Hydrogen sulfide in water

About six months ago, my friends asked me to help determine what water treatment system to chose for a private house. The source of water supply is a 40 meters deep-well. Initially, I assumed that the water will have a composition inherent to groundwater. So, the well is considered to be artesian if it has protection against pollution by surface waters, i.e. it has no contact with the atmosphere and is not polluted with different organics. As a rule, the protection of artesian water provides a layer of clay. A layer of dense clay begins at a depth of an average of 25 to 75 meters. Water can be considered artesian if it located deeper than this layer of clay. In such water there is practically no organic matter and oxygen. If the mineral composition is right (salt content of water should be less than 1000 mg / l), this water is considered drinking.

On the territory of the Saratov region artesian water begins at a depth of 100 m, so I assumed that the well with a depth of 40 meters is likely to be groundwater. That is, it will be contaminated with organic matter and oxygen will be in the water, because it has contact with surface waters which are rich in organic matter and oxygen.

I strongly recommended to make a full water analysis for the compliance with the requirements of SanPiN “Drinking water”, but … as you know it is easier said than done.

So, the water from the well was brought for analysis. I test water for the adjustment of water-chemical mode of boiler rooms. Therefore, I can make an analysis with sufficient accuracy on the following parameters salinity, hardness, alkalinity, pH, iron, oxygen, CO2, ORP.

As a result of the analyses, the following data were obtained:

Salinity – 261 mg/l

Hardness – 2,6 mg-eq/l

Alkalinity – 1,0 mg-eq/l

рН – 5,8

ОRP   + 28 mВ

Fe – less 0,1 mg/l

СО2 – 133 mg/l

As we can see from the analyses the water has a low pH. This is lower than the requirement in SanPiN (6,0-9,0). This suggests that the water is saturated with carbon dioxide (CO2). If the surface water pH is usually 7.6-8.0 (with the concentration of CO2 in the water is 2.5-5 mg/l), then the concentration of CO2 in this well is 133 mg/l!!! Because of this, there is a low pH of water. It should be added that LSI index of the water is minus 2.2. This indicates a strong corrosion activity of the water. If we take into account that this water contains almost no oxygen (redox potential of only + 28) and the fact that CO2 does not leave the water into the atmosphere, we can say that this water is artesian. Probably, the layer of dense clay is very close to the surface at the location of the house.

I proposed to produce non-pressure aeration of water to raise the pH. Since there is quite a lot CO2 in water, it would be easily removed by the usual spraying of water in a certain volume and, accordingly, the pH of the water would immediately increase to acceptable values.

You can verify this simply by pouring this water into the beaker and measure the pH. Initially, the pH value was 5.8. After 8 hours, the pH value was 6.4 and after 14 hours 6.8. The pH value of 6.4 corresponds to the concentration of CO2 in water is 33 mg/l, and pH value of 6.8 – CO2 concentration of 17 mg/l. Then, even after 2 days, the pH value increased only up to 7.1, which corresponds to the concentration of CO2 in water-6.8 mg/l. Theoretically, CO2 in water and in the atmosphere should be the same, i.e. equal to 0.4 mg/l, but such parameters can be achieved only with intensive removal of CO2 from water (for example, aeration or spraying in the air volume).

We can say that the more carbon dioxide there is in the water, the more effective it will be removed from the water. In this case, I think the usual non-pressure aeration is enough.

But if the water is aerated, such water will be saturated with air oxygen, as well as polluted with various substances which contained the air contains. Therefore, it is necessary to use at least ultraviolet disinfection and a filter with active carbon after non-pressure aeration.

As a result, the owners decided not to pure the water at all. A fine filter cartridge was installed directly in the well. Then the water under pressure enters the accumulator tank, which primarily functions as a water reserve tank (tank of 150 liters). The accumulator tank has no contact with the atmosphere. Then the tank water is supplied for consumption.

So, at first everything was fine. Summer, irrigation, large water flow through the system. In December 2018, the owners appealed to me with a complaint about the smell of hydrogen sulfide from the water. Moreover, the water right from the well did not smell of hydrogen sulfide, and after the accumulator tank had a stable smell of hydrogen sulfide.

They brought me the water with the smell of hydrogen sulfide for re-analysis. As a result, the analysis of water was almost identical to the first analysis of water except iron. The concentration of iron in the second sample was 0.4 mg/l. It is probably because of corrosion of metal pipes due to low pH of water. But this is quite obvious. Much more interesting is the fact of the appearance of hydrogen sulfide in water. Here it is necessary to discuss it in detail.

Where may you receive a sulfide in the water, if it’s not in the source water?

It is obvious that in this case hydrogen sulfide, or rather sulfide ion, appears in the water as a result of the life of anaerobic bacteria. These are bacteria that appear in water in the presence of organic matter and the absence of gaseous oxygen. If there is gaseous oxygen and organic matter in the water, aerobic bacteria develop. In fact, both aerobic and anaerobic microorganisms use oxygen in their life. But only aerobic bacteria take oxygen from the air, and anaerobic from inorganic compounds such as NОх, SO4, etc.

We can say that if there is organic matter in the water and water has contact with air, then aerobic bacteria develop in such water, gradually oxidizing organic matter, using air oxygen for this purpose.

If there is an organic substance in the water and the water does not have contact with air oxygen, then anaerobic bacteria develop in such water, using oxygen, which is part of the anions (i.e. part of the mineral composition of the water), to oxidize the organic matter.

In this case, I think the following process takes place:

Water entering the pipeline system with the accumulator tank is polluted with organic matter. This may have happened only at one time (for example, the pipeline system was not pre-washed before the first start-up).    However, organic matter was sufficient for the development of anaerobic bacteria. There is no oxygen in the water and, accordingly, a type of bacteria using oxygen of sulfate SO4 for their life activity has developed. Sulfate in the source water is not less than 25 mg/L. Although I don’t do tests for sulfate, but in this case it’s obvious.

Accordingly, it turns out that if the sulfate SO4 will give oxygen it will sulfide ion S2-. Moreover, sulfide ion in water exists in three forms and depends on the pH of the water. So when the pH is 10.5, the water actually contains only the sulfide S2 – and hydrosulfide HS-. At pH 7, it exists in the form of sulfide and hydrosulfide about 47% and hydrogen sulfide, respectively, 53 %. At water pH of 6.0 sulfide and hydrosulfide in water is only 10% and 90% hydrogen sulfide.

In our case, the pH of water 5,8, respectively, almost all sulfide exists in the form of hydrogen sulfide gas. It is worth noting that if the pH of the water would be, say, 9,0 that hydrogen sulfide would be 1%, and sulfide 99 %, but still there is no smell of hydrogen sulfide it only masked the presence of sulfide in the water and such water containing a strong reducing agent, will have a strong corrosion aggressiveness, especially with respect to non-ferrous metal.

How can we fight bacteria that produce hydrogen sulfide?

First of all, there should be no organic matter in the water or air oxygen.

The most obvious solution in this case is the installation of non-pressure aeration. In this case 2 problems are solved at once. Firstly, excess carbon dioxide is removed from the water and, accordingly, the pH of the water grows, secondly, the water is saturated with oxygen and, accordingly, there will be no anaerobic bacteria producing sulfide ion in the water.

But there is a drawback. Water in the aeration process will be exposed to secondary air pollution, and installation of ultraviolet and coal will be required. And the tank of the aeration system will have to be cleaned from time to time.

Therefore, to begin with, a simple decision was made to disinfect the water supply system and a storage tank with a solution of hydrogen peroxide. After disinfection, the smell of hydrogen sulfide from the water disappeared.

Now we just have to watch. If the smell of hydrogen sulfide appears again, probably, after all, we will have to do non-pressure aeration.

And the pH value of the water can be increased without aeration, just it needs a filter with calcite and it shout be added from time to time.


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