Optimized H₂S dosing control after treatment centralization in Aalborg

Background
Hydrogen sulfide (H₂S) is a persistent challenge in sewer networks and utilities often rely on chemical dosing to control the toxic gas and thereby reduce the risk of odor and corrosion challenges.

However, setting the optimal dosing rate for chemicals is difficult without reliable data. Sewer systems are complex and dynamic, with H₂S levels varying across the network depending on an array of factors including flow, retention times, and wastewater composition. Without understanding these variations, H₂S management risks becoming reactive, leading to odor issues that require time-consuming intervention from service personnel, premature asset deterioration caused by corrosion, and a risk of excessive chemical consumption.

Challenge
The Danish utility Aalborg Forsyning operates an extensive sewer network in Aalborg, Northern Jutland, and ongoing infrastructure investments have impacted the utility’s H₂S challenges. These include the centralization of wastewater treatment at two large-scale treatment plants located in the eastern and western parts of the city, as well as a sewer separation project in which stormwater is diverted into dedicated pipelines instead of being combined with wastewater. While these initiatives bring about several benefits, they have also led to longer transport distances and increased retention times in parts of the network; conditions that favor the formation of H₂S.

The main culprits are two force main lines outside Aalborg that transport sewage from surrounding catchment areas to the two treatment plants in the city of Aalborg.

• Nibe to Aalborg West WWTP
  A former treatment plant in Nibe has been converted to a lift station, resulting in long transport distances towards the western treatment plant in Aalborg. This line also carries a significant share of industrial sewage, further contributing to elevated and fluctuating H₂S levels if left untreated.
• Hals to Aalborg East WWTP
  This 43 km line transports domestic wastewater from small towns and a vacation home area near Hals, where flow conditions vary significantly. In the off-season, extended retention times create favorable conditions for H₂S formation.

Solution
To address these H₂S challenges, Aalborg Forsyning established several strategically placed chemical dosing stations using a mix of both nitrate and iron sulfates along both of the force main lines. To ensure effective control of these dosing stations, seven SulfiLogger™ H₂S sensors were installed at strategic sites, and the utility uses the sensors’ real-time data as a direct input for setting and adjusting dosing rates.

A SulfiLogger™ H₂S sensor before installation in a lift station well near Nibe.

The sensors measure directly in the wastewater and provide continuous, real-time data, which is integrated into the utility’s SCADA system via a cloud solution. This allows operators to monitor H₂S levels closely and use the data actively in daily operations. Based on real-time insights, dosing rates across the different stations are continuously adjusted to match actual conditions and cost-optimize chemical use.

The online liquid-phase H₂S data plays a key role in the setup, as explained by Michael Lystrup from Aalborg Forsyning:

“The SulfiLogger sensors measure directly in the liquid phase. They are the only sensors that can be placed directly in the wastewater, where they give us the most reliable picture of the actual H₂S concentration in the sewage where we measure. It is very valuable that we can dose based on the wastewater that is in the pipes right now, rather than what was there last week, as would be the case with traditional gas loggers”.

Results
With continuous H₂S data from multiple points, Aalborg Forsyning has moved from reactive to proactive H₂S management. Operators can now see the immediate effect of dosing and continuously refine dosing strategies. This reduces the need for reactive interventions, as issues can be addressed before they develop into odor complaints from residents.

At the same time, dosing is adjusted to match actual conditions; applied when needed and reduced when possible, resulting in a more cost-efficient chemical use. By maintaining stable H₂S levels, the utility also lowers the risk of corrosion, helping to protect infrastructure and extend asset lifetime of concrete pipes and other costly infrastructure assets.