Closed, Illegal, and Abandoned (CIA) Disposal Sites

Continuous Landfill Gas Monitoring for Structures Located Near Landfills and Disposal Sites (Part 2)

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Continuous Gas Monitoring System Components

The major components of a continuous gas monitoring system are:

• Gas Sensors
• Automatic Controller
• Data Acquisition System (memory)
• Software

Gas Sensors. Gas sensors are placed in a monitored space to detect the presence and concentration of landfill gas (methane). Typically sensors come in two types: (1) catalytic bead and (2) infrared (IR) analyzer. The catalytic bead sensor detects combustible gases by measuring changes in resistance of a heated wire element which is responsive to exposure to a combustible gas. Infrared analyzer sensors impart ionized energy to a gas and observe an emission spectral response, which has a signature which is compared to known gas signatures. Some features of gas sensors are:

• Detect presence and measure concentrations of combustible gas.
• Generally combustible gas indicator (CGI) instruments calibrated to “known” gas (methane).
• Two types: catalytic bead or infrared analyzer sensor.
• IR sensors do not require oxygen.
• Explosion-proof housing.
• 4-20 mA (milliamp) output signal.
• Multigas plug.
• Wireless models available.
• Cost: $300-$1500.

Figure 3. Combustible Gas Sensors: Catalytic Bead (left) and Infrared Analyzer

Controllers. Controllers are designed to provide power (when hardwired) to and receive an input signal (4-20 mA) from a sensor. The 4-20 mA signal, based on user-specified conditions, can then be stored to a data acquisition system, and provide an output control signal that can actuate an electric switch or control voltage to sound an alarm or turn on a ventilation system, or dial a phone number and send an electronic message to another information system. Several manufacturers produce controllers for multiple industrial and commercial applications. Some features of controllers are:

• Controller used to “receive” 4-20 mA sensor output signal, store in data acquisition unit (if present), and control “output” devices such as alarms, electric switches, solenoid valves or other “controls”, e.g. enable heating, ventilation, and air conditioning system (HVAC).
• Controller requires power source (120V/60 Hz).
• Controllers can have single-channel, 4-channel, 8-channel, 16-channel, and multichannel sensor inputs and output control (cost goes up though); for a typical closed disposal site, a 16-channel should suffice.
• Controllers generally cost between $500 and $3500.

Figure 4. Multichannel Programmable Logic Controller Model

Data Acquisition/Data Logger Systems. Data acquisition systems provide memory to store gas monitoring data points, which can be logged at specified times, intervals, and frequencies to allow a time-trend analysis (see figure below) to be performed for comparison to regulatory thresholds. The time-trend analysis provides data which can be used to determine if a disposal site is in compliance with regulatory gas levels (1.25 percent in structures and 5 percent at the property boundary).

Figure 5. Graph Showing Time-Trend Analysis for Gas Concentrations (Newport Terrace Condominiums, Newport Beach, Calif., Methane Gas Readings--Monthly Trend)

Data acquisition and logger system features are as follows:

• Loggers provide memory to store data points produced by 4-20 mA measurements (gas concentration measurement normally 0 to 5 percent concentration of gas by volume in air) taken by gas sensor.
• Loggers can be programmed to query and store data at user defined time points and frequencies, e.g. 1 measurement per hour, 24 hours per day, 7 days per week, and 365 days per year.
• Logger programs can output data to spreadsheets for documentation and data analysis, e.g. graphical representation (measurement versus time) for time-trend analysis.
• Provides documentation and defensible evidence for determining 1.25 percent in structures and 5 percent at the perimeter boundary. Manufacturers: Logic Beach, Omni Instruments, DataQ, HOBO, WebDAQ, Yokogawa, MadgeTech.
• Data Logger may require: software for PC interface, additional memory, modem, wireless transmitter (these are not included in base unit cost).
• Data Logger requires power (120V or 24V source), phone line connection.
• Data Loggers with multichannel capability, PC interface software, memory and modem can run between $1500 and $3500.
• See the Data Logger Store.

Figure 6. Data Logger and Data Acquisition System

Continuous Gas Monitoring System Components (Continued on next section...)

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Last updated: April 18, 2008