pH correction/dosing 1

[ivanhoe374]

Does anyone have a feel for the amount of caustic(50%) i will need to add to wastewater to raise the pH from say 5-7.

I dont have enough information on the wastewater to look at detailed water chemistry.

 

[m777182]

The amount depends on buffer capacity of your waste water. The most simple way to estimate it is to measure it by an experiment and it does not involve a detailed water chemistry.
m777182

 

[25362]

Is this a batch operation or a continuous one ? m777182 has given you good advice.

There are cases, however, where the demands to treat wastewater of varying flow rates and pH values as well as an everchanging combination of sources may complicate the problem.

If you are dealing with situations as these I suggest to take a look at Process Control Systems by F.G. Shinskey (McGraw-Hill), Chapter Ten: "Controlling Chemical Reactions" on the requirements for pH control.

BTW, the most common basic reagents used for the purpose are 10% caustic and lime, both approximately 2.8 N.
Lime, has the advantage of having a limited solubility, keeping the pH of the neutralised solution from exceeding 12.3.

Another sidepoint of interest is that acids are not necessarily needed to make pH values lower than 7. Even Na₂SO₄ and NaCl, both 0.1M solutions, have pH values of 6.1 and 6.4, respectively.

Temperatures also affect the pH. At 45^oC, the pH=7, at 25 deg C, drops to 6.7, meaning the amount of [H₃O]⁺ ions has doubled to 2*10^-7.

 

[25362]

Another side note: salts of transition-metal ions, at the same above 0.1M concentration in water, have low pH values.

FeCl₃: pH=2.0
AlCl₃: pH=3.0
Cu(NO₃)₂: pH=4.0​

 

[ivanhoe374]

thanks for the replies,

The water is from the wash down of trains.

We will be adding ferric chloride prior to a clarifier and this is what we believe will lower the pH.

Since this water will be recycled the pH correction is required.

The problem the plant isnt installed, neither is the train wash itself so it is hard to get a water sample to test on.

Im just trying to size a dosing pump/tank for the caustic which we will be using to raise the pH.

flow of the water is only about 700lph.

 

[25362]

As a ballpark estimate, if it were only ferric chloride (MW:162.2) in water, which produces one hydronium per molecule, NaOH (MW:40) would be needed in the amount of about 40/162.2, or about one part NaOH to four parts of ferric chloride.

 

[tickle]

I recently did some work on increasing the pH of stormwater for aquifer storage and the chem inj people suggested that MgOH is used in lieu of NaOH. It has safety and environmental indications and has a better "neutralisaion" capacity.

Have a look at orica's website (ww.orica-chloralkali.com) for more information - search for MHL

 

[OilBoiler]

ivanhoe374,

I will suggest you to go ahead and grab several samples and do some titrations for it with a lower strength of caustic and then do the equivalence for the 50% caustic. I've been dealing with wastewater for a while and I've noticed that you can easily underestimate or overestimate the amounts needed for chemicals, because pH is such a tricky measure (logarithmic). If you're sizing control valves or you just want to know what amount you need of caustic, I strongly suggest titrations.

In fact, when you do the titrations, you'll find that 50% caustic is very strong and you may need lower strength (even 25%, is too strong for titrations) so you can get more accurate results.

This may be a good site for normality/morality and % strength equivalences. You can also refer to many other sites or basic chemistry books for all that information.

http://www.furrowpump.com/Applications/JarTesting/...

Good luck!

 

[25362]

Digital valves may be a good solution to control variable pH levels. Ask the main valve manufacturers.

 

[ivanhoe374]

I have gone with a pH moniter/controller with PID control with a variable speed 5.5L/hr dosing pump with 100:1 turn down. Hopefully it all works out!

Will do some tests when we get some samples! only problem is the plant train wash isnt even installed yet

 

[DeltaCascade]

Due to evaporation and recycle, depending upon your washing system's blowdown and make-up water flow rates (cycles of concentration), you may find that pH actually rises due to concentration of make-up water alkalinity. What you are washing out will also be of importance, no doubt.

Tough when you dont have samples ... or know what you are washing.

Nonetheless,pH control of clarification itself (your recycle stream is feedstock) is a useful parameter (re its affect on coagulation and flocculation and clarification performance). Having a small plastic diaphram metering pump to feed diluted caustic (from tote or barrel), or H2SO4, if anything, or as your needs may dictate, is not a big deal. Perhaps just make sure that you have room for it at a later date.
You likely may also need to chlorinate, with alkaline bleach or acidic Cl2 gas, for example. So leaving room for pH adjustment wouldnt hurt, yes.
Titrate samples when the plant is running .. then add the pH control?
How about assuming 20% of system volume make-up each week, with blowdown of water with zero alkalinity, and size based on neutralizing make-up water alkalinity ... like, with acid. Like a cooling tower.

Just a thot....