Aeration and iron removal

[Prashant2022]

Good evening,

I was looking to see if anyone else has experience with an industrial water treatment system functioning better after the removal of aeration treatment. This has been confirmed at multiple locations across my country. The filters seem to be removing iron better without the aeration process. Albeit, there is some contact aeration prior to filtering. The aerators we have in operation are tray aerators. The water then moves to a retention tank and then directly to the filters (where manganese dioxide and sand are typically used). I am lost as to why the iron removal is better without the aerators from groundwater sources.

 

[bimr]

Really impossible to answer your question without a water analysis.

Aerating trays are used to add oxygen for oxidation of iron. The trays are also used for carbon dioxide, hydrogen sulfide, and manganese oxidation.

You fail to mention whether this is surface or well water and whether you are using chemical treatment.

Note that a significant amount of oxygen occurs when the water in the retention tank is exposed to the atmosphere.

You also don't define what functioning better means.

Are you filtering the water?

 

[Prashant2022]

Right, so I made the post expecting further questions. To answer yours:

- The trays I'm referring to are used to oxidise iron from groundwater sources. Carbon dioxide and hydrogen sulphide are minimal and there is no concern of manganese presence.
- There is no chemical added to aid in oxidation. The aerators are supposed to be the main source of oxidation. Currently, with them being by-passed, the only source of oxygen for oxidising iron is the contact with air in the retention tanks (that are open to atmosphere).
- The retention tanks are followed by filtration with the media as mentioned above. By 'better' I mean that there is breakthrough of iron after the filters when the aerators are active (iron values above 1 mg/L). When the aerators are by-passed, there is no by-pass of iron and the final water is conforming to our guidelines (<0.3mg/L).
- The groundwater iron values range from 2 mg/L to 5 mg/L.

 

[EdStainless]

Have you checked the pH?
Is the force aeriated water lower pH?
Might this lead to more dissolved Fe?
Or have you looked at other ions in the solution, is there a competition factor involved?

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P.E. Metallurgy, consulting work welcomed

 

[Prashant2022]

The pH at all locations is around 6.5. No significant deviations from that. The highest is 7 and the lowest is 6. Generally, after aeration, the pH rises slightly (due to the removal of dissolved CO2). This rise is around 0.1-0.2 pH units. I have checked dissolved oxygen levels after the aerators and they show about 5 mg/L to 7 mg/L which indicates that adequate oxygen has been incorporated in the water. Additionally, in around 60% of the samples, the ferrous iron in the retention tank is very low (<0.2 mg/L). I'm unsure what ions to look for in the solution aside from what I've already mentioned.

 

[bimr]

Removal of dissolved iron is time and pH dependent. It is preferable to have a retention basin with a detention period of not less than 30 minutes. Keeping the pH above 7.5 will also afford a good factor of safety in the removal of iron by aeration.