Sand Filter with Limestone 3

[ozfish]

I intend on running ground water with low pH, high C02 and high iron concentrations through a sand filter with limestone media to allow me to effectively remove the CO2 and hence oxidise the Fe2.

Has anybody any sugesstions or experience in doing this?

 

[lyonramirez]

Limestone media could work for pH adjustment but I dont think it would work as filtration media to retain iron oxide particles. I wonder how it would be to backwash this type of media. I recomend you to remove CO2 first by clasic aeration which will help also for iron oxidation, then use sand filtration. Also you could evaluate biological iron removal by sand filtration.

 

[ozfish]

The ground water is has very little in terms of suspended solids. The limestone is purely to add CACO3 to the water to allow complete oxidation after aeration.

 

[bimr]

"When iron-bearing waters are treated alkali without aeration or the use of other oxidizing agents, objectionable amounts of iron may be present in filter effluents as: ferrous carbonate or ferrous hydroxide". Taken from Nordell's book. Probably will not work.

 

[MikeJefferies]

It will work, but not for very long. The iron will coat the limestone, because of a boundary layer effect. This will quickly prevent further interaction between the CO2 and the media.

 

[ozfish]

Thanks for your replies.

Can I further clarify a few more clues.

The ground water which has very low DO levels will pass through the vessel prior to aeration. I am thinking that this will minimise any oxidation of iron in the vessel.

After water leaves the vessel at it will then pass through an aerator which will strip the C02, oxidise the iron and then pass through a secondary filter to remove the iron oxide.

I have tried this using an open top container to hold the limestone. It works for short periods (several months) - the pH after aeration increases to 7.5 - 8 and oxidation is sucessfull. After several months there is a buildup in the limestone holding tank at the surface (where it is in contact with air. I am not sure what the builduopis precisely. I thought it was iron oxide? - Maybe it is as "bimr" suggested "ferrous carbonate"

Mike - if there is little o2 present as it passes through the limestone do you still believe the iron will coat the limestone?

 

[MikeJefferies]

I really don't know. The water I had experience on was a surface water, which turned limestone brown even after an RO plant.

The other thing you may get is ferric hydroxide precipitation, due to local high pH's at the surface of the media.

I'm a bit confused now as to what you are trying to achieve ? Why bother with the limestone, why not just go for aeration and filtration ? Or maybe chlorination or pottasium permanganate dosing to oxidise the iron ?

 

[ozfish]

Thanks for the reply Mike.

The Fe2+ in the water is 45mg/l. I found that aeration alone of would only take the pH to 6.5 which meant the oxidation process was slow. I have very effective floating polymer filters to remove the iron hydroxide.

After the polmer filter I do use potassium permanganate to remove final iron and manganese. To use it before hand would be cost prohibative and inefficient.

 

[bimr]

I am also not sure what you are trying to do. From your first post, I thought you were trying to take out the iron with a neutralizing filter. Now, I understand that you are just trying to raise the pH.

The iron is present in the raw water in the form of ferrous bicarbonate.

The neutralizing filter or calcite filter (I assume that's what you are talking about when you say "limestone".) will generally raise the pH. The use of an aeration tower will also raise the pH. The pH can only be raised high enough if you have appreciable amounts of bicarbonate alkalinity in the water. You have not said what the level of alkalinity is, but I doubt that you have adequate alkalinity present to get the pH high enough.

Note that the resultant pH after the neutralizing filter (or aeration tower) is dependant on the concentration of the alkalinity in the raw water.

At a pH of 7.0 and above, a practically complete removal of iron is effected. When I say that, I mean if you have a pH of 7 and an aerated water, the iron concentration after filtering will be very low. I might add that keeping the pH above 7.5 is generally recommended since this pH will afford a good factor of safety in the removal of iron by aeration, settling, and filtration.

So, a target pH of 7.5 is what you are trying to achieve. If the resultant pH after aeration is below 7.5, you will need to add alkalinty to the aerated water. The alkalinity should be added into the basin below the aeration tower.

A neutralizing filter will probably not take the pH up to 7.5 so the use of a neutralizing filter is not going to help you remove the iron. This is the reason a neutralizing filter treatment approach is not used.

A couple of other comments:

Another disadvantage of a neutralizing filter is that it raises the hardness of the water so that where softening is practiced, the costs of the softening are increased. Don't know if this is a concern to you.

The neutralizing (calcite) filter should not foul with iron as long as you keep the air out of solution. I will respectfully disagree with Jeffries in that the neutralizing filter will continue to work since the calcite is dissolving. This is just not a good application for a neutralizing filter.

One treatment approach that you may consider is to use a water softener. I have seen many successfull municipal plants where such levels of iron are removed with ion exchange. The secret to the approach is to keep the air out. If you keep the air out, the iron will remain in solution and will be removed with the ion exchange process. The reason will also not foul.

You might also consider the use of a greensand filter approach.

 

[MikeJefferies]

45mg/l ? Wow, that's a lot. Given the previous reply, I'd just use a caustic dosing set - cheaper capital and running costs, and you can get the pH to any level you choose.

You may be correct about the calcite unit continuing to work, I've only one site to go on and we definitely saw a drop off in calcite uptake which was coincidental with iron pick up on the calcite. I've assumed the two were linked. I guess if the backwash is effective, and causes a scouring effect between the calcite particles, then the iron will shed off and be removed.

 

[ozfish]

Thanks for the advice!
I have got the system running but I do have some glitches - as bimr predicted.

The water leaves the bore and passes the vessel with limestone, it has no or very little contact with O2 at this stage. It leaves the vessel and passes through an aerator and and enters into a floating polymer filter.

The pH after aeration is approx 6.5 - the pH of the filtrate drops to 6.1 -6.2?????

The alkalinity at this point is 60-70ppm. Could this be what you where advising of?

If the media was contaminated with iron bacteria would this cause the pH to drop?

I look forward to your replies.

Regards

PS Greensand filters with such high iron and manganese was not effective. I may have to eventually use a caustic dosing set - but am trying to avoid the use of to many chemicals.

 

[bimr]

My point was that you have to get the pH to at least 7.5 to ensure that all of the iron is out of solution in order that the iron can be filtered out.

A calcite filter is not able to take the pH up to 7.5.

The sample may be absorping CO2 from the air, which is causing the pH to drop. Iron bacteria does not usually have such a dramatic effect on the pH.

 

[ozfish]

Thanks again Bimr
I am going to try a dosing with caustic soda after the water has past an aerator.

Should I just be monitoring pH for correct dose to reach 7.5

 

[bimr]

Yes, you should be monitoring the pH to get to a pH of 7.5.