Hydrogen sulfide is a problematic gas in wastewater collection systems and in some areas of wastewater treatment plants.

H2S has an odor of rotten eggs that can be troublesome if released into residential neighborhoods or near facilities such as office parks, hospitals or schools. It is also quite toxic and can pose a hazard to technicians working on equipment.

The gas is also corrosive, and if high concentrations persist it can do severe damage to equipment and concrete structures, imposing high costs for repair or replacement. For all these reasons, it is important to monitor H2S concentrations accurately and consistently, so that hazards can be detected, and proper actions taken.

For this purpose, CAS Dataloggers offers the OdaLog family of instruments. It includes portable gas loggers designed mainly for the wastewater industry and manufactured by App-Tek International, a company that specializes in gas detection for harsh environments. The devices are mainly used near sources of H2S, like wastewater pumping stations and receiving manholes, inside sewer collection lines, and near facilities such as primary clarifiers in treatment plants.

The products range from portable odor survey units to larger online general-area and process monitors. A range of accessories can help provide a complete H2S monitoring solution. Terry Nagy, engineering manager with CAS DataLoggers, talked about the technology in an interview with Treatment Plant Operator.

:  How severe is the H2S problem in the wastewater industry?

Nagy: It varies from location to location. A major issue is corrosion. Based on the design of the system and the way the wastewater flows, certain locations have serious issues with damage to the infrastructure because of H2S.

: Where is that problem potentially the most serious?

Nagy: It is the most severe in relation to force mains. If the conditions are not right, large amounts of H2S can be generated within those force mains. It gets trapped in the waste stream, and then it outgases when it hits the discharge point, and that’s where the corrosion can occur. H2S will degrade concrete, metal, and even some kinds of plastics. It can deteriorate pumps, concrete, manholes and other structures.

: Why is it important to monitor the concentrations accurately?

Nagy: One case involves working in areas where H2S is present. In those cases, monitoring is important for personal safety. The OSHA ceiling concentration limit for an eight-hour shift is 20 parts per million.

: What is another situation in which accurate measurement is important?

Nagy: One example is where you’re treating a wastewater stream to eliminate the bacteria that generate the H2S. Monitoring is critical in those cases because you’re looking to determine what is causing the bacteria to thrive. If you know where it’s being generated and why, then you can either treat the flow or try to minimize the conditions that are causing the gas to be generated.

: Is H2S encountered in wastewater treatment plants as well as in collection systems? 

Nagy: Yes, and one issue in those cases is odor. It’s often at the very low end, 20 parts per million or less, but you might have complaints from the neighborhood under some conditions, such as at certain times of the year, or if the wind is blowing in a particular direction. So that’s a case where a plant might consider monitoring.

: How do the OdaLog devices work?

Nagy: They have an electrochemical sensor inside. A catalytic reaction of the H2S with water generates a tiny current in an electrode. That current is then converted into a gas level. A calibration process is required to determine the correct relationship between the current and the gas concentration.

: Where would these sensors typically be mounted?

Nagy: Where the concern is corrosion, they often suspend a unit inside a manhole. The unit has a metal loop on top for fastening it to a cable or a rope and suspending it in the air. For odor applications, units are often attached near a fence or a guardrail at the perimeter of the treatment facility, to monitor the concentration that is escaping into the local environment.

: Are the sensors mounted permanently?

Nagy: No. Typically, the manufacturer recommends that they be kept in service for about two weeks. Then you rotate the unit out for reconditioning and replace it with another unit. You might be doing continuous monitoring, but you’re using multiple units and swapping them out over time.

: What is involved in reconditioning the units?

Nagy: The chemical reaction in the unit creates sulfuric acid, and after a while the unit gets saturated. So you pull the unit out of service, put it in a dry air environment, and the reaction goes the other way. It dries the sensor out, and then you can put it back in service.

: What kinds of accessories can be added as part of a monitoring solution?

Nagy: A complete system might include a 4-20mA adapter that can be used to feed the data into a control system. In that case, you might place an Odalog unit near the source of the H2S. It would monitor the level and send a signal that would be fed into a PLC controlling a pump to regulate chemical dosing.

: What is the benefit of that type of installation?

Nagy: It might be a case where you have to change the chemical dose based on the H2S level. Without some kind of a sensor-based control, you would probably dose at a worst-case rate, which might cost a lot of money. By having the monitor, you can adjust the dosing based on the gas level. So you’re making much more efficient use of the chemical you’re applying.

: How is the data collected from the OdaLog units?

Nagy: One type stores the data in memory and needs to be connected to a laptop via infrared or Bluetooth for the data to be extracted. We also have a unit with a built-in cellular modem that can automatically forward the data to a server. That type might be placed in a location far from the main facility or otherwise difficult to access. You set the unit up and it will periodically push the data to your server for review and analysis.

: How might sensors be set up to monitor an odor issue in a community?

Nagy: You might have several units spread around from points on the collection system, following the path of the wastewater. In this way you could map the H2S levels at multiple locations without having to go out and download the data from each logger. You could build a map of where the problem starts or where the biggest part of the problem is located. 

: What is happening with further development of the OdaLog line?

Nagy: The manufacturer has been making product upgrades. They added the Bluetooth capability several months ago, and that definitely makes it easier to retrieve the data. Looking forward, they intend to incorporate the latest sensing technologies to improve performance and make the devices easier to use.