Project Description

The shortcoming of H2S gas sensors in wastewater applications

A recent measurement campaign performed by Unisense and the Danish water utility Hedensted Spildevand highlights how traditional gas phase H2S sensors struggle to provide a clear picture of the presence of hydrogen sulfide in wastewater collection networks. Mitigative actions relying solely on indirect, local gas phase measurements could therefore simply move the H2S problem further downstream in the sewer network rather than solving it at the source as intended.

Background and solution
At a pump station sump in an area commonly associated with H2S problems, Unisense installed a SulfiLoggerTM H2S sensor directly in the raw wastewater to supplement an existing H2S gas sensor installed just below the cover. Both sensors were connected to Hedensted Spildevand’s SCADA system and provided online H2S data as shown in the graph.

Results
Although both data set were correlated, the graph showed a clear difference in the sheer level of detail between the two measurement approaches. The SulfiLoggerTM H2S sensor provided a complete picture of all the daily fluctuations in dissolved sulfide in the raw wastewater, while the traditional gas sensor only managed to register spikes.

The traditional gas sensor struggled to detect the daily variations, as heavy ventilation of the air and the long distance between the gas sensor and the wastewater at most times diluted the hydrogen sulfide concentration in the gas well below the 1 ppm detection limit of the sensor. The gas sensor did measure correctly throughout the entire campaign, but the graph highlights a fundamental shortcoming of using indirect gas phase H2S sensors as a proxy measurement in wastewater applications.

The SulfiLogger™ sensor was connected to an extension pipe, connected to the wire mesh and lowered down into the raw sewage.

Perspective
Reliable gas phase measurements depend on several factors including air ventilation, water turbulence and the sheer distance between the sensor and the wastewater. Relying solely on data from gas sensors for mitigative actions, utilities could thus be inclined to conclude that the concentration of H2S is non-existing at times, even though large quantities might remain in the sewage only to be transported further downstream in the network.