hydrogen sulfide removal in high pH well water 1

[MsPersistence]

Reference thread164-129243

We have been asked to assist the community we live in (24 homes) with a source water problem. The water has a pH of 9.1 - 9.2 and sulfide value of around 8.0 ppm. At present the current operator is dosing about 43 ppm of chlorine, the water is gassing off noticable foul odour. The water is then stored in 40,000 gallons of storage, with an approximate retention time of 11 days. The gas, of course, as well as remaining sulfate reducing bacteria makes it's way into the residences. I'm not concerned about the fluoride levels of 4.2 ppm at this point since each home has an RO for drinking water, solely due to the fluoride levels. The elemental sulfur has been sludging into the bottom of the water storage reservoir, (for how long, I don't know) and of course partially making it's way to the homes too. (I'm allergic to sulfa so no wonder I've been feeling sick since we moved here haha) To futher compound the issue, the dibromo's presently exceed our 16 ppb limit (at 26 ppb) and the last set of THM's came back at 63 ppb (our limit is currently 100) I do not know if the operator was dosing at 43 ppm when that THM level of 63 ppb was reported, however the dibromos already exceed the limit so I really want to get away from using this much chlorine for oxidation.

chlorides are around 83 ppm, sodium at 198 ppm and carbonates at about 200 ppm , and TOC at 3.7 ppm TDS from what I recall is nearing 500 mg/L

The only good think I can come up with so far is that the source water would be an excellent candidate for onsite sodium hypochlorite generation!!! (probably too complex for this situation though)

Some tests using aeration and carbonation have been done (by others) - some worked, some didn't. Issues (as laid out in prior thread from 2005) were correct in stating that the CO2 will gas off before the H2S does. (thus leaving the pH back on the rise, the opposite of what we need). That is exactly what happened in these tests done by others.

We piloted an oxygen generator on one of our sites last year. (not this site) It worked GREAT! We went through 4 variations of the pilot process (varying types of equipment) and in the end removed 100% of iron, manganese, TOC with great turbidity & colour reductions, and significant improvements in transmissivity.

So I was thinking - what if we first reduced the pH from 9 to 5.5 (not sure how yet) the flow rate is about 8 gpm.

Then we gas off as much H2S as possible (not sure if there is a practical and economical unit for this available commercially) I know I know - a vent pipe through the roof, right - but I don't know if this is environmentally feasible or safe. So we need environmentally feasible and safe.

Then we go through the micro aeration unit (it does not like chemicals apparantly so I'm not sure how it will handle the pH adjusted water - I am thinking the probes will simply require more maintenance.) The idea is to oxidize any remaining sulfur to elemental.

THen we chlorinate to control of any remaining sulfate reducing bacteria (~ 5ppm or so)

THen we go through pressure filter combo unit with KDF 55 and possibly GAC to remove the elemental sulfur and remove some (or all) chlorine with the added bonus of TOC removal (25 - 30 % with GAC)

Then more chlorination and pH adjustment to clearwell.

The disposal field for the site no longer is working and the backwash holding tank has limited capacity. So RO is not really an option here. ANd we also need to really think about how to keep the amounts of backwash water down.

So what I need is advice. Will this work? What are the pro's and con's and potential problems? Is there a better method? If this process might work, what is the best way to go from pH 9 to 5.5? Is pH 5.5 the "right" number? How do we scrub the H2S?? (economically?) How do I keep the plant from blowing up? We deal with Gas Analytical so I am certain they have a good H2S monitor we could spec. Is KDF 55 the right media? I am thinking most any media would be good! I was also thinking we might get real lucky and have a side benefit of fluoride redutions! (though I haven't yet looked into this at all) Is there a better way to control the sulfate reducing bacteria? How should we bring the pH back up to 7 or 7.5 from 5.5? Will this happen partially with the gassing off since pH is really just a measure of the hydrogen ion and they will be gassed off??

Thank you all - I look forward to your responses

My second idea would be to do pH adjustment first, with aeration second and try and gass off ALL the H2S either in tanks inside the building or take this huge treated water storage tank,split it into a raw and treated reservoir and insert aeration tubes into the raw side, vent it, pH adjust it, and have it act like a clarifier for any deposited sludge. By continuing adding oxygen to it, and adjusting the pH, the H2S should be "forced" to gass off. (shouldn't it?) THen we pump from the raw water storage back through the plant, perhaps through media filters or even Harmsco bag filters (SS cleanable) we even have 3 HomeSpring ultrafilters that would work from GE. The idea is to ensure any remaining, oxidized elemental sulfur does not get into the treated storage - then pH adjustment, chlorination and treated water storage. Then once a year or every coupld of years we simply vacuum off the bottom of the raw water storage reservoir or have the reservoir professionally cleaned.

 

[bimr]

A complete water analysis is needed to develop a treatment scheme. Do you know the concentration of iron, manganese, suspended solids, calcium, magnesium, and alkalinity? Are there any other water quality parameters that are a concern?

Have you talked to the well driller about the possiblity of getting better water quality with a new well?

 

[MsPersistence]

The latter did cross my mind!

I will post the water analysis shortly

I realize I am trying to look for a simple solution for something that is probably quite complex. Thanks

 

[MsPersistence]

RAW WATER
Fluoride 4.43 mg/L
pH 9.05
Sulfide as S 8.5 mg/L (in lab with field fixing agent)
(We have a Hach field testing kit on order)
Turbidity 1.3 NTU

TREATED WATER (so the Raw water, with Chlorine added only)
Calcium 1.7 mg/L
Iron < 0.03 mg/L
Magnesium 0.1 mg/L
Manganese < 0.01 mg/L
Potassium < 0.8
Sodium 198 mg/L
Bicarbonates 201 mg/L
Bromides < 0.1 mg/L
Carbonates 39 mg/L
Chlorides 82 mg/L
Fluorides 4.47 mg/L
Nitrates as N 0.4 mg/L
Nitrites as N < 0.02 mg/L
NO3 + NO2 as N 0.4 mg/L
Sulfates 70 mg/L

Electrical Conductivity 897 uS/cm
pH 8.95
Hardness (as CaCO3) 5 mg/L
Total Alkaliknity (as CaCO3) 243 mg/L
P-Alkalinity (as CaCO3) 20 mg/L
Hydroxide (as CaCO3) 0 mg/L
Total Dissolved Solids (calculated) 502 mg/L
Sulfides as S 0.4 mg/L
Turbidity 0.6 NTu
Ammonia Nitrogen 1.3 mg/L
Total Organic Carbon 3.7 mg/L

THM's 63 ppb
Bromodichloromethane (BDCM) 26 ppb

I forgot to mention there are two large pressure filters in the facility as well as a municipal chemical approval to use KMNO4. Both filters are plugged off and media is pretty much a block of concrete - we are unsure whether these could be utilized in the future, however we could treat the "blocks" with a light acid to see if they would break down so we could have the media sucked out.

I look forward to receipt of your responses. Thanking you in advance.

 

[bimr]

I would recommend a continuous potassium permanganate oxidation with manganese greensand filtration to remove hydrogen sulfide.

A potassium permanganate solution can be injected into the water with a small chemical feed pump installed ahead of a holding tank that provides at least 15 minutes of contact time. The oxidized sulfur particles can then be removed using a manganese greensand filter. The filter media also allows for polishing of unoxidized hydrogen sulfide.

You should contact the equipment supplier and have them repair or replace the equipment. Add a small contact tank.

You should chlorinate after the filtration step.

Several reasons for the recommendation:

1. The treatment plant flow is very small and you need a simple system. You are not going to get a PHD to operate this system.

2. Acid feeding to strip sulfide may be successful. However, you will be opening the water to the atmosphere, potentially adding more airborne contaminants. Hydrogen sulfide will give off an odor as well from the vent. You would also have to repump after the aerator.

3. The CO2 in the water is probably not that high since the pH is elevated. There is little reason to air strip the CO2, and therefore no reason to install an aerator.

4. Raising and lowering the pH with acid and caustic is going to be to complex for such a small system.

5. You probably do not want to add the chlorine on the raw water side because the chlorine will produce the THM's. Better to add the chlorine after the water is filtered.

6. You will also have to add a huge quantity of chlorine to oxidize the ammonia. You are better off using less chlorine and using chloramination.

Good luck.

 

[bimr]

One last item. You would have to add a large amount of acid (over 200 mg/l) in order to lower the pH to air strip the hydrogen sulfide. Basically, it would involve removal of all of the alkalinity.

 

[MsPersistence]

1) We were not concerned about CO2. That was a recommendation by another party. I doubt there is much CO2 and it was recommended to introduce it to lower the pH

2) The aeration was for oxidation. My orignal idea was simply to use fine microbubbles to aerate and then filter out the sulfur particles. I did not realize that H2S would not fully oxidize until it reached a pH around 5.5

3) Do you have any other ideas besides pot-perm?? We really try to stay away from pot-perm if we can. The current operator does not want to use pot perm.

I was thinking perhaps we should try shock chlorinating the well to get the bacteria numbers down in the aquifer and first see if we can maintain control that way. But I am concerned about the residual chlorinated water, frequency of shock chlorination required, and the potential to produce even more THM's. I cannot understand how a well such as this has such high TOC.

Do you have any other ideas or recommendations?

I look forward to hearing from you. Thanks.

 

[bimr]

1) CO2 will not significantly lower the pH. If you want to lower the pH, you will need to add an acid. The acid is going to convert the alkalinity to CO2.

2) Air oxidation of hydrogen sulfide is a very slow process.

3) What is the problem with KMO4? It is a common water treatment chemical and safer than chlorine.

4) You did not mention that you have problems with the well. If you have well contamination, you will need to disinfect the well. I don't understand how you can operate if the well is not disinfected.

5) Once the well is disinfected, you should not add chlorine until the water has undergone treatment. That way, you will avoid THM formation.

6) Here's another process. Aeration to take out a little hydrogen sulfide (maybe 1-2 mg/l hydrogen sulfide removed with aeration). Then chlorinate to oxidize the rest of the hydrogen sulfide, then filter.

Disadvantage of this process is high chlorine usage & THM formation

 

[MsPersistence]

I do not know the source of the H2S - I am thinking that there is a tonne of sulphate reducing bacteria in the aquifer. They enjoy the anaerobic conditions and thrive, producing H2S in the meantime. However, the TOC is probably an indication of an old organic bed of some kind. I do not know the depth of the well or the construction (sandstone, shale, etc) however the river is only 1 km away. I do think the well is very deep. I am thinking of obtaining more information on the well and also geting the SRB BART kit from Hach to see if it is SRB. I like the ideas you presented. We need to keep it simple. The dibromos may be over currently but perhaps we can deal with the TOC using another method which we have found successful.