August 16th, 2018
Wastewater Treatment and Hazardous Gases
The operation of Waste Water Treatment (WWT) facilities has evolved into a very technical 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 liable for significant fines for releasing effluent from improperly treated waste streams.
The modern WWT collects waste streams and processes them in enclosed vessels and inside buildings which help to contain odors. These enclosures also present the risk that hazardous gases may be contained and workers may be exposed 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 may originate from aeration tanks, sludge digester tanks, and the deodorizing plant within the WWT treatment plant.
- Hydrogen sulfide: a colorless gas, known for its rotten egg smell, produced by the biological reduction of sulfates and the decomposition of organic material
- Chlorine: often used in water purification. Typically added as sodium hypochlorite (“liquid bleach”)
- Carbon monoxide and carbon dioxide: released from decomposition of organic material
- Methane – 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 that the process can be controlled and the exposure risk eliminated before harm to employees or damage to 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 are Hazardous Gases Detected (How do they work)?
Toxic gases: 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).
Combustible gas: detected by Infra-Red (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 Draeger Gas Detectors?
Draeger Safety, Inc. makes the world’s finest gas detectors. Draeger is a 140 year old company still owned by the founding Draeger family. This family driven business has been a leader in gas detection and is committed to constant improvement in reliability, repeatability, and accuracy.
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. Draeger makes their own sensors and is one of the few companies which does. This means Draeger can control quality and innovate constantly without depending on others. The Draeger gas detectors are designed to operate in the harshest of environments, reliably, and with exceedingly long life. Draeger gas detectors are not the least expensive to buy but their long service life makes them the least expensive to own.
Draeger Gas Detection in Wastewater Treatment
- Toxic Gas: Draeger 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.
- Combustible 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 Draeger 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:
- The large graphic backlit display shows 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 analogue output with relays, it also offers 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 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 industrial environments. The unique 4 beam signal stabilizing system makes the sensor resistant to dust or dirt deposits on the optical surfaces. Environmental and ageing effects are compensated ensuring long term, drift free operation.