Gas Monitoring for Wastewater, Sewers and Utility Vaults

Question 1

Why do sewers need continuous monitoring rather than a single pre entry test?

Accepted Answer

Because a sewer cannot be isolated, so the atmosphere you tested is not the atmosphere that will be there in ten minutes. 29 CFR 1910.146(d)(5)(i) contains a specific rule for exactly this situation. Where isolation of the space is infeasible because the space is large or is part of a continuous system, such as a sewer, pre entry testing is performed to the extent feasible before entry is authorized, and if entry is authorized, entry conditions are continuously monitored in the areas where authorized entrants are working. The physical reason is upstream. A slug of septic wastewater, a discharge from an industrial connection, a pump starting somewhere else on the system, or simply a change in flow can push a completely different atmosphere past a crew that tested clean twenty minutes earlier. Nothing about the test was wrong. The system moved. 8 CCR 5157 Appendix E adds that in large bore lines that cannot be isolated, the lead entrant should carry a monitor ahead of the crew, so the instrument meets the change before the people do.

Question 2

What gases are you looking for in a wet well or a manhole?

Accepted Answer

Hydrogen sulfide, methane, and oxygen deficiency. Usually all three at once, and they are not in the same part of the space. Hydrogen sulfide comes from anaerobic bacterial reduction of sulfate in sewage, which is favored by low flow, long retention and warm weather. It has a molecular weight of 34 against 29 for air, so it collects low, in the bottom of a wet well, in the invert, in a sump. Methane comes from the same anaerobic decomposition and has a molecular weight of 16, so it does the opposite and collects at the top of an enclosed headspace, under a cover, at the crown of a pipe. Oxygen falls for two reasons together: bacteria consume it, and the gases above displace it. The practical consequence is that a sample drawn at one depth can miss two of the three. Test at the top, the middle and the bottom of the space before anyone enters.

Question 3

What alarm setpoints should a sewer crew use?

Accepted Answer

In California, 8 CCR 5157 Appendix E states them: oxygen below 19.5 percent, combustible gas at or above 10 percent of the lower explosive limit, and hydrogen sulfide or carbon monoxide at their respective exposure limits. Those respective limits are the California ones, which are stricter than federal. 8 CCR 5155 Table AC-1 sets hydrogen sulfide at a 10 ppm ceiling with a 15 ppm short term limit, and carbon monoxide at 25 ppm as an eight hour time weighted average with a 200 ppm ceiling. Federally, hydrogen sulfide has a 20 ppm ceiling with a 50 ppm peak, and carbon monoxide is 50 ppm. Note how low the combustible setpoint is. Ten percent of the lower explosive limit is one tenth of the concentration at which the atmosphere would burn, which is the standard margin in this work and is not the factory default on every instrument. An instrument that arrived on federal defaults needs its alarms reset before a California crew carries it.

Question 4

Should a lift station have a fixed gas detection system?

Accepted Answer

It is common, sensible practice, and it is not a code mandate. We will not tell you NFPA 820 requires it, because it does not. NFPA 820 is the standard for fire protection in wastewater treatment and collection facilities, and it is widely and wrongly described in marketing material as a gas detection standard. What it actually governs is hazardous area electrical classification, ventilation rates, and separation between classified and unclassified spaces. It is a classification and ventilation document. The OSHA obligation is to test and monitor for entry, which is satisfied with a portable instrument carried by the crew. A permanently mounted detector at a lift station earns its place for different reasons: it can run the ventilation on demand instead of continuously, it gives an alarm to somebody at the plant rather than only to whoever is standing at the hatch, and it provides a record between visits. Those are good reasons. They are just not a citation, and a distributor who tells you otherwise is not someone to take regulatory advice from.

Question 5

What about utility vaults and telecom manholes?

Accepted Answer

Same three gases, different source, and one addition worth taking seriously. An electrical or telecom vault in a street has no sewage in it, but it is a below grade enclosure in a right of way, and natural gas from a leaking main migrates through soil and along the outside of conduit runs into exactly these spaces. A vault can fill with methane from a leak half a block away that nobody has reported. Oxygen deficiency is also common in vaults from rusting steel, standing water and poor ventilation, and carbon monoxide can arrive from a generator or a vehicle running at the surface directly above the opening. 29 CFR 1910.269(e) sets requirements for enclosed spaces in electric power generation, transmission and distribution work, including testing oxygen before flammable gas. The equipment answer is the same as for sewer work: a pumped instrument to test from the street, and a worn instrument for the duration.

Your cart is empty

Your cart

Estimated total