The operation of Waste Water Treatment (WWT) facilities has evolved into a very technically engineered process. Years ago, the process was governed by the adage “dilution is the solution”. This is not adequate for dense, large cities, nor is it legal under current regulations. Cities can be fined for releasing effluent from improperly treated waste streams.
The modern WWT collects waste streams and processes them in enclosed vessels inside buildings which help to contain odors. These enclosures also present the risk that hazardous gases may be contained exposing workers to dangerous concentrations of these gases. Also, hazardous gases may be introduced when illegal dumping occurs upstream of a WWT. The hazards are twofold: toxic gases and combustible gases.
Toxic gases in WWT: hydrogen sulfide (H2S), chlorine (Cl), carbon monoxide (CO) and carbon dioxide (CO2). These gases can originate from aeration tanks, sludge digester tanks, and the deodorizing plant within the WWT treatment facility.
- Hydrogen sulfide is a colorless gas, known for its rotten egg smell. It is produced by the biological reduction of sulfates and the decomposition of organic material.
- Chlorine is often used in water purification. Typically, it is added as sodium hypochlorite, or liquid bleach.
- Carbon monoxide and carbon dioxide are released from the decomposition of organic material.
- Methane is emitted during the handling and treatment of municipal wastewater through the anaerobic decomposition of organic material.
Detection of Hazardous Gases
Safety is paramount in modern facilities. Gas detection is required to assure that any leak or exposure is detected quickly so the process can be controlled and the exposure risk eliminated before harming employees or damaging the facility. The gas detection equipment is usually connected to alarm horns and warning lights as well as a central computer that records and forwards the alarm condition.
Gas Detector Features and Installation
Usually, one detector is used for a single gas or gas type (e.g. combustible, or chloride, etc.); multiple gas hazards usually require multiple gas detectors.
- Only one detector may be necessary in the area of a hazardous gas (more units may be required depending on the size of the area, the possible failure mechanisms that could release gas, the gas(es) of interest, etc.)
- Detectors may be located in hazardous areas and provide alarms prior to a worker entering the area, or detectors may be mounted in areas usually inaccessible to employees.
- A detector has the ability to implement alarms or mitigating actions through discrete or common relays, either locally or plant-wide.
- Useful for detecting combustible gases since it can operate continuously near potential leak sources and the potential ignition sources.
- Detectors can monitor for upset process conditions releasing toxic gases thus allowing immediate corrective action.
How do gas detectors work ?
Toxic gases are detected by electrochemical sensors. These are essentially fuel cells composed of noble metal electrodes in an electrolyte. When a gas is detected the cell generates a small current proportional to the concentration of the gas present. That current is amplified to provide a signal to the safety system (alarm lights, horns, computer, etc.).
Combustible gases are detected by infrared (IR) sensors. The presence of combustible gas attenuates the strength of a unique infrared wavelength. The detector compares the strength of the IR which has been exposed to the environment to an IR signal that has been isolated. The difference in signal strength is proportional to the amount of combustible gas present and creates a signal that communicates the amount of gas present.
Why Use Dräger Gas Detectors?
The heart of a gas detector is the sensor. If the gas is not detected accurately and repeatedly then there is nothing that can be done to improve the operation of the device. Dräger makes their own sensors and is one of the few companies which does. This means Dräger can control quality and innovate constantly without depending on others. The Dräger gas detectors are designed to operate in the harshest of environments, reliably, and with exceedingly long life. Dräger gas detectors are not the least expensive to buy but their long service life makes them the least expensive to own.
Dräger Gas Detection in Wastewater Treatment
- Dräger Polytron 8100 gas detector explosion-proof transmitter for the detection of toxic gases or oxygen. It uses a high-performance plug and play electrochemical DrägerSensor® to detect a specific gas.
- The Polytron® 8700 is Dräger’s top of the line explosion-proof transmitter for the detection of combustible gases.
Both of these are part of the Dräger Polytron 8000 series. All transmitters in this series have the same design and user interface. This allows for uniform operation with reduced training and maintenance requirements. The Polytron 8000 series units feature:
- A large graphic backlit display showing status information clearly and in an easy-to-use format.
- The measured gas concentration, selected gas type, and measuring unit are displayed during normal operation.
- Besides a 3 wire 4 to 20 mA analog output with relays, they also offer Modbus and Fieldbus protocol making it compatible with most control systems.
- Colored LEDs (green, yellow and red) provide additional alarm and status information.
- Operated by means of a magnetic wand over contact surfaces.
- Three relays for controlling external equipment
- Safe, robust housing for every application
- Electrochemical sensors: These long-life sensors provide continuous detection even under the harshest conditions. DrägerSensors offer the industry’s widest temperature and humidity range between -40°C to +65°C (-40°F to +150°F). The built-in memory contains all calibration and configuration information, therefore the sensor ships to you pre-calibrated and is ready for immediate operation. An intelligent sensor self-test function allows for predictive maintenance.
- Infra-Red (IR) sensors – PIR7000: stainless steel, 316L enclosure and drift-free optics for industrial environments. The unique 4 beam signal stabilizing system makes the sensor resistant to dust or dirt deposits on the optical surfaces. Environmental and aging effects are compensated ensuring long term, drift-free operation.
If you are interested in Dräger Gas Detectors for your facility, reach out to our team of safety experts today to learn more!
