Liquid-Phase Online Measurements of Hydrogen Sulfide for Odor and Corrosion Control and Mitigation: 3 Use-Cases

SUMMARY
With the development of liquid-phase measurement technologies, wastewater professionals are now reassessing the limitations of gas-phase-only sensors in the pursuit of more reliable readings for more productive and flexible H2S control. In addition, the location of where the measurement of H2S takes place can have a major impact on the accuracy of the reading, and therefore impact the ability to manage the problem in a cost-effective manner. This presentation will summarize the gas- and liquid-phase measurement data from several operational facilities, demonstrate the sensor's ability to aid in the mitigation of corrosion, odor, and process issues resulting from H2S, and present the operational advantages of liquid-phase H2S measurement technology.
INTRODUCTION It is generally accepted that the most impactful approach is to neutralize the H2S while still in the liquid phase, prior to conversion into the gas phase. Electrochemical sensors that accurately monitor H2S content in the liquid make it easier to monitor precise levels via existing in-plant SCADA and PLC systems or cloud-based systems. Compared to gas-phase measurement in the air above, continuous liquid-phase measurement provides the true H2S concentration in the water itself. This is because the rate at which H2S transfers from the liquid phase to gas phase-and the levels of concentration detectable in the air-can vary based on temperature, turbulence that accelerates H2S off-gassing from the water, the distance between the sensor and the water surface, and ventilation of the air where H2S is measured. Continuous monitoring of the liquid-phase H2S not only allows collections and treatment plant operators real-time visibility into the true concentration of H2S in water, but also serves as a useful leading indicator for timely process control and early warnings for potential slug loading events.
METHODS AND RESULTS This paper will discuss three applications where the use of liquid-phase H2S measurements was implemented. Use-case 1: Collections System Monitoring of a Manhole: Liquid-phase vs. Gas-phase The first use case is the implementation of the liquid-phase measurement in a side-by-side comparison with traditional gas-phase measurement in a collections system manhole. This utility elected to invest in online monitoring at key locations within the collection system to monitor for odor and corrosion risk. They identified a manhole for their evaluation and installed 3 sensors (1 liquid-phase and 2 gas-phase) in the manhole. The liquid phase unit was installed directly in the wastewater flow and the gas-phase units were installed just above the water level and near the top of the manhole, respectively. The relative concentrations of H2S were measured and plotted over a 24-hour period. The liquid-phase measurements demonstrated higher sensitivity to varying conditions and captured peak H2S concentration events, which gas-phase measurements struggled to detect unless influenced by turbulence from the pumping activity in the system. For gas-phase measurements taken near the top of the manhole, peak H2S concentrations were almost completely missed. All gas-phase measurements resulted in significantly lower H2S values and periodic dropouts. Inserting a liquid-phase sensor directly into the wastewater flow enabled a more complete representation of the true concentration of H2S present. This contrasts favorably against gas-phase-only sensors that under-represented the scope of the problem due to the large distance between the water and the sensor, as well as the potential interference from turbulence. Use-case 2: Onsite Odor Mitigation (at the WWRF) & Chemical Optimization The second use case highlights real-world results using both the liquid-phase and gas-phase abilities of the sensor to help automate and optimize chemical dosing for odor control. A US water utility wanted to optimize the dosing of ferric chloride in the primary clarifier to reduce the burden on their air scrubber and minimize overall odors. In this case, the facility was balancing chemical dosing for chemically enhanced primary treatment (CEPT) and for odor control. Based solely on traditional gas-phase H2S monitoring for chemical dosing control, the facility suspected they were occasionally over-dosing chemicals leading to excessive chemical costs. They also suspected they were missing H2S spikes, leading to breakthrough odor events. Additionally, the traditional gas-phase sensors required high levels of maintenance and frequent sensor cycling (2x/week). To gain continuous visibility into H2S loading entering the plant, they installed two (2) Hach H2S sensors – one measuring the liquid-phase H2S in the grit chamber, downstream from the dosing location of the ferric chloride and the other measuring gas-phase H2S directly in the primary clarifier air scrubber. The measurement values were plotted and evaluated over several days. The data from these sensors was used to optimize chemical dosing, leading to a 25% reduction in chemical use, while also optimizing scrubber efficiency for maximum odor control. This case demonstrates that a dynamic, sensor-controlled dosing strategy can enable utilities to optimize the effectiveness of H2S odor management activities and reduce operational costs.
Use-case 3: H2S Monitoring at WWTP Inlet – Pressurized vs. Gravity Collections System The third use case highlights variable measurements of H2S and how concentrations can be influenced by the nature of the collection system conveyance. This treatment facility was interested in pinpointing the various sources of H2S. Treatment staff wanted to characterize the influent from their two main lines-a pressurized line and a gravitational line. The facility installed two (2) Hach H2S sensors, one at each of the inlet sources. The data was collected over a week, revealing distinctly different H2S profiles from each sewer system (Figure 3). The gravitational line demonstrated frequent and substantial spikes of H2S entering the treatment plant, exceeding 4 mg/L several times during one week of monitoring. The pressurized line, however, demonstrated a highly predictable and consistently low H2S pattern, consistently below 0.4 mg/L. This comparison provides insights into downstream impacts of various types of collection systems and provides valuable data for utilities looking to maximize asset lifespan and minimize odors.
CONCLUSIONS In comparison with traditional gas-phase measurements, liquid-phase and dual-phase (liquid and gas) H2S monitoring can improve the operator's capacity to pinpoint H2S hotspots, optimize and automate H2S mitigation efforts, and continuously monitor H2S concentrations in collection systems and treatment facilities. With the demonstrated versatility, accuracy, and reliability of liquid-phase H2S sensors, collection system and treatment system operators have an effective means of ensuring worker safety, minimizing odor issues, and protecting wastewater infrastructure from corrosion. Because H2S concentrations differ significantly between liquid-phase and gas-phase monitoring locations, having the ability to continuously monitor the actual concentration in the water maximizes a utility's ability to manage all H2S-related aspects of collection system and treatment plant operation. Low-maintenance sensors permanently installed in the wastewater flow and seamlessly integrated into existing SCADA and PLC systems-or enabled by battery-powered remote-monitoring devices connected to the cloud-can extend that control to remote locations wherever H2S problems exist.