Oct 14, 2019

Successful H2S Scrubber Gas Monitoring

This article originally appeared in the October 2019 edition of Water & Wastes Digest as "Scrubbing Sulfur."

The typical wastewater treatment plant scrubber system.

With the increased concern about clean air, many wastewater treatment plants have added scrubber systems and biofilters to control odors and pollution from hydrogen sulfide (H2S) and other waste biogases. Known for its rotten egg smell, H2S is a colorless, clear toxic gas that is a potential danger to employees and frequently is the source of obnoxious odor complaints from nearby downwind residents and businesses. 

The Dangerous Qualities of H2S

With its toxic, combustible, flammable and corrosive properties, H2S gas is first and foremost a plant safety issue in municipal wastewater treatment plants and for some industrial businesses, such as food processing plants and dairies. Beyond the rotten egg smell jokes, the toxicity dangers of high H2S exposure and/or explosions or fires are real and no laughing matter to those responsible for workplace safety and pollution monitoring.

H2S gas occurs in nature with the breakdown of plant and animal matter, including the human waste that is treated by municipal wastewater treatment systems. Once released into the air, H2S gas can be present for weeks as an annoying air pollutant. While not technically a greenhouse gas, it has been dubbed the “other greenhouse gas” by some because it tends to get trapped within the atmosphere. 

There are two primary wastewater treatment methods in use today where H2S gas is present at municipal plants, anaerobic digestion ponds and digester gas tank systems. 

The toxic H2S waste gas associated with these treatment processes can be a significant safety hazard and a source of corrosion that also affects the maintenance and life cycle of piping, scrubbers and other plant equipment.

Dangerous H₂S employee exposure, according to the U.S. National Institute for Occupational Health and Safety (NIOSH), can cause irritation to the eyes and respiratory system. At higher exposure levels, it can result in apnea, coma, convulsions, dizziness, headache, weakness, irritability, insomnia and stomach upset; and if exposure is from liquid: frostbite. The severity of symptoms depends on level of exposure, duration and type of work being done. 

While the odor of H2S gas initially is distinctive, the human sense of smell quickly can be overcome (immune), leaving people in jeopardy of severe exposure. OSHA warns that worker exposure to H2S gas is not to exceed the 20 parts per million (ppm) level during a shift with few exceptions, and the U.S. EPA requires monitoring and reporting of H2S gas levels because of its toxicity and adverse effect on aquatic life.

Stench in the Midwest

A U.S. Midwestern city’s municipal wastewater treatment plant management team recently decided to upgrade its existing pollution control system to better treat H2S gas. The treatment plant relies on a carbon bed dry scrubber system technology, rather than the wet scrubber technology alternative that utilizes water-based treatment fluids.

There are multiple types and manufacturers of dry and wet H2S scrubber systems. They all have one thing in common, however: the more accurately and consistently H2S gas is measured at the system inlets and outlets, the more efficient the systems are at remediating the gas at the lowest operational cost. The cost of running scrubber units can be significant in terms of the system chemicals, analyzers and other instrumentation, electric power, system maintenance and staff time.

In seeking to optimize the treatment facility’s scrubber operations, the plant engineers contacted MSA for safety expertise in monitoring H2S and other toxic and combustible gases. The treatment plant engineers realized accurate measurement of H2S gas was the key to optimizing the performance of their existing scrubber system.

H2S Scrubber Monitor Proposal

Figure 3-1. A schematic showing the sample sensor, pipe nipple and scrubber piping in relation to the flow through the system.
Figure 3-1. A schematic showing the sample sensor, pipe nipple and scrubber piping in relation to the flow through the system.

After reviewing the plant’s installed scrubber and requirements for H2S gas sensing, the MSA applications team recommended a combination monitoring solution. The proposed scrubber H2S monitoring system (Figure 2, page 51) would include individual gas monitors on the scrubber’s large diameter intake pipes and a full sample draw system on the scrubber’s outlet piping to assure air quality after treatment (Figure 3-1, page 49, and Figure 3-2, page 51).

Figure 3-2. A schematic showing the location of the sample tap assembly and sensor monitor assembly in a scrubber monitoring system.
Figure 3-2. A schematic showing the location of the sample tap assembly and sensor monitor assembly in a scrubber monitoring system.

This recommended end-to-end type of system offers valuable H2S gas level data to the plant’s control system for real-time monitoring and scrubber system adjustment during the treatment process. The post-treatment testing pre-engineered sample draw system supports plants with high detection range requirements, which could exceed 500 ppm H2S concentrations. 

The plant team accepted the H2S gas monitoring system recommendations, which incorporate the installation of Ultima X5000 H2S gas detectors (Figure 4). This suite of H2S gas detection and monitoring products can be used as an ambient point detector, in-situ pipe or vent monitor, or within an extractive flow panel system. 

Sensors like this identify worrying levels of H2S and alert workers nearby with sounds and lights.
Sensors like this identify worrying levels of H2S and alert workers nearby with sounds and lights.

This detector measures H2S in a choice of three standard ranges: 0 to 10, 0 to 50 or 0 to 100 ppm. The response time is less than 23 seconds, with repeatability of ±3% and zero drift per year at less than 1% of full scale. The advanced design of these gas sensors extends calibration cycles for as long as 18 months, includes self-diagnostics that actively monitor sensor integrity and compensate for environmental factors and electrochemical sensor drift. 

The detector’s built-in display features organic LED (OLED) technology, with full word text in nine languages. Bright green, yellow and red status LEDs provide extreme visibility. A touch-button interface offers a tool-free user experience and Bluetooth wireless communications are optional for staff remote interface capability to support detectors placed in crowded equipment locations or at high locations that require scaffolding and worker safety harnesses.

Global hazardous area approvals are provided for these detectors. They include Class I and Class II, Div 1; ATEX, CSA, IECEx and CE Marketing. They also are SIL-2 rated for safety integrated systems (SIS) applications requiring superior reliability.

The detector and the associated flow panel designs were developed to withstand harsh environments and sample conditions and can operate where explosive atmospheres potentially are present. They are designed for performance in high-moisture sample environments, such as wastewater treatment plants and are expected to operate at least one year without manual intervention. 

Although these flow panel systems can operate independently, ideally they are started up in conjunction with the operation of the odor scrubber. These ambient point detectors operate continuously to monitor for H2S leaks around process equipment, wet-wells and holding tanks. 

Conclusions

There are a number of benefits derived from H2S scrubber gas monitoring systems. These gas monitoring systems ensure odor scrubbers meet EPA requirements, provide early warning alerts to the potential breakthrough of carbon bed scrubbers, help control chemical feeds for wet scrubbers to conserve expensive chemicals and help eliminate odor complaints from the community.

The Midwestern city’s H2S gas monitoring system for its scrubber units has been in operation for more than one year. There were no significant problems during system installation and start-up, and plant engineers have not reported false alarms. They said the accuracy of gas measurement has helped improve the operation of the scrubber system and lowered annual facility costs.

 

About the author

Tim Wolk is water and wastewater market manager for MSA - The Safety Company. Wolk can be reached at [email protected].

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