In short
- H2S needs alkaline, oxidising chemistry. Ammonia needs acid chemistry.
- Where both are present, a two-stage scrubber treats ammonia first, then H2S.
- Wet chemical scrubbers cope best with high or variable loads; other options suit steadier, lighter duties.
Hydrogen sulphide and ammonia account for a large share of odour problems at wastewater, food, rendering, composting and chemical sites. They are also workplace hazards in their own right. This article compares the main ways to remove them, with a focus on when a chemical scrubber is the right answer.
Know the gases
| Gas | Smell | UK workplace limit (EH40) | Chemistry that removes it |
|---|---|---|---|
| Hydrogen sulphide (H2S) | Rotten eggs | 5 ppm long term, 10 ppm short term | Alkaline liquor plus an oxidant |
| Ammonia (NH3) | Sharp, pungent | 25 ppm long term, 35 ppm short term | Dilute acid liquor |
Odour complaints usually begin at concentrations far below these workplace limits, so a site can be safe for staff and still be a nuisance to neighbours.
How a two-stage chemical scrubber removes both
Ammonia is removed with a dilute sulphuric acid liquor. The two react to form ammonium sulphate, a non-volatile soluble salt that leaves with the bleed. Hydrogen sulphide is removed with sodium hydroxide and sodium hypochlorite: the caustic neutralises it and the hypochlorite oxidises the sulphur to soluble sulphate. Because the products do not come back out of solution, the liquor can be recirculated efficiently.
The two chemistries cannot share a sump, so the scrubber is built as two stages in series. Ammonia is normally removed first, in the acid stage, so it does not consume reagent in the alkaline stage.
Getting odour complaints?
Tell us the source and what you have measured. We will tell you which stages you need.
Other removal options
| Option | Best suited to | Watch out for |
|---|---|---|
| Two-stage wet chemical scrubber | High, variable or mixed loads | Chemical handling and running costs |
| Biofilter or bio-scrubber | Moderate, steady loads | Slow response to load swings; media replacement |
| Activated carbon or impregnated media | Low loads, polishing | Fast exhaustion at high load; moisture sensitivity |
| Catalytic iron media | Roughing ahead of a polishing stage | Partial removal only |
Our comparison of scrubbers, carbon filters and biofilters goes into this choice in more detail.
Engineer's note
Odour loads at wastewater and food sites are rarely steady. We ask for a few days of logged H2S readings rather than a single spot measurement, because the peak, not the average, is what reaches the neighbours.
Mercaptans and other sulphur compounds
Many odours involve organic sulphur compounds such as mercaptans as well as H2S. The oxidising alkaline stage handles many of them, but some need extra contact time or a polishing stage. A gas analysis before design is the best way to avoid surprises.
Planning odour control at a works or plant? Our odour control page shows the two-stage designs we build.
See odour control scrubbersFor sites where acid gases, ammonia and sulphur compounds all appear, our chemical scrubbers combine the stages in one package.
Common questions
Can one scrubber remove both H2S and ammonia?
Yes, as a two-stage unit. Each stage runs its own chemistry: acid for ammonia, alkaline and oxidising for H2S.
Why is ammonia removed first?
If ammonia reached the alkaline stage it would not be captured well, and in the acid stage it is removed cleanly as ammonium sulphate.
Is a biofilter cheaper than a chemical scrubber?
Biofilters often have lower whole-life costs on moderate, steady loads. Where loads are high or change quickly, a chemical scrubber is usually more reliable.