water collected in the waste ponds is the wastewater from all of our processes. Briefly, we have flotation and bleaching processes, which means that the reagents from both processes will be present. The residence time and the compounding effect in the ponds is so great that it's nearly impossible to control the chemical composition of the influent. Point 4 of your discussion is very helpful.
5. Caustic addition point may require more mixing at one of the two points. The flow is turbulent within the transfer pipes. However, the residence time may be a greater issue at this point. Once the treated water is transferred into the second pond, though, it's just sits there. Aeration may be required in this second pond after caustic treatment to make sure that no side reactions with CO2 are taking place.
6. pH control system is in order since the last inspection. We did have the instrumentation problem last time when we've had first experience with lack of pH control.


Some more questions:

1. Would it be better to have feed forward control vs. feedback? I.e. - measuring incoming water pH and dosing the caustic based on the titration requirement. Maybe, we'll need to perform some sort of automatic titration periodically if the incoming water quality changes.
2. Is there a way to address the residence time issue without putting in a treatment plant?
3. Treated water sample was caught at 8.8 pH and was allowed to sit in my office. For about 2 weeks now, the pH has dropped to 6.8 very slowly. I've gotten a small aquarium pump to aerate the 1-gal bucket of water. pH went up slightly, to about 7.1 and stayed there for about 3 days now. I'll turn off the aeration and see whether the pH starts dropping again. Water coloration changed, also, from greenish tint to slightly brown, but the water is much clearer now with aeration.
4. It took 0.75 mL of 25% caustic to bring the pH up to 7.4 (5-gal bucket of 3.9 influent water was caught and treated in field conditions). Theoretically (based on pure water), only 0.12 mL would be required to do the same thing. So our caustic requirement is 6 times as much as what ideal conditions would predict.
5. I will put in place a static mixer at the caustic addition points. Also, I will propose aeration between discharge point to the second pond (treated water), and discharge to the creek to prevent further pH drop due to CO2 absorption. At least, this may help the pH stay steady at a certain point.
6. To close a gap between the set point (8-9 pH) and the process variable at the discharge to the creek, I will have to provide a solution around the neutralization efficiency (treatment plant? feedforward control equipped with automatic titrations). Do you have any recommendations around #6? Do you know of any reliable equipment to perform the titrations automatically?

Thank you so muich for your time.

Veta

 

[moltenmetal]

Hmm- greenish, turning to brown over time, clearing after aeration- is there any iron present in your water? If so, the greenish (iron II) material may be oxidizing slowly, which will lower the pH as a result. That would explain a slow pH reduction on sitting, and the clearing after aeration.

The difference between your "pure water" base titration result and your actual base requirement is the result of buffering capacity of the water. To understand what is buffering the pH (i.e. resisting the neutralization), we'd need to know more about your water composition, and have a titration curve.

Assuming there's no oxidation, re-dissolution or hydrolysis going on, straight acid/base reactions are relatively fast. The weak acid neutralization that's resulting in the buffering you're observing is somewhat slower, so maybe you need to move your pH probe further down the pipe. Mixing helps, and you should definitely put at least a static mixer in the line between your pH probe and your base addition point, preferably as close to the addition point as possible. This will slow the response of the pH controller, so you'll need to re-tune the controller.

If your flow is changing and you're measuring the flow, you should consider using a very simple model-based controller to de-couple the flow component from the pH control loop: i.e. if 50 gpm of flow requires 1 gal/hr of caustic, it stands to reason that 25 gpm of flow will require 0.5 gal/hr of caustic- you don't need to (and shouldn't) rely on PID parameters to decouple such a simple relationship. The PID parameters will then be used to control the actual variability of the water's composition, measurement variability etc.

In my experience, the number one problem with pH control systems isn't reaction rate- it's pH probe failure. From my experience, pH probes become slower and slower to respond the longer they're in service: i.e. they'll still read pH 4 in a pH 4 buffer, but you'll have to wait an hour or a day instead of a minute before the meter will show the correct reading. Inexperienced technicians don't notice, because they're impatient, and the probe will still seem to calibrate during a two-point calibration if it's particularly slow to respond, especially if they don't bother to check a third point. If they don't wait long enough for the reading to truly stabilize, they may push the "calibrate" button too soon- and not detect a defective probe. Depending on the design, the composition of the water they're in etc., pH probes can last periods of time from hours to years before they become too slow to respond properly and hence need to be replaced.

 

[ChrisDerrick]

Veta,

I think Moltenmetal is correct with respect to the Iron II oxidizing to Iron III. I have a similar situation in which the pH of a treated wastewater decreases with time. In my case we treat wastewater high in Iron II. Bleach is added to oxidize H2S and Oil and Grease is removed with a water treatment polymer, pH is adjusted with NaOH. The bleach oxidizes the Iron II to Iron III. The Iron III react with the OH- to form Fe(OH)3. Fe(OH)3 is fairly insoluble but redissolves as pH is lowered below about 7.5. Finally, the Fe(OH)3 is coagulated by the residual polymer and is prevented from disassociating as the pH drops. As a result the pH can dip as low as 5.0 or so even after entering the holding tank at pH 9.0.

Having said all that I would think the Iron reaction is propably not the signigicant factor in your pH change. As Saxon and Moltenmetal suggest you should look at your controls, reaction time, mixing, buffering capacity, etc. first. However, measuring the Iron content of the water is relatively easy and inexpensive. If you are still having problems after optimizing the caustic addition, pH measurement, etc. then investigating the iron reaction may be worth the time a d effort.

Hope this was usefull

 

[redlabel]

what is a more cost effective for oxidation of the iron bleach or hydrogen peroxide?

 

[Veta]

Bleach is definitely more economical.

I appreciate everyone's responses. I am working on optimizing the rest of the system's components. Iron did not seem to be present in large quantities based on my ICP data. We have sodium, sulfur and calcium in decreasing content order.

I've caught a sample of the treated water in a plastic, 1-gallon bucket, and held it in my office monitoring its pH. It's been dropping steadily now for two weeks. I've tried different things with it: closing the lid and opening back up (pH seemed to stop dropping when lid was closed and resumed its drop with an open lid), aeration (downward pH trend reversed by 0.2 pH points and held there for 4 days before going down again). pH had levelled at 5.1 from 8.8.
I believe CO2 has a lot to do with it. We have an anionic polymer that was dumped into wastewater system during the trial. It has a tendency to produce CO2 at breakdown. Lid opening and closing alone with aeration would support CO2 acting up in the system. However, I will definitely follow up on the iron theory.

Thanks again everyone! Please feel free to share any more ideas that you have.

Veta

 

[moltenmetal]

If you're confident that the suspicious green/brown colour change on oxidation isn't due to iron, have you considered biological degradation as a possibility? Your pH decrease seems to be related to or controlled by the presence of oxygen, yet the pH does not fall continuously when you air sparge the sample (i.e. when the CO2 generated is stripped from solution). If something (polymer or otherwise) in your water were aerobically degrading to CO2, perhaps this might explain the behaviour.

 

[DeltaCascade]

Does the bleach oxidant produce elemental sulphur as a byproduct? Sounds like good "bug food". Are sulphates present? Perhaps, as Moletenmetal mentioned, you have microbiological activity, such as sulphate reducing bacteria (which like anaeoribic oxygen poor conditions)? Nitrifying bacteria may also produce low pH.
Faculative and aerobic bacteria may oxidize organics, perhaps explaining the clearing of the water. Perhaps your water treatment copmany, like a Betz or a Nalco, can help you do SRB and aerobic plate count tests to determine microbiological activity.

For what its worth, I have seen an effluent pond where pH drifted up, but not down. Demineralization by ion exchange plant mixed spent acid regenerant on top of spent caustic regenerant in a batch neutralization sump (instead of the other way around). This caused precipitation of alkaline salts ... the salts later redissolved in the downstream pond, slowly raising the pond's pH. Maybe its similar, except with acidic precipitates from some other upstream process, as our friends mentioned above. Perhaps another potential culprit is alum (aluminu sulphate, Al2(SO4)3), particularly if the carbonate alkalinity of the water (and hence its carbonate buffering capacity) is low?

Good luck!