Sand Filter with Limestone
Sand Filter with Limestone
(OP)
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?
Has anybody any sugesstions or experience in doing this?





RE: Sand Filter with Limestone
RE: Sand Filter with Limestone
RE: Sand Filter with Limestone
RE: Sand Filter with Limestone
RE: Sand Filter with Limestone
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?
RE: Sand Filter with Limestone
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 ?
RE: Sand Filter with Limestone
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.
RE: Sand Filter with Limestone
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.
RE: Sand Filter with Limestone
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.
RE: Sand Filter with Limestone
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.
RE: Sand Filter with Limestone
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.
RE: Sand Filter with Limestone
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
RE: Sand Filter with Limestone