Time required for Iron oxidation 5

[DJBess]

I am working with a small municipality upgrading their water treatment system. Their existing system pumps water from wells (.135 MGD, 1.5 ppm Fe, negligible Mn)over an aluminum induced draft aerator (rated 300 gpm), into a detention tank (11,500 gals), from which their high service pumps draw the water, pump it through pressure filters and on to Cl, distributions sytem, etc. I'm wondering if any of you can explain to me or suggest a good resource to understand the chemical/phsical aspects of what is going on. Obviously, in the big picture, we're simply aerating the water to entrain oxygen, which then, in the detention thank, oxidizes the dissolved iron to rust which can be separated by filtration. But how does one determine how big the aerator really must be? How much oxygen are we wanting to entrain? And how do we predict the detention time required? Another thing I wonder is why there is no mixing at all in such detention tanks? Nearly all of my experience has been with waste treatment and I look at this arrangement and think the reaction time could be accelerated by some gentle mixing. Yet no one seems to do that with this type of water treatment system. Curious.

This is important because I need to advise them whether they need to replace their existing aerator and give them some idea of the capacity of the aerator/detention tank combination.

Would appreciate any advice any of you can give me. Thanks.

 

[semo]

A good resource is "Water Quality & Treatment" by AWWA through McGraw Hill.

There are a lot of variables in iron/manganese removal which include pH, organic iron, alkalinity, entrained gases (CO2,H2S, etc.). They all can play a part in the treatment process; but, I'll go over some basics.

By MO standards (from 10 States Standards), the detention time is regulated by the amount of iron present unless you do pilot studies. With iron under 4 mg/l, a detention time of 2 hrs is required. The forced or induced draft aerator must have a loading rate of 1 to 5 gpm/sq ft of total tray area.

The aerator adds oxygen to the water to oxidize the soluable (ferrous) iron to an insoluable (ferric) state where it can settle out. Chlorine is typically added near the aerator to assist in this oxidation process. I've even seen plants using chlorine instead of aeration.

The detention tank then allows a calm (no mixing) zone where the small light iron particles are allowed to settle. Some detention tanks (clarifiers) utilize mixing as they add polymers to the water to help flocculate these small light particles. The settled water is drawn from the top of the tank and taken to the filters.

If the system is not operating properly, I would find out where the problem is. You should perform tests at various locations in the system to see if each individual process is performing properly. Then correct the system from there.

 

[DJBess]

Thanks, Semo. Big help. I can't believe I didn't think to check Ten State. Duh. Too many "irons" in the fire! I am still curious on the mixing question. In the aeration/detention setup we have, there is no sedimentation. The detention time serves only to give the oxidation reaction time to go to completion. As I look at it, it appears to me that we basically are just getting the ferrous in contact with the dissolved oxygen to produce the ferric. Why wouldn't we want to input some mixing energy to encourage the reaction? Seems like we could get more reaction in the same size basin if it were mixed. Hmmmm. Just curious. Would be interested in yours or anyone else's thoughts.

 

[BobPE]

DJBess:

Mixing wont help, you goal in aeration is to get the oxygen into solution where it reacts with the iron, velocity gradients don't matter. Lots of small bubbles (large surface area), slow moving water column (ample time to enter into solution) is the driving operation principal in thin film transfer. Usually, areators are designed using rules of thumb (which are scary in their own right,) as semo referenced from 10 states. I very rarely find these systems sized correctly...usually, they are too small....

BobPE

 

[semo]

I agree with BobPE. I think mixing will have very little (if any) affect on the oxidation process. I've heard others have also had theories on that; but, not seen any results of testing to prove or disprove it.

The aerators also seem to do more for the process by stripping CO2 and raising the pH. My experience has been that Cl proves to be better oxidant for Fe, especially if there is any organic iron.

Even though standards require long detention times for settling, I've found that little settling occurs unless a flocculant is used. The filters do the majority of the removal in these types of systems. The job is really to get the iron oxidized so that the filters can remove it.

 

[mdszj]

Hello all

While doing a net search on iron oxidation and removal for a groundwater pump/treat application I am working on, I came across this thread. After reading it I have a couple questions, since I am also trying to optimize Fe removal in my application.

1) From the thread above, sounds like a detention time of at least 2 hr is needed for Fe under 4 mg/L (my water has ~30 mg/L Fe, including 20 mg/L soluble Fe). Does anyone know if oxidation using peroxide would be faster in comparison to aeration? The detention time in the clarifier is 15-30 minutes depending on flow rate so I would need to complete the reaction faster.

2) Does the 2 hr figure represent time needed for the chemical oxidation reaction only or does it also include settling time? (Note: we are also using anionic flocculant for settling).

3) We are curently using NaOH addition to pH 10 (followed by anionic floc addition), and Fe is the only metal to be concerned with. All regulated metals are well below their discharge limits. I think we need to run some jar tests to verify the pH (seems very high for Fe removal). I would also like to do tests to evaluate oxygen addition. Question: does oxygen addition also require pH adjustment or does it work by itself? I realize that each water is case-specific and that is why I want to jar test first.

Any info is appreciated on this, thx.

 

[semo]

MDSZJ,

Normally, it doesn't take 2 hrs to oxidize iron. As I said there are several variables though. The pH will have an affect on the oxidation rate, (longer for lower pH). I would want to see it around 7.8-8.1. Depending on the hardness, raising it to 10 can cause other changes in the treatment; but, will have little effect on the Fe except to waste caustic and possibly cause a water that is scale forming. I don't know your whole process or water analysis so that might not be.

I'm sure your polymer will work best at a particular pH too. Jar tests will show this.

You can check the oxidation by sampling after the aerator and running the water through a 0.45 micron filter. Then test the filtered water for iron. This result will show how much iron is not oxidized. You can also age the samples prior to filtration to see how long the oxidation process takes.

Are you adding any Cl2? That will also increase the oxidation rate. I've never tried peroxide so won't say either way; but, I seriously doubt it is cost effective.

I'm assuming your clarifier is a solids contact type. Settling, will take place in your clarifier with the aid of the polymer. How readily it settles really depends on the clarifier upflow rate rather than the detention time. If your upflow rate is over 1 gpm/sf you probably won't see much settling. The floc will overflow your weirs and be removed by the filters. 0.75 gpm/sf is a good target upflow rate.

Testing is the only way to verify what is happening in your system. When you make a change in chemicals/process control, make them one at a time and give plenty of time for the change to take affect. Sample, test, then move on to the next change. If you make more than one change at one time, you don't know exactly what worked or didn't work.

 

[Dmitri]

Halo to everyone,

I am wondering if someone uses a biological removal of As, Fe, Mn and NH3 from well water?

Some papers introduces these methods but there is not enough information of for full scale up e.g. ratio between As and Fe e.t.c.

Any suggestion would be helpful.

 

[mdszj]

Semo

Thanks for the info. I agree about the Ca - we just got water sample results back from samples I collected at various points throughout the system. When the pH was raised to 10, the soluble Ca was reduced from ~28 mg/L to about 17 mg/L. The Ca must have precipitated out. Additionally, we have been getting our lamella clarifier plates plugged up with solids, which requires a lot of draining, cleaninig, etc. ALso, I found out that precipitation of Ca at high pH often is delayed, (due to insufficient charge neutralization?) which may help explain the plates fouling. I also want to look at floc polymer dose - that may be too high.

I noticed in the thread that chlorine will help increase oxidation reaction of Fe. Depending on my pH test results I may check out bleach addition along with the best pH. Do you know if there is a pH range where bleach is most effective for this, or is it ok to just go with my optimum pH? I guess we can always test to find this out.

Also can you tell me how to calculate clarifier upflow rate?

thx for any info.

 

[DJBess]

mdsjz -

I don't know the chemistry of the Fe - Cl precipitation reaction but I'm sure you could call Tonka or US Filter and they could tell you.

 

[semo]

The bleach will help ozidize the Fe. I don't know how much quicker it will work at the higher pH's (never tested it); but, I doubt much. I would recommend the pH be in the 7-8 range (probably closer to 8). The iron should oxidize readily at that range. I also imagine your polymer works better in that range also.

As you mentioned, I would also jar test the poly dose to optimize it. That is an expensive chemical to waste and too much will shorten the filter run times.

There are other threads with Cl dosages for iron. javascriptpenindex(450,350,'

http://www.tipmaster.com/includes/refinfo.cfm?w=450&h=350')

Your upflow rate is calculated by dividing the flow rate (gpm) by the surface area of the clarifier at the top of the sludge blanket.

 

[ChrisDerrick]

DJBess,

There is some good information on the reaction of iron and peroxide at h202.com/applications/industrialwastewater/fentonsreagent.hmtl

Also there is some good information on the relationship of Iron(II)Hydroxide and Iron(III)Hydroxide as a function of oxidation state and solubility at the web site of 2 The 4 Technology Solutions. I think the web site is 2the4.com.

Hope this helps

Chris Derrick

 

[bimr]

Sorry for the late reply.

First, I would have to correct an earlier post. Recommended Standards for Water Works (Formerly titled 10 States) only calls out 30 minutes of detention time rather than 2 hours (that was earlier quoted by a poster). It's on page 75.

Mixing will not do anything for you as far as iron removal goes because the water in the detention tank is no longer aerated.

The aerators are used to remove methane, hydrogen sulfide, and carbon dioxide as well as to saturate the water with oxygen.

Keeping the pH at or above 7.5 will allow a good factor of safety in the removal of iron by aeration, settling and filtration. All of the iron will be removed with a detention period of less than 15 minutes at that pH. There is no reason to with any higher pH.

A good book that you might try to locate is "water Treatment" by Eskel Nordell.

 

[DJBess]

bimr,

Thanks for your response. Will look into the book.

I am still curious on the mixing. I have read all of the above responses but they don't exactly answer my question (still appreciate them -- sometimes gives me ideas for other things I'm working on!)

What I am thinking is simply that the aerator has entrained oxygen in the water -- That is nice but there still must be phsical contact between the reduced iron molecules and the dissolved oxygen. It seems to me that, if one introduced both into a completely quiescent pool, the reaction would have to take longer than if some mechanical energy was introduced to bring the two into contact. Or perhaps the reaction is so aggressive it doesn't need any encouragement?

This is totally a matter of engineering curiosity to me. The system in place works fine. But I'm still curious. Wish I understood the chemistry itself, reaction rates, etc. Thanks for any input.

 

[bimr]

Lets try again. Remember that the water is being pumped from an anaerobic well. As soon as the water is exposed to air, the oxidation of iron will initiated.

Oxygen is dissolved (rather than entrained) in the aerator. The splashing and cascading effect in the aerator will thoroughly mix the water and oxygen prior to the water entering the detention tank. Think of the aerator as being a rapid mix tank with oxygen being added.

Sometimes, a clarifier instead of a detention tank is situated underneath the aerator. If you are using a clarifier in that application, you would not want any mixing in the clarifier.

Theoretically, 1 mg/l of DO will oxidize 7 mg/l of iron. The solubility of oxygen from air dissolved in water is about 7 ml per liter at 59 Deg F, which is about 10 mg/l. So you can see that water saturated with dissolved air at about 60 Deg F contains enough oxygen to oxidize some 70mg/l of iron.

This oxidation is very slow at low pH values and rapid at high pH values. At a pH of 5, only about 1/10 the iron will be oxidized in 15 minutes. At a pH of 6, 1/2 of the iron will be oxidized in 15 minutes. At a pH of 7 and above, essentially all of the iron will be oxidized within 15 minutes.

In summary, you have an excess of oxygen added in the aerator and mixing is not critical because the reaction occurs rapidly.

 

[DJBess]

Thanks, bimr! That is exactly what I wanted to know. Appreciate your help.

is your Nordell book is pretty old (like 1961?). Must be an "Oldie but Goodie"? Oftentimes it seems to me that very old engineering books are better than newer ones. Newer ones can of course address technology advances, but, so much comes down to fundamentals. And that is where the old ones shine. Am I onto the right one? And any other comments? The only one I found they wanted $50.

 

[bimr]

You are correct. It is an "Oldie but Goodie". It is only deficient in the coverage of membranes. However, it can hardly be faulted for that since membranes were commercialized after the book was published!

 

[DJBess]

bimr

I did find a Nordell. Thanks for the recommendation!