Gas detector for landfills and wastewater: methane, H2S and oxygen, and which monitors fit the work

What is in landfill gas and sewer gas?

Typical landfill gas is 45 to 60 percent methane, 40 to 60 percent carbon dioxide, 2 to 5 percent nitrogen, 0.1 to 1 percent oxygen, 0.1 to 1 percent ammonia, up to 1 percent sulfides including hydrogen sulfide, up to 0.2 percent hydrogen, up to 0.2 percent carbon monoxide, and a fraction of a percent of other organic compounds. The methane is lighter than air and moves upward and outward through the soil; the carbon dioxide is heavier than air and collects underground and in low enclosed spaces. The most common places for gas to collect off the landfill are basements, crawl spaces and buried utility entry points.

Sewer gas is the same chemistry on a smaller scale with more hydrogen sulfide, because wastewater carries sulfate and the bacteria in a stagnant line or a wet well convert it. H2S is denser than air and settles to the bottom of the space; the smell of rotten eggs is detectable at parts-per-billion levels and the sense of smell fatigues quickly, so the absence of smell proves nothing. The NIOSH Pocket Guide warns directly that smell cannot be relied on to detect continuous exposure.

Why does a standard 4-gas monitor struggle with raw landfill gas?

The LEL sensor in most 4-gas monitors is a catalytic bead. It burns a little of the gas on a hot element and reads the heat, so it needs oxygen to work and it is built to read 0 to 100 percent of the LEL, which for methane is 0 to 5 percent by volume. Raw landfill gas is 45 to 60 percent methane with almost no oxygen. A catalytic sensor in that gas can read low or go to a fault, and manufacturers’ instructions say so. An infrared methane sensor works the other way: methane absorbs infrared light, the sensor measures how much is absorbed, it needs no oxygen, and the autoranging versions read percent LEL at the low end and percent by volume at the high end. That is the sensor landfill gas crews, gas-well technicians and digester operators carry.

For a manhole, wet well or vault, where the question is whether the air is safe to breathe and will not burn, the standard 4-gas set of LEL, oxygen, H2S and CO is the right instrument and is what 1910.146 testing is usually done with. The two jobs need two sensors, and a plant that does both usually carries both: a pumped 4-gas unit for entries and an IR methane unit for the gas system.

In what order is a space tested, and when?

1. Before entry, from outside. Lower the probe or the pumped monitor into the space and test at the top, middle and bottom; heavy gases sit low and methane rises. The entrant or their representative has the right to watch.
2. Oxygen first. Many combustible sensors read wrong in low oxygen, so the oxygen number comes first and decides whether the LEL reading can be trusted.
3. Combustible gas second. Percent LEL. Above 10 percent, the space is hazardous and is not entered until ventilation brings it down.
4. Toxic gases third. H2S and CO at minimum for wastewater; the permit lists any others the employer has identified.
5. Ventilate, then retest. Under the alternate-entry procedure, continuous forced-air ventilation must clear any hazardous atmosphere before anyone enters, and the air is retested periodically to confirm it is staying clear.
6. Keep testing during the entry. Test or monitor as necessary to confirm acceptable conditions are being maintained; hand the entrant the results as they come. A wearable monitor on the entrant is how most programs meet this.
7. Write it down. The permit records the results of the initial and periodic tests with the tester’s name or initials and the time of each test.

What else does the standard put on the employer?

• Evaluate the workplace to find permit-required confined spaces; at a wastewater plant that is most wet wells, digesters, manholes, vaults and tanks.
• Use a calibrated direct-reading instrument. The word calibrated is in the standard; a bump test before each day’s use and calibration on the manufacturer’s schedule are how that is shown.
• Reevaluate the space in the presence of any entrant who believes the first evaluation was inadequate.
• Keep canceled permits at least 1 year and note on them any problems encountered, so the program can be revised.
• A gas well field, a leachate line or a flare station outside a confined space is still covered by the exposure limits in Table Z-1 and Z-2; the confined-space rule adds the testing procedure, it does not replace the limits.