After complete oxidation, Sulfides "reappear" after Multi Media Filters.

You might try searching this site as this subject has been repeatedly discussed.

Hydrogen sulfide takes a very long time to oxidize. How much time are you providing?

How much hydrogen sulfide is present in the raw water and how is it being analyzed?

How is the hydrogen sulfide being analyzed after the filters?

I apologize if this has been discussed already. I was sifting through the posts for a while, and I unable to find anything similar to my problem.

We have two sediment tanks both have large volumes (30,000 gal together), Depending on the in fluent flow (which varies), the residence time in the tanks can be anywhere around 50-80 minutes.

The average concentration of the sulfides coming in is 15-20 ppm. The inflow ranges from 500-600 gal/min on an average day. We use a total sulfides kit using a dropper method, to eyeball the approx concentration.

After the filters we use a Colorimeter (la Motte), which is able to detect sulfides at low levels (<1.5 ppm). I also use this to test the effluent coming from the sediment tanks (which is usually 0.00 ppm). Every once in a while our peroxide pump will get air locked overnight (maybe once every two months or so), so the hydrogen sulfide isn't reduced over night and enter the filters, not sure if this has anything to do with it.

I've attached a picture of our process/setup.

A little more input on the filters: They are multi media filters with anthracite, fine and course garnet. My predecessor was not operating these filters correctly. Inside the second sediment tank, we have two float switches that were connected to two different pumps. Normally only one pump would turn and if the influent flow rose to higher levels, the 2nd pump would kick on. The problem with this is how the backwash works with these filters. There is a restrictor valve on the filters. You would adjust the valve to the maximum flow before seeing media loss in the backwash effluent. So if my predecessor calibrated the restrictor valve based on the flow of one pump turning on (700 GPM), then if two pumps turned on (1400 GPM) chances are we be losing media (the backwash flow would double). Or vise-versa, if he had calibrated the valve to two pumps turning on (which I believe to be the case), then every time the backwash sequence started with only one pump (which was usually the case, maybe 90% of the time) then the backwash would only be half of what it should be (meaning it wasn't cleaning much at all). So this went on for several months I'd imagine before I joined. Sure enough we had a pressure differential problem. The filters were getting massively clogged (massive flow loss). To remedy this I was able to get both pumps on one float switch (one constant flowrate to adjust the restrictor valve), gave each media pod a chlorine shock, and increased the backwash to the recommended amount. I have ran these filters like this for several weeks now, and the sulfides "reappearing" problem is still occurring.

Thanks for the reply, If you need more information just let me know. Any help/direction you can give would be very much appreciated.

I'd suggest you take samples and then read them about 3 times to make sure your testing is consistent then submit the same exact samples to a testing lab to confirm your testing process.

I'd do this on a low detected output day and on a high detected output day. The high or low levels could be measurement issues that will otherwise drive you insane.

Keith Cress
kcress -

The system that you have is not optimal. Is there a method to remove the suspended solids from the sediment tanks?

The influent suspended solids level (as represented by the 126 NTU) is too high for the filters, the level should be less than 50 mg/L.

The effluent suspended solids level (as represented by the 11 NTU) indicates a problem with the filters.

Have you filtered the effluent suspended solids and then measured the sulfide concentration?

I completely agree with you with the "non-optimalness" of the plant, I more or less inherited this plant (and it's problems).

There's two sediment tanks in series, both with the standard underflow/over flow weirs. The bridge between the two tanks has the water fall a few feet into sediment tank 2 (to premote aeration). The final 3rd section of the sediment tank 2 has a water fall over the overflow weir and down about 6-7 feet (further aeration) to the "holding chamber" where the pump floats are located and active when the chamber becomes filled to a certain level (~35% of the tanks height). So I'm not sure if that deals with the suspended solids.

The turbidity coming into the sediment tanks its pretty low, ~10 NTU. I believe the reason the water becomes so murky (white) and why turbidity jumps so high is the elemental sulfur that's produced during the oxidation of the H2S by the H2O2. So the filters are catching / removing the elemental sulfur, I'm just confused as to why the elemental sulfur goes back into a sulfide form during/after the filters.

UPDATE: Over the weekend, I chlorine shocked all 7 Multi media filters and left them bathing over night (based on the recommended 20oz of chlorine per ft^2 of filtration area. Today when I checked the effluent of the multi media filters, it was 0.02 ppm sulfides. So we may have this problem wrapped up hopefully. The turbidity was still around 10 NTU though. So I'm thinking that there was some sort of organic build up (possibly tar, read below) within the tanks that was promoting the elemental sulfur back into a sulfide form? Does that make any sense?

On a further note, possibly related, when I opened up the multi media filters I noticed there was extremely sticky oil (more like tar, as we are treating ground water near tar pits...). I'm curious to know how the oil/tar is getting passed the sediment tanks. We have an oil boom position in the first sediment tank (in the middle of tank 1, between the over and underflow weirs). Shouldn't the oil/tar float, and be absorbed by it? I added a 2nd oil boom and placed it directly above the first overflow weir in the sediment tank 1, my line of thought being that the water must flow over the overflow weir, and if there is tar flowing with it it would flow very close to the oil boom. Any one face any oil/tar related problems like this? If this is what caused the problem i'd like to prevent it in the future.

Thanks again for the help, let me know if you need more information.

I would opine that once the sulfides are oxidized, the sulfides will not dissolve back into solution.

The oxidized sulfides should be a dark color not white.

It would take a miracle to remove all of the oil with an oil boom or two. Unless the pH is down around 5 or so, some of the oil will be emulsified and dissolved.

Unfortunately, you will suffer operating such a poorly designed filter. The filters in particular will be a maintenance headache.

Thanks bmir, headache is an understatement...

I was under the impression that the fully oxidized hydrogen sulfides turns into elemental sulfur (the milky white color), or is fully oxidized H2S a sulfate?

I was doing alot of reading through previous posts and I'm thinking I may have a serious case of Sulfate Reducing Bacteria forming within the mutli media tanks. I'm inclined to think this for a couple of reasons and please tell me if I get something wrong here.

1) My effluent from the sediment tanks is reading 0.00 ppm sulfides, while my effluent from the multi media is reading 0.40 ppm sulfides. It could explain how my fully oxidized sulfides (sulfate?) is reducing back into hydrogen sulfide (pretty sure that the sulfides I'm reading in the MM effluent is H2S due to the rotten egg smell).

2) My two booms alone can't prevent all of the organic material from entering the MM filters. So the sludge/oil that enters the MM filters could be breeding the colonies?

3) After my chlorine shock, the sulfides in my effluent dropped and stayed stagnate for a couple of days, then sure enough started rising again. Like the SRB colony was damaged and regrew?

I did not mention this in my earlier posts but after our MM filters we have 4 GAC towers (2 leads, 2 lags), as I wanted to first fix the MM filters. The effluent from the GAC's has also been a headache. The sulfides in our final effluent are around 1-2 ppm (strong odor). The carbon is well before its estimated breakthrough (thanks to my predecessor)and I'm thinking this is due to the fact that it has been adsorbing the H2S that was reforming from the SRB's in the MM filters. From what I read, adsorbing H2S is a huge waste for your carbon, as it blocks many of the pores (earlier breakthrough?).

I was wondering though... Is it possible for SRB's to grow within the GAC towers? I know we need to replace the carbon, but it is still adsorbing some VOC's we need to eliminate (1,1-DCE's). I don't really want to replace the carbon until I have the MM filters figured out, as we'd just waste the carbon again.

How would I go about removing the SRB's (if thats what it is) from the GAC's when the activated carbon adsorbs chlorine? We use hydrogen peroxide for our oxidizer, could I just massively up the dose for a while and kill the buggers in the MM and GACs?

As always any input / advice would be greatly appreciated! Thanks again.

The H2S will be air oxidized to sulfide in the tank. The sulfides will then be oxidized by the ozone to sulfates.

Increasing the peroxide dose will should end the possibility of SRB's.