High pH 2

[longbow75]

We have a filter plant which has three 300 gpm pureflow filter vessels and trying to treat groundwater which has Fe Mg and As. Our influent pH is 7 however effluent pH is 10.
The only chemical we add to the influent is NaOCl.Initially I thought it is because of the filter media. It is brand new and the filter media is PF80-120, PF 250, PM100 and PF600. (I wasn't able to have pureflow explain me what they were)To my understanding, it is some form of AlSiO4.
What may be causing this pH spike?
Any thought would be appreciated. Plz let me now if you need more info.

 

[kenvlach]

I have an MSDS for sodium hypochlorite solution which shows pH 12.5-13.5. You may be using a lot, or the incoming water may be low in buffering capacity.

 

[25362]

I see that kenvlach is right.
Sodium hypochlorite dissociates in water 100%. The ion Na⁺ doesn't form any acidic hydrate. The OCl^- ion is the conjugate base of HOCl. Thus, for example, a 0.1M solution would produce 0.1M [OCl^-].

A hypochlorous acid solution at 25^oC has K_a = 3[×]10^-8, meaning its K_b = 0.33[×]10^-6.

A 0.1M [OCl^-] solution would result in:

[OH^-] = [HOCl] ~ (0.33[×]10^-6[×]0.1)^0.5 = 1.8[×]10^-4​

which gives a pOH = 3.74 and a pH = 10.26.

 

[bimr]

Kenvlach is partially correct in that sodium hypochlorite may raise the pH.

However, it is incorrect that sodium hypochlorite will have a significant effect on pH in water treatment applications.

"The reaction of hypochlorites and water forms both hypochlorous acid and sodium hydroxide or calcium hydroxide. This causes little change in pH."

http://www.ge-water.net/handbook/cooling_water_systems/ch_27_chlorine.jsp

One part of chlorine will oxidize about 1.6 mg/l of iron. You probably only have 3-6 mg/l of iron.

You should also note that the iron oxidation reaction time varies with pH as well as with the chemical used for ozidation.

I would suspect that the media that you are using for filtration is causing the change in pH.

 

[longbow75]

Thanks for the replies. I was able to contact pureflow and they confirmed pH raise and told that it'd take 2-4 weeks to stabilize.
1 ppm NaOCl would increase pH to only about 9 even if there was no Fe and Mg present. That's an interesting result though that there is some type of media reaction going on!!
I though about adding something else upstream to stabilize the media faster but did not get response from Pureflow. Does anybody know what PM100 is?

 

[kenvlach]

"...Iron and Manganese pressure filtration system...the Pureflow© process that included pretreatment with Chlorine gas and Sulfur Dioxide gas. Each chemical addition was followed by a one (1) minute mixing / reaction vessel and then by a pressure filter that included our PM-100 proprietary, highly adsorptive catalytic media..."

http://www.wateronline.com/ecommcenters/pfdiv.html

For more info, see

http://www.pfdiv.com/

 

[bimr]

The media is probably made using a caustic (NaOH) wash process. The caustic will cause the iron and oil to solubolize. You probably need to rinse the media to waste to neutralize the media.

The last time that I looked at the calendar, it was the year 2007. Go back to the supplier and have them furnish you a MSDS on the media. There are no secrets anymore and it is probably illegal to sell the media without a MSDS. The supplier probably is trying to keep the media a secret so that you can get a competitive price on the media.

 

[longbow75]

This is what I got...


" Thanks for your phone call regarding the Pureflow Filtration system located at the ....... well water treatment facility. In reference to the pH increasing from 7.0 (raw water) to 10.0 (filtered water), I offer the following information:
a. The pH increase is caused by the Pureflow Pm-100 catalytic adsorptive media. The increase in pH is common and is dependent on the raw water of each well.

b) This pH increase is temporary and will be reduced when the PM-100 media becomes conditioned by the Iron / Manganese in the raw water which causes a "patina" to develop on the surface of the media. When the PM-100 media develops the coating the pH will be reduced to just above the raw water pH.
c) PM-100 adsorptive / catalytic filter media operates as follows:
Iron and Manganese, along with other dissolved contaminants such as Arsenic, Hydrogen Sulfide, organic carbons, etc., are oxidized by chemical pretreatment, followed by an inline static mixer. The Iron and Manganese are precipitated as metal hydroxides. The Arsenic is removed by co-precipitation with the Iron and / or an adsorptive reaction. The Iron and Manganese are filtered on the PM-100 media where an adsorptive reaction occurs on the water / media interface. There, localized zones of an increased pH assure the formation, and the maintenance, of an active, adsorbent, hydroxide floc. The PM-100 media does not require regeneration and is backwashed at a rate of 20 gpm / ft2 of media, with filtered water.

d) If the temporary increase in filter effluent pH is unacceptable we offer the following remedies:
· Install a temporary acid feed pump / tank to lower the pH (Citric or Muriatic Acid can be used).
NOTE: Pureflow will provide the chemical feed pump and tank at no charge.
· Feed Ferric Chloride ahead of the filter (if there is little or no Iron in the raw water). This will cause the media to become conditioned faster and may lower the pH of the filtered water.
· Blend the filtered water with another water that has a lower pH until the media becomes conditioned.
e) As the PM-100 media becomes conditioned with the Iron / Manganese the pH will gradually come down to a level similar to the raw water.
f) PM-100 media will remove the Iron / Manganese in the raw water to levels well below the EPA / DHS MCL for each contaminant and most often reduces the Iron / Manganese to (or near) non-detectable levels, typically at filter loading rates of up to 10 – 15 gpm / ft2 of media. PM-100 is a permanent media, is not coated (and, therefore, there is no coating to wear off) and does not require Potassium Permanganate feed as a regenerant.
We appreciate your concerns and will offer any assistance you need to resolve the above referenced subject. Please call if you have any questions regarding this report.
Regards,
...... "

 

[longbow75]

Thanks guys!!!
I believe I'll look into trying to expediate the process since we can't add acid (no permit) and don't have waste tanks.

 

[sbush]

I experienced a similar situation a few years ago at a municipal water plant where we installed pressure filters and a catalytic media, known as Birm, for iron and manganese removal. No chemicals of any kind were fed ahead of the filters, only dissolved oxygen from a pressurized air saturator system. The Birm has an aluminium silicate substrate that is factory-coated with manganese dioxide. The manufacturer, Clack Corp., is tight lipped about the formulation; however, they told me that a "binder" material is used in the coating process. Upon start-up, we saw a similar pH rise (from 7 to 10+), which put the plant out of conformance with municipal drinking water regulations. Like you, we contacted Clack and were told that an initial pH rise was normal and that simply flushing would make it go away. After million of gallons and weeks flushing, the municipality demanded that we change the filter media. We changed to a silica sand/anthracite media coated, in situ, with manganese dioxide. Problem solved. A settlement was reached out of court.

S. Bush

http://www.water-eg.com

 

[bimr]

I agree with sbush. Maybe you should think about changing the media. Any media will work to take out the iron. It will be more difficult to remove the arsenic.