H2S odour in wastewater treatment: where it comes from and how to tackle it

In wastewater treatment, the smell of rotten eggs can almost always be traced back to one substance: H2S, or hydrogen sulphide. For local residents it is mainly a nuisance, but for installations and staff it is downright dangerous. Whether it concerns a municipal sewage treatment plant, a pumping station or an industrial wastewater treatment plant: if you want to tackle H2S odour, you need to know where the gas is produced and which filtration solution fits.

How is H2S formed in wastewater?

Wastewater is often transported under anaerobic conditions, meaning with very little oxygen. Under these conditions, bacteria break down organic material and produce H2S. In long pressure mains and sewer systems, the concentration can rise considerably. As soon as the water is released, for example in a receiving pit, the gas enters the air.

Which process steps cause the most odour nuisance?

The largest odour sources are mainly found at the start of the treatment process:

• Receiving pit: this is where the wastewater flows in and is effectively stripped. This often produces the highest H2S concentrations.
• Buffer tank: this tank evens out fluctuations in flow and concentration. Those fluctuations are large: concentrations vary with temperature, time of day and, for example, the moment in the morning when everyone flushes at the same time.
• Sludge treatment: the separated solids give off a strong odour.
• DAF units: in industrial treatment plants, small air bubbles carry solids to the surface where they are skimmed off. These bubbles also release H2S.

In a decentralised setup, we give exactly these steps a higher filtration capacity, so the odour filters are placed where the load is greatest.

The two-stage approach: bulk filter plus colour indicator

For H2S at treatment plants, we usually apply a two-stage setup in an odour control system:

1. Impregnated activated carbon as the bulk medium. H2S is an acidic gas and can be captured chemically very well with carbon impregnated with an alkaline substance. This reaction is irreversible, so the gas is not released again. Laboratory values of 30 to 40 percent by weight are often quoted: 100 kg of carbon could then absorb 30 to 40 kg of H2S. We deliberately calculate conservatively with around 25%, so the promised service life is also achieved in practice.
2. A top layer of potassium permanganate medium. This chemical filter medium also captures H2S, mercaptans and other sulphides, and has a useful property: it changes colour. From purple to dark brown to light yellow and eventually white. When the top layer starts to discolour, you know the bulk carbon underneath is becoming saturated.

This colour indicator is important because with activated carbon you cannot measure how close you are to saturation. Without an indicator, you only notice when odour breaks through. With the top layer, you can plan replacement before local residents smell anything.

Is ammonia present alongside H2S? At low concentrations, the two-stage setup is usually sufficient. At higher concentrations, we add a specific medium for ammonia, such as acid-impregnated carbon from our range of activated carbon filter media.

H2S: not just an odour problem

H2S is called a silent killer for good reason. At low concentrations you smell it immediately, but at higher concentrations your sense of smell is numbed and you no longer notice the gas. Prolonged exposure from around 100 ppm is harmful, and at around 1000 ppm, a level that does occur in practice, exposure is immediately fatal. Our technicians therefore follow strict safety instructions when replacing filter media: first measure the concentration with an H2S meter, and only then start work with the right personal protective equipment.

H2S also damages installations. In control cabinets and electronics, it causes corrosion that cannot be reversed. That calls for a different, proactive approach. Read more on our page about corrosion control.

Responding quickly to odour nuisance

A pumping station in the middle of a residential area is critical: if it starts to smell, you want the problem solved the same day. With our own vacuum truck, we replace filter media quickly and ergonomically, without depending on the availability of third parties. Under one of our contracts with a water authority, we even guarantee to resolve an odour problem within one day.

Conclusion: tackling H2S odour in a targeted and safe way

H2S odour in wastewater treatment mainly arises at the start of the process and requires a filtration solution designed for it. With a two-stage setup of impregnated carbon and a colour-changing top layer, you stay in control of service life and prevent breakthrough. Curious what this means for your treatment plant or pumping station? Take a look at our solutions for water treatment and sewage works or contact us for free advice.