Raschig USA Inc.

Odor Control H 2 S / Sewer Gas Wet Scrubbing Explained

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Courtesy of Raschig USA Inc.

H2S / Sewer Gas / Rotten Egg Smell Control
Hydrogen Sulfide, chemical formula H2S. also commonly referred to as 'sewer gas' is frequently a nuisance problem at waste water treatment facilities - and a source of complaints from neighbors of such facilities.

Additionally, H2S not only stinks, at high concentrations it incapacitates, blinds and eventually will kill if allowed to concentrate fiulher. And as it is heavier than air. if not controlled it can under proper circumstances collect undetected in low places such as basements, septic tanks and even semi-enclosed areas to dangerous concentrations. And, nature is cruel - at dangerous concentrations H2S is odorless!

So control, i.e.. destruction, of H2S gas has a significant safety benefit in addition to odor elimination.

Packed Bed / Wet Scrubber Control Option
There are several techniques available to destroy H2S in a gas stream. Generally, packed bed / wet scrubbing is used when the air flow is relatively large. ~ 10.000 cfin or more, and the H2S concentration in that gas stream is relatively modest. - 50 ppmv or less. For such conditions counter-current or horizontal backed bed wet scrubber works well. The foul air containing the smelly gas is passed into and up through the packed tower as a scrubbing solution of an oxidizing agent at high pH is passed down and out of the tower. A typical oxidizer is simple bleach, sodium hypochlorite. The overall chemical reaction is: blowdown is consumed oxidizing H2S in the Ist stage scrubber. So 100% utilization of NaOCl (bleach) is achieved.

Note: In addition to the above chemistry, there is another reaction that occurs between H2S and NaOCl:

H2S + 4NaOCl + 2NaOH -> Na2S04 + 4NaCl + 2H20 (pH - 9)

The above odor control can be accomplished in a single stage scrubber. However, oxidizing agent (bleach) is the most expensive chemical agent used in the above example. And if only a single stage scrubber is used it must therefore be overdosed. So as a result often odor control at municipal WWTPs. and other locations, is achieved using two stages of packed bed / wet scrubbers in series. In such an instance the chemistry shown above is broken into two steps:

Stage 1: H2S + NaOH -> 2Na2S + H20 @ high pH. typically 10 or above Stage 2: In this stage, the solubilized S'2 is oxidized by bleach to sulfate ion (pH - 9)

The bleed or 'blowdown* of the 2nd stage is directed to the 1st stage sump. In this way no expensive oxidizing agent is sent down the drain. Rather, any residual NaOCl in the 2nd stage

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