Project Description

Case study

Liquid-phase H2S sensor cuts end-of-pipe dosing by 50%

The SulfiLoggerTM H2S sensor has demonstrated its ability to optimize H2S management activities and reduce operational costs. A Danish water utility used the continuous signal from a SulfiLoggerTM sensor as a direct control input for the dosing of iron salts. By dynamically adjusting the chemical dosing rate to match real-time H2S sensor data, the effectiveness of the dosing system improved while chemical consumption dropped by 50%.

Background
Hydrogen sulfide (H2S) causes severe problems in collection systems when wastewater is pumped over long distances. To limit rotten-egg odors and to mitigate premature asset deterioration, utilities often add neutralization agents to the wastewater. However, without a dynamic overview of the H2S concentrations in the wastewater, the optimal chemical dosing rate remains unknown. This lack of information implies that there will be either under- or overdosing and time consuming optimizations.

Challenge
A Danish water utility wanted to optimize the dosing of ferrous sulfates (FeSO4) in a force main discharge well to reduce the consumption of chemicals and to improve the mitigation of potential H2S related odor and corrosion issues in the collection system.

Solution
A small, self-contained dosing system was installed at the force main discharge well consisting of a SulfiLoggerTM H2S sensor, a dosing pump, and a chemical tank. In this setup, the SulfiLoggerTM sensor’s real-time H2S signal was used as a dynamic control input for the dosing pump. By measuring directly in the raw wastewater at the end-of-pipe transition inside the well, the SulfiLoggerTM sensor was able to quickly detect changes in the composition of the wastewater and thereby allow the fast reacting chemicals to be added in just the right quantity. The dosing rate was simply proportional to the H2S signal.

To measure the effect of the dosing setup, an additional SulfiLoggerTM sensor was installed in the sewage in a manhole 1.2 km downstream in the gravity system, and using these two measurement points, different dosing strategies were implemented and compared.

Wastewater

  • Reduce chemical consumption
  • Mitigate H2S induced corrosion in the collection system
  • No odor complaints

Sensor-controlled dosing of FeSO4 end-of-pipe using continuous, liquid-phase H2S measurements

  • 50% reduction in chemical use
  • Zero H2S downstream from the dosing point
  • Increased lifespan of sewer assets (Corrosion potential eliminated)
  • No odor complaints

The SulfiLogger™ sensor delivered continuous H₂S data to a chemical dosing pump (1), which dynamically adjusted the rate of chemical injection proportional to the sensor signal (2). The effectiveness of the system was measured and compared to other approaches using a second SulfiLogger™ sensor measuring in the sewage 1.2 km further downstream.

H2S mitigation effectiveness

Chemical consumption

Results
With dynamic H2S sensor-controlled dosing, the consumption of chemicals was optimized, and all downstream H2S problems were fully mitigated since only negligible H2S levels were observed.

With a constant dosing strategy, even using twice the daily amount of chemicals used for the sensor-controlled dosing strategy, the dosing was unable to fully neutralize the H2S spikes above 1mg/L.

Without any dosing, the majority of the dissolved H2S detected end-of-pipe was transported to the downstream verification site 20 mins later, where odor and corrosion issues would persevere.

The pitfalls of constant dosing
Constant chemical dosing – the dominant strategy used for the dosing of iron salts – is a simple but inefficient approach to H2S mitigation. The fundamental shortcoming of this strategy is that H2S is a dynamic variable – not a constant – and as the composition of the wastewater changes, a constant dosage is excessive throughout long periods of the day, yet also incapable of fully neutralizing the effect of H2S spikes. The constant dosing strategy also fails to account for shifts in the magnitude of H2S variations caused by factors including pump operation settings, changing seasons, varying temperatures, and heavy precipitation.

What are the benefits of dosing chemicals end-of-pipe?

  • Automatically dose chemicals in direct proportion to the H2S sensor signal as illustrated in this case for cost-effective H2S mitigation
  • Quickly detect and neutralize unforeseen spikes in H2S that would otherwise cause odor and corrosion problems
  • Dose chemicals completely independent of complex factors influencing the production of H2S in the pressurized system such as the wastewater’s hydraulic retention time, different sources of sewage (industrial and residential wastewater), and the effect of branched connections

Savings potential
The H2S sensor-controlled dosing strategy improved the effectiveness of the dosing system, thereby minimizing the impact of corrosion and odor issues, while using 50% less chemicals compared to a constant dosing strategy. This case has proven that a dynamic, sensor-controlled dosing strategy – using the SulfiLoggerTM sensor – can enable utilities to optimize the effectiveness of H2S management activities and reduce costs.

The SulfiLogger™ sensor measured directly in the raw sewage in the inlet of the well.

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