Local problem or full sulfide potential?

Did you know that it is possible to get significantly different H2S measurements inside the exact same sewer manhole at the exact same time? It is!

In this insights story, we will show how your choice in measurement location and media has a big influence on your data. This emphasizes why your measurement approach should be determined by the purpose of your measurement campaign.

Measuring the full sulfide potential

The traditional approach to hydrogen sulfide monitoring in sewer networks is gas loggers installed in the headspace just below manhole covers. Although this approach is excellent for documenting local odor problems, it provides limited insights into the scope of the downstream sulfide risk. The gas loggers only reveal the local problem, because they only measure degassed H2S without accounting for the share of dissolved sulfide that is transported further downstream with the sewage.

As hydrogen sulfide is formed and transported in the wastewater – and only a fraction is released into the air in the headspace – the best insights into the full potential of the sulfide challenge is achieved by measuring the dissolved gas directly in the sewage. This measurement approach is possible with the introduction of sensors for continuous, liquid-phase monitoring.

Same well, different results

Is there a noticeable difference between measuring hydrogen sulfide in the air or in the sewage? To answer that question, we decided to install two SulfiLoggerTM sensors inside the same discharge well, one measuring directly in the wastewater (black) and one measuring in the air below the manhole cover (red). The data from the two sensors paint two very different pictures about what is going on in what is essentially the same place. Both graphs show that a periodic rise in hydrogen sulfide concentration follows general pumping rhythms as shown in the periodic spikes on both graphs. But while the sensor measuring in the headspace shows very short peaks in the H2S concentration followed by long periods of almost no sulfide impact, the sensor measuring in the wastewater instead shows that there are significant concentrations of H2S being transported downstream with the sewage in the entire timespan of the graph.

This data set illustrates how you would be inclined to reach two very different conclusions about the state of the problem based on which data set was presented to you – and that obviously implies that your course of action or recommendation would be very different.


Your data’s ability to give valuable insights depends on the purpose of your measurement campaign. Gas-phase measurements performed in the headspace below manhole covers are indeed the best choice for documenting local odor problems, but for a wider range of advanced applications such as chemical dosing control or critical infrastructure decision-making, liquid-phase measurements – or gas-phase measurements performed much closer to the sewage – provide more reliable insights into the full scope and dynamics of the hydrogen sulfide challenge.

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