Floc Forming After Aeration To Reduce Hydrogen Sulfide

[sbush]

We are treating 650 gpm of well water to reduce hydrogen sulfide odor. Immediately after aeration (bubble-tray type), we are seeing a feathery, white "floc" that resembles surface water treatment floc, however there are no coagulants added to this feedwater. Only aeration, no chemicals, not even chlorine. The pH is 7.2. We have no analytical results for the quantity of H₂S however there is a noticeable sulfur odor in the aerator off gas.

This floc is causing the macroporous anion resins that we use to reduce 18 units of color (tannin) from the feedwater to cake up. The resin bed will solidify to the point where it will not fluidize during backwash mode. The resin simply breaks/cracks up into basketball sized chunks after backwashing and remains that way. The resins will return to individual beads after we manually rake them out. However, they will solidify again in a week or so, as the differential pressure loss across the bed increases about 0.5 psi per day. Right now, we are backwashing every day to maintain a relatively clean pressure drop of 5 psid (delta-P). The resins have not lost their ability to reduce color, so long as they aren't given a chance to become caked and chunks have formed.

There is also a jelly-like slime (biofilm?) on the wetted surfaces of the ground storage tank and color removal treatment vessels. The slime appears to be clear to gray-ish and, in some spots, pink-ish in color. Samples that I've collected in a sealed jar give off a sulfur smell after a short while in storage, and seem to turn black at the center of the mass.

I am hoping to hear from anyone who has direct experience with this "sulfur-floc". Not necessarily in a color removal system, but perhaps simply an aeration/detention/filtration systems. What is this floc? Does it clog sand filters? Does it correlate to the formation of surface biofilm? Can it be prevented by other means or removed without filtration?

I'm looking for firsthand knowledge and experience. No Google-ease please. Thank you.

 

[bimr]

Would have to suspect that the white floc is calcium carbonate that precipitates after the carbon dioxide is released in the aeration, The release of the carbon dioxide is causing the pH to increase.

Aeration to remove hydrogen sulfide does not typically produce satisfactory results. CO2 is also released concurrently with the hydrogen sulfide, raising the pH and causing the hydrogen sulfide to stay in the water. H2S(g) <====> H2S(aq) When dissolved in water, hydrogen sulfide is partially ionized, so that it exists as a mixture of H2S and HS -. The proportions depend principally upon the pH of the solution. 61% is the hydrogen sulfide ionized at a pH of 7.2, which means that you have only removed 39% with the aearation.

Without benefit of a complete water analysis, it is somewhat of a guess.

The slime is probably mold and/or anaerobic bacteria that may reduce the hydrogen sulfide to sulfur.

 

[QualityTime]

As bimr said you need to do an analysis of what is in the water. I have designed a municipal water treatment plant before where ground water was the source. Iron and manganese were issues. Given what I am saying below as a matter of interest I would like to know what is in the water
1. Groundwater is can be high in manganese, when you aerate the water SOME of the manganese will drop out. The speed of the reaction is dependent upon water pH, time and temperature. Manganese is black and my guess is that this is at the centre of the black slime. Municipal drinking water supplies require no more than 0.05 mg/L manganese in the water. The slime itself, as bimr suggest, is bacteria or iron bacteria. You have to chlorinate the water to control its growth.
2. As far as for the “white floc” a picture of what you are saying that you see would be helpful

 

[sbush]

The last two paragraphs of my post are what I'm really looking for.

 

[QualityTime]

Is this a new installation or a recent problem? Also, I don’t think anybody can help you unless you can provide more water quality information.
I understand that macroporous anion resins are used to remove ORGANICS from the water supply. It seems like your “sulfur floc” is reacting with the anion resins. The common theme with any type of aerator is that bugs (and resulting slime) can grow in the aerator or in downstream tankage. Bugs are ORGANIC. All of the aerator literature talks about treatment of the water to kill the bugs after aeration. In absence of anymore information being supplied by you I would look at cleaning out your aerator and tankage first. Also look at the floc under a microscope and tell us what you see

 

[QualityTime]

I could show you a picture black slime and grey slime on a fine air bubble diffuser from an aeration tank. The cause of the black and grey slime is low DO (<0.8 mg./L). when the DO is 1.5 mg/L or greater the we see a (normal) light brown washable film on the surface of the diffuser

 

[bimr]

The floc is calcium carbonate which will filter out with the typical media filter. Most people would use an air scour with the media filter to assist in backwashing the floc material out of the media filter.

If you want to prevent the white floc from forming, lower the pH. You are raising the pH in the aerator.

It is quite unusual to have tannin in a ground water well, it is normally seen in surface water.

 

[QualityTime]

Bimr, groundwater is hard, therefore I doubt very much aeration has elevated the pH enough to precipitate calcium carbonate. Besides calcium carbonae drops like a rock...and it does not form like a flocc

 

[bimr]

QualityTime, in the future, please do not respond to my posts. You typically state that you are an expert in water treatment, but then demonstrate a lack of water treatment knowledge by asking questions regarding basic facts of water treatment that a newby should be familiar with.

If you desire to increase your water treatment knowledge, review these articles, among others.

"Concentration of carbon dioxide varies widely in groundwater, but the levels are usually higher than in surface water. Water from a deep well normally contains less than 50 mg/l, but a shallow well can have a much higher level, up to 50 to 300 mg/l."

http://webcache.googleusercontent.c...m_prep/aeration.htm+&cd=2&hl=en&ct=clnk&gl=us

"When CO2 is lost, the solubility of calcite drops, and calcite crystallizes from the water. This is the mechanism by which speleothems (stalactites etc.) grow within caves, or travertine (calcite crusts) are formed at surface - as shown above at Travertine Falls in the Grand Canyon."

http://web.viu.ca/earle/geol304/304g.pdf

The pH is a function of the balance between alkalinity and carbon dioxide. If you have a well water with high alkalinity, then the carbon dioxide concentration will also be elevated. The carbon dioxide readily air strips, while the alkalinity does not. That is why the pH increases.

http://www.hwa.com.au/wp-content/uploads/2013/06/Clara-Paper.pdf

 

[QualityTime]

Bmir, I think your post says a lot about how highly you think of yourself

 

[QualityTime]

One thing you might consider doing is checking the pH of the incoming water and the pH of the air stripped water. To make sure we are talking apples with apples please make sure the water is the same temperature when the ph is measured. You can do this by keeping it in the refrigerator. If there is a lot of alkalinity in the water the alkalinity acts as a buffer to resist pH change whether it is towards the alklaine direction or the acidic direction

I assume you have some sort of tank where the air stripped water collects. If it is calcium carbonate precipitation you will see it collecting in the bottom of a tank. It will also collect on the walls of the piping and it will eventually plug the pipe.

 

[QualityTime]

sbush, I assume you know what calcium carbonate sludge looks like. It is the same stuff that collects and coats the inside of your tea kettle. It will also collect on the walls of your piping. This is the practical method of determinining if you have calcium carbonate precipitation. Groind water is very hard and it has a lot of alkalinity and I personally doubt that a pH change has occurred but I could be wrong. To get a pH change the residence time in the tray aerator has to be significant. At 650 gpm this must be one big tray aerator. The fact that you have a pH of 7.2 is a clue to what I suspect. Again look for biological growth in the water coming out of your tray aerator and in your holding tank. Have you ever opened up the tray aerator to do maintenance?

 

[QualityTime]

This is an interesting problem. It would be nice to hear from you on your findings/further investigation!

 

[sbush]

Thank you both for your interest. I've learned that the white floc is elemental sulfur. It is filterable using conventional sand media. If the feedwater pH were lowered sufficiently, as BIMR mentions, elemental sulfur wouldn't form after aeration.

 

[bimr]

Since I have been working out of town, I have not had access to my usual reference materials. I now have had some time to review this issue.

Attached are a few pages that document what you are experiencing. See the bottom of page 110.

A portion of the hydrogen sulfide is being air stripped as the water passes through the aerator. Some of the hydrogen sulfide passes through the aerator since the pH is too high to allow the hydrogen sulfide to be completely air stripped.

A portion of the hydrogen sulfide that has passed through the aerator is being oxidized by oxygen to elemental sulfur (colloidal sulfur).

What is puzzling is that the oxidation of hydrogen sulfide is a very slow process and would normally be something that is experienced with a long detention reservoir, not in an aerator where the retention time is extremely short. You have not described a system that has a long retention time.

 

[QualityTime]

Somehow I feel I am owed an apology for some of the harsh words in one post

Sbush, I though sulfer was yellow so I am not sure how you can get a white feathery floc? The answer may lie in the concentration but I though I would ask.

 

[sbush]

Detention time is 30 minutes. Serpentine flow path.

 

[QualityTime]

If you do a google search under "hydrogen sulphide removal from ground water + aeration byproducts" you will find a number of good articles and you will find the cause of why your filter is growing (as I suspected and communicated in my earlier post...bacteria). The solutions are discussed. Have a good day

 

[QualityTime]

Hi, you can also do a google search under "hydrogen sulphide removal from ground water". There are lots of good articles and you can put the pieces together as to what is happening and what your possible solutions are. I am curious as to why water quality studies were not done prior to installing the equpment?

 

[QualityTime]

I searched the internet and found "Innovative Hydrogen Sulfide Treatment Methods: Moving Beyond Packed Tower Aeration". I thought it was a good article.

http://www.bluetoad.com/display_article.php?id=432925

"....Anion Exchange Similar to cation exchange, anion exchange can be used to remove negatively charged dissolved solutes such as total organic carbon and sulfides, but with regard to sulfides, the removal efficiencies are variable and the resin can foul because of growth of sulfur-related bacteria. Also, salt regeneration of the resin is required, which will add cost and impact downstream wastewater discharges. Since the county was concerned about increasing the conductivity of discharges to its publicly owned treatment works, this option was not selected for field testing.

.....Anion exchange is another promising treatment method for removal of hydrogen sulfide. While it has not yet been employed solely for sulfides removal at a full scale, bench and pilot scale studies have shown that it is effective.

As discussed previously, an impact of using an anion exchange process is that salt must be brought on site for regeneration and subsequent disposal that will elevate conductivity levels for the municipal sewer system, which is of concern to the county. Regrowth associated with sulfide-reducing bacteria and other biological species may increase the likelihood of bio-fouling, causing operational problems when relying on anion exchange for sulfide removal (personal conversation with Audrey Levine of the EPA, Washington, D.C.). Based on the information gathered and analyzed, UCF developed a testing plan and protocol that was designed to evaluate the following short-listed technologies: mixed-oxidants, ferrate, Electromedia® filtration and MnO2 filtration...."