Wastewater Neutralization

Does the PH meter have a control output to change the flow of caustic? if this tank is to run unattended, yet must be compliant at all time with water emission standards you will need a PH meter on the output that will shut it off if the PH goes too acid.

With a 6% acid feed and 20% caustic

and the reaction heat is 13,700 calories for each equivalent weight.

so HCL = 36.5 grams/ew, 6% = 1.6ew/liter

and NaOh = 40 grams/ew 20% = 5ew/liter

So one liter of caustic will take 3.2 liters of acid = 4.2 liters = 4200 ML

and we have 5 EW reacting = 13,700 x 5 = 68,500 calories divide by 4200 =16C temp rise.

PVC should be OK as long as the inputs are fairly close to room temperature(20C + 16C = 37C = body heat. Any increase in the input temp will add directly to the output temp.

There are many small PH controllers that you can use here that cost $300-500 or so and have a control output to turn a valve on or off. The valves might cost more than the controllers.

SImplest way is to have the controller turn on a pump and add caustic if PH goes too high, but you must measure after you add the caustic and you can have another in the tank. Feeding the caustic into the draw of the eductors and monitoring the tank PH is another way as long as you make the eductors mix the tank thoroughly and have your controller measure the exit PH.

With 45,000 pounds to change it will change slowly, and you should have a separate pump to feed tank water through the eductors at a good rate to avoid spots of high concentration of either stream. At this temp platic pumps work fine and they never corrode, and you can get immersible ones

Thanks for the temperature rise calculations. They confirm that the plastic pipe should be OK.

According to my titration curve the pH does change slowly at first. On the scenario I gave, it takes almost 2 hours to get to a pH of 2. But in less than 2 minutes it goes from 2 to 11. That is the portion of the curve I am worried about. To keep the costs down, I plan on only installing a one-sided neutralization process.

I design & build these all the time for places like MIT (a current project) and Cornell. I hope you have a plastic holding tank and plastic pipeing to the system. Otherwise you could be facing more problems in the near future.

Most states require a pH higher than 5.5 and lower than 11.5 for discharge. Do you have state a discharge pemit? What does it say the limits need to be? It would be very odd if you were allowed to discharge a pH limit of 2. That is so low, you are likely to kill off an activated sludge treatment system downstream. The bugs hate large swings in pH, but will tolerate a swing to the high side better than to the low side.

You will need a way mix the tank. It might take two as this is an odd shape. I have used eductors with submersible pumps and have a ratio of 6/1 or so. 1 gallon pushed through the eductor mixes with 6 gallons of tank water. Don't mix NaOH in the piping outside of the tank. The heat can be enough to affect PVC cement. Use thermo welded plastic pipe, socket or butt. A short piece of PP pipe could be adapted to fit the mixing pump, hold a chemical injector in the flow, and end by passing the water through the eductor, increasing overall tank mixing and treatment in one operation.

A pH meter will be needed to control the chemical addition. Need to treat any HF acid? Then you will need a special probe. I might go to a 50% NaOH unless the caustic is free to you. The cost is about the same, but you can treat more water. Test the tank pH and control to that reading. Record by chart recorder the discharge pH so you can allways show regulators you were in compliance.

Send me your e-mail if you want me to supply the controls, components and instructions.

Hi Sparkchaser,

Thanks so much for your offer of help. Your experience is invaluable for the success of my project. There is one thing you mentioned that I am not familiar with and that is a "chemical injector". Is there a make and model number you would recommend?

I was in error when I said, "discharging off site." What I should have said was discharging from my area of responsibility. There is a further treatment of this waste before it ends up in the river. But I am not responsible for that treatment. My responsibility is to get the pH between 2 and 11. They are responsible to meet the permit requirements. I think the first step in their treatment process is further neutralization. I would guess they have pH limits so that their neutralization step works effectively at getting a good pH on the water before being exposed to the bugs.

All current and planned piping is /was fiberglass or CPVC both on the tank inlet and outlet. I will consider your recommendation for the PP. Have you ever used Kynar in such an application? I am considering Kynar for the tank eductors.

The underground tank I was describing is double walled fiberglass.

I only have to deal with HCl, no HF.

We currently purchase 50% caustic but dilute it to 20% for storage so that we don't have to worry so much about freezing issues. So since it is readily available, that is what I intend on using.

I plan on installing pH monitoring and control.

Thanks again. I look forward to your reply.

The below ground part is worrisome. Here, waste holding of the type described requires an above ground tank contained inside a dike wall. The tank and all piping would be ptfe lined.

That said, the discharge pump would have automatic valves able to place the discharge into recirculation. The pH would be controlled while in recirculation. You would need a pH meter transmitter and separate controller, or a pH meter with integral controller. You'll need the help of the Instrument Engineer (A sales rep) in selecting the pH instrument and probe. There are many different designs for different applications to choose from.

Keep in mind that pH probes are expensive and require periodic adjustment and replacement. You can minimize replacement frequency by paying particular attention to getting the right probe for your application. Also, if you can design the pH measurement as a low pressure slipstream to the recirculating flow, your probe should last longer. Consider yourself lucky if you can get more than a year out of a probe.

Good luck!

It sounds like you are not in control of your Wastewater Neutralization process. The mixing may be of poor quality and pH probe placement might not be yielding the most accurate reading.

You asked about the chemical injector. I must assume you don't have one. The injector is installed at the end of the line. It acts as a back pressure valve, keeping the line flooded with chemical. The injector is held closed by a spring and the pump must over come a few PSI of pressure before any chemical is added to the water. The advantages are the line is always pre-charged with chemical and when the pump turns on, chemical is added right away. Additionally, the line cannot drain down after the pump turns off, or worse, siphon chemical out of the holding tank and into the water. NaOH is pretty viscous but cut to 20%, might siphon well. The injector also acts as a check valve to prevent Process water from flowing backwards into the chemical line. The missing (broken?) injector is the likely cause of most of the over treatment you are seeing.

pH is not a linear scale but logarithmic. The volume of chemical needed to move the pH one point is variable. It takes much less to move from pH 5 to pH 6 than from pH 2 to pH 3.

Holler with more questions...Matt

Matt,

You are correct. I don't have control over my Wastewater Neutralization process. Reason is, I don't as of yet have an automated one. And we don't currently have any mixing to speak of. I am in the process of designing an automated system and upgrading the mixing. I just want to make sure I install something that will consistently achieve the objective (pH between 2 and 11) with little, if any, operator intervention. Right now, we are doing everything manually.

We currently don't have a pH meter on the tank I am looking to do the neutralization in. So any suggestions you have in that regards would be greatly appreciated. Currently, our pH meters are downstream.

You are also correct in assuming that I don't have a chemical injector. Could you please recommend a make and model number for my application. I see http://www.dosatronusa.com/ on the web. My piping is only 2".

Thanks a bunch for your help, John.

Check this page. http://www.gfsignet.com/applications/IWW_Neutralization.htm

I sell GF Signet and could set you up with an automatic system to meet your needs.

Dear CF Signet Salesperson,

Thanks for your offer. I am in central Michigan. Are you the local rep.? If so, please send me a private email with you phone number so that we can discuss. If the system is suitable for an industrial environment, then I am interested in getting a budgetary quote.

I'm not a wastewater guy but I've got a suggestion.

You said that your job is, basically, to bring your effluent to between 2 and 11 ph and send it to another part of the facility or process where they treat it further for eventual discharge to the river.

If you're pumping the effluent to the other facility, could you inject your NaOH before the pump (which then acts like a mixer-pump) and monitor the pH at the outlet and use that to control the amount of NaOH you are injecting? Your caustic needs time to react with the effluent, I know, but at least you can bring it to within certain limits that you know will result in satisfactory pH levels leaving your part of the facility.

You will still need to stir your tank so that the distribution of acid is more or less even which will help with consistency of control. Your controller will also need to be specially configured for pH control which is non-linear.

'Just a suggestion. I don't even know if what I'm suggesting is feasible in wastewater treatment but I'd like to know anyway.

Hi Vulcan, Thanks for the suggestion. I have experience with injecting the caustic on the inlet of a pump for pH control. It works well. I plan on doing the same for this application. It is also good to be reminded of the need for special configuration on the pH control loop. Thanks. John