Iron and H2S removal 3

[Watro]

Hellow Gents

We are designing a Brackish water RO plant, feed water from wells. Flow rate is 300 gpm
The feed water has an Iron 5 ppm, H2S ~50 ppm. pH is 6.8, TDS is ~13,000 ppm, Temp: 25 - 36 deg C
Can we oxidize the Iron completely with NaOCl, then acidify to air strip H2S
Other downstream steps are 300m Filter, Ultra filtration, single pass RO and Re-Mineralization at last.

Alternate design is to keep passing the feed water under anaerobic condition to RO then to air strip the H2S downstream.
In such case, not sure how the Iron would react in RO. Is this be filtered out in UF due to insoluble condition? Risk is any O2 ingress will form Ferric salts and colloidal sulfur?

Can we remove H2S to maximum level in single stripper? I mean H2S <0.1 ppm

How the Ultra-filtration would react with Iron particles. Is this can be backwashed/cleaned with chemicals?

Thank you
Niss

 

[ashtree]

What you are proposing is theoretically possible but likely to be very difficult in practice.
Dosing with NaOCl will oxidise both Fe and H2S at the same time at the Ph you are talking about but you will have significant turbidity. Iron will oxidise quite quickly, a few minutes at the most and pH around neutral will be okay, but sulphur is a little more complex.
The oxidisation of H2S proceeds best between a pH of 6.5 to 8.5.
The theoretical amount of chlorine to oxidize 1 mg/l of Hydrogen Sulfide to elemental Sulfur requires about 2 mg/l of chlorine but its normal to dose an extra 15% to ensure there is adequate chlorine. A contact time well beyond 20 minutes will be required

Chlorine oxidation of Hydrogen Sulfide to elemental sulfur produces an undesirable chemical turbidity consisting of very fine particles. The UF process should remove these particles but they will be very difficult to clean from the membrane. This turbidity will be white if the sulfur is completely oxidised or yellowish if not complete. Another issue with chlorine oxidation is that partially oxidized and oxidized sulfur in the finished water can then be easily converted back into H2S by sulfur reducing bacteria that are not completely destroyed through disinfection.

Hydrogen Sulfide may be completely oxidized to a soluble sulfate form using chlorine in an aerobic environment. The oxidation of Hydrogen Sulfide to Sulfur may take simultaneously or proceed at a slower rate depending on the pH and concentration of the reactants. Under most conditions a minimum of 20 minutes of contact will be required for complete sulfate conversion. The optimal secondary pH range is narrower than the primary range and is from 6.5 to 7.3.
The chemical equation is shown below:
4Cl2 +S2 + 4H2O → 8HCl + SO4-2
Note that Oxygen is provided by the water molecules. If oxygen is present in the water the sulfate reaction will more readily occur. To oxidize Hydrogen Sulfide completely to the sulfate form, 8.32 mg/l of Chlorine are needed for each mg/l of Hydrogen Sulfide oxidized but you would normally dose a littlle extra.

In saying all that i think the big risk is that you won't be able to oxidise 100% of the iron to a particulate form and sufficent will pass through to the RO. Given that you are going to have to use a high dose of NaOCl to oxidise both the H2S and the Fe tyou will have to use a dechlorinating agent to after the UF to protect the RO from chlorine damage. If you carry any chlorine through you are likely to damage the RO membranes.

I have seen keeping the the unoxidised iron in solution under anaerobic conditions work successfully in plants with maybe 0.5ppm of iron but even then they get a small amount of iron fouling. If something goes wrong 5ppm will quickly clog the membranes.
Air stripping of H2S may well work but it will stink. To go from 50ppm to less than 0.1 ppm in a single stripper will be difficult as this represents 99.8% removal. Likewise any residual iron will clog the air stripper media.


Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[ashtree]

Have you looked at Greensand filtration as an option. It will remove iron and H2S as well as the turbidity. The bed volumes will be critical to avoid very frequent regenerations and you will probably still need the UF to get a sufficently low SDI for the RO.

Just to correct my post above. Where i have said "Hydrogen Sulfide may be completely oxidized to a soluble sulfate form using chlorine in an aerobic environment." I meant to say in an anaerobic environment.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[bimr]

then acidify to air strip H2S

This option will not work.

 

[Watro]

Thank you Ash for your detailed post

I believe then stripping should be upstream prior to Iron oxidation.
We may add additional scrubber to get rid of the H2S odors removed from stripper. Strippers will preferably be 2 stage for complete removal. Followed by Iron oxidation

Thank you
Niss

 

[ashtree]

No i don't believe you can air strip the H2S first because that will oxidise the iron. Even if you strongly acidified first you would be likely to still oxidise enough iron in the airstripping process to foul the system particularly if you were using a packed tower degasser.

My opening statement still applies "What you are proposing is theoretically possible but likely to be very difficult in practice."

If you are going to use chlorine as the oxidant than i think you will have to plan on removing both the H2S and the iron at the same time and then attempt to remove the particulate iron and sulphur which may be tough.

I would suggest that you thoroughly test this in the lab before you commit to it.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[Watro]

Thank you again.
Lab tests possibility are limited WRT the site location and present project status

Please advise if below sequence make any sense.

Hypo (dosage only for Iron to Ferric) -> Bag filter -> Ultra Filtration -> Break tank -> Feed pump -> Air stripper (2 stages) -> RO Feed pump -> H2SO4 dosing -> 1m Cartridge filter -> ROHP -> RO Membranes -> Limestone contactor

In worst case we will add BIRM media filter to remove Iron followed by Stripper

 

[ashtree]

I don't think it will work because you will oxidise iron and H2S at the same time when you first add chlorine.
The bag filter is likely to foul quickly given the iron and H2S concentrations.

Why not try something like this: Greensand then UF, then maybe air stripping(maybe only one stage) , and RO.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[Watro]

Thank you Ash