Engineering Evaluation Case: An Odor Scrubber Removes H₂S but Organic Mercaptan Odor Remains

An odor-control system may show excellent hydrogen sulfide removal while people downstream still detect a strong sulfur odor.

The immediate reaction is often:

“The scrubber is not working.”

The tower may actually be removing H₂S very well.

The remaining odor can come from different reduced-sulfur compounds such as mercaptans, which may not respond to the same scrubbing chemistry in the same way.

Project Situation

Consider an odor-control packed scrubber treating gas from:

• wastewater;
• sludge;
• organic waste;
• industrial process vents.

The inlet contains:

• H₂S;
• low concentrations of organic reduced-sulfur compounds.

The caustic scrubber achieves a strong reduction in measured H₂S.

Odor complaints continue.

The customer considers:

• more packing;
• higher liquid circulation.

Before modifying the tower, identify the residual compounds.

Odor Intensity and H₂S Concentration Are Not the Same Metric

Different sulfur compounds have different:

• odor thresholds;
• chemical behavior.

A very low concentration of another sulfur compound can remain noticeable even when H₂S is greatly reduced.

Therefore, an H₂S analyzer alone may not describe total odor performance.

Caustic Chemistry Is Not Equally Effective for Every Compound

NaOH is effective for many acid-gas duties.

Organic sulfur compounds can require different chemistry depending on:

• molecular structure;
• pH;
• oxidation state.

A packing cannot make one reagent chemically selective for every odorant.

The remaining odor may therefore be a chemistry problem rather than a contact-area problem.

Oxidation May Change Removal Behavior

Some odor-control systems use oxidizing chemistry to improve treatment of certain reduced-sulfur compounds.

Introducing an oxidant changes:

• material compatibility;
• reaction products;
• chemical consumption.

The exact process should be designed for the identified contaminants.

The tower supplier should not assume an oxidant merely because “odor remains.”

Gas Analysis Should Go Beyond H₂S

A useful investigation can identify whether the residual gas contains compounds such as:

• methyl mercaptan;
• other reduced-sulfur species;
• VOCs.

The exact analytical scope depends on the process.

Without identification, the project can spend money optimizing the wrong pollutant.

More Packing Helps Only When Mass Transfer Is the Limitation

If the existing liquid chemistry can absorb or react with the residual compound but contact is insufficient, more packing may help.

If the chemistry is fundamentally weak, additional height can have diminishing benefit.

This is the same principle seen in difficult VOC absorption but applied to a specific odor-control mixture.

Liquid Aging Can Change Selectivity

A recirculating odor scrubber accumulates reaction products.

Its:

• pH;
• oxidation-reduction condition;
• dissolved solids

can change.

A system that removed several sulfur compounds well with fresh liquid may become more selective toward only one as the solution ages.

Two-Stage Treatment May Be More Effective

Different contaminants may benefit from different:

• pH;
• reagent;
• oxidation conditions.

A two-stage system can sometimes provide better control than forcing all compounds through one liquid chemistry.

Whether that is justified depends on the actual composition and required outlet.

Activated Carbon or Other Polishing May Have a Role

Where residual odor compounds remain at low concentration, another polishing technology may be considered downstream.

This does not mean the packed scrubber failed.

It may mean the process requires:

• bulk removal stage;
• polishing stage.

The downstream technology should be selected from the residual gas composition.

Demister Performance Still Matters

A person may smell chemical carryover from droplets rather than residual gas alone.

Therefore, verify:

• mist eliminator condition;
• droplet carryover

before attributing every odor to gas-phase mercaptans.

What Should Be Monitored?

Useful data include:

• inlet H₂S;
• outlet H₂S;
• broader sulfur-species analysis when needed;
• pH;
• oxidant level if used;
• gas flow;
• liquid flow.

Engineering Takeaway

Odor treatment should be based on the compounds responsible for the odor, not only the easiest sulfur species to measure.