Today’s round of questions, my smart-aleck replies and the real answers:
Question: It’s my understanding that methane gas is 25 times more potent than CO₂ in trapping atmospheric heat as related to global warming and climate change. Methane gas can escape from a variety of sources. One of those sources can be sewerage treatment facilities. Is the Metropolitan Sewerage District that serves the greater Asheville area monitoring methane gas that might escape from the processing of raw sewerage? If they are monitoring gas levels, what amount of gas is escaping? If they’re not monitoring, why not? Also, does the Buncombe County Landfill on its former site or current site monitor levels of methane gas that might escape, and if so, what do they find? Once again if not, why not?
My answer: Thank god no one monitors me for escaped gas.
Real answer: We’ll start with Buncombe County on this one, as their answer is simpler. Kristy Smith, solid waste manager with the Buncombe County’s Solid Waste Department, said the department “voluntarily invests in landfill gas collection and control through its landfill gas-to-energy facility.”
“This facility has been operating at the landfill for over a decade and actively destroys the methane gas that is naturally produced from decaying garbage, significantly reducing Greenhouse Gas Emissions,” Smith said via email. “The generator onsite combusts methane and produces renewable electricity which goes directly to Duke Energy’s power lines onsite.”
Buncombe County, as the reader stated, does have an old, closed landfill, and a newer, still-active one. Also, that figure of methane being 25 times more potent than CO₂ comes from the Environmental Protection Agency website, which also notes, “Over the last two centuries, methane concentrations in the atmosphere have more than doubled, largely due to human-related activities.”
But back to the local landfills.
“Both the retired old landfill and the active landfill have two federal and state permits regulating the monitoring and reporting of methane gas,” Smith said. “Buncombe County Solid Waste is in compliance with all state and federal regulations.”
At the Metropolitan Sewerage District, Hunter Carson, engineering director for MSD, acknowledged that wastewater treatment facilities and collection systems “can be a major source of methane.”
“However, MSD’s French Broad River Water Reclamation Facility is not one of them,” Carson said. “This is due to the treatment processes used to stabilize wastewater solids at its plant. Methane generation in the collection system is also minimized due to the nature of MSD’s system.”
Fair warning here: We’re about to take a deep dive into the wastewater treatment process. Horrible choice of words there … sorry.
How about a “deep examination?” OK, not much better. But Carson really did go all in on this one, and I’ve got to say it’s strangely fascinating.
“Wastewater contains fecal matter and organics such as food waste, ammonia, soaps, oils, paper, carbohydrates and proteins,” Carson said. “Bacteria and other microorganisms play a vital role in wastewater treatment by consuming the organic matter present in the waste stream.”
He also provided this quick biology lesson: “Aerobic” bacteria use oxygen in the decomposition process and can deplete the wastewater of its dissolved oxygen, creating an “anaerobic” environment, one void of oxygen.
“It is under these conditions that anaerobic bacteria produce methane and other gaseous byproducts,” Carson said. “Thankfully, our mountain topography in Asheville helps reduce the level of methane generated.”
That’s because MSD’s collection system mostly consists of gravity sewer lines that follow the topography. Steep slopes mean “high velocities, low travel times, and oxygenated conditions for our wastewater.” That means a dissolved oxygen concentration that curbs methane generation.
This next part might be a little TMI for some folks, but Carson pointed out that in wastewater treatment, organic and inorganic solids are separated from the wastewater stream “and then thickened, stabilized, and disposed of.”
“Collectively, these processes are referred to as ‘solids handling,’” Carson said. “It is these solids handling processes that generate methane at wastewater treatment plants, and specifically a process called ‘anaerobic digestion.’”
That type of decomposition typically is completed in airtight vessels, with no oxygen. The process “stabilizes” the solids by killing dangerous pathogens before they are disposed of, typically by applying to land.
This process is not done at the Woodfin plant, but it used to be. If you’ve ever noticed the geodesic domes along Riverside Drive in Woodfin, those are MSD’s old anaerobic digesters that operated between 1967 and 1992.
“Inside the aluminum dome was an expandable bladder that would capture methane gas generated from the process,” Carson said. “The gas was used to fuel hot water boilers.”
In 1992, MSD transitioned away from anaerobic digestion to thermal oxidation of its solids, via sewage sludge incineration. That eliminated major sources of methane generation at the plant.
“Methane generation at MSD’s Water Reclamation Facility (WRF) is negligible and for that reason, not monitored,” Carson said. “Today, the old digester tanks are used for equalization storage to help mitigate wastewater flows into the plant during wet weather events.”
Carson explained that MSD also used to use methane from the county’s old landfill.
“In the late 1990s, a landfill gas collection system was installed at the old Buncombe County landfill to harvest the natural gas byproduct, a mixture of approximately 50 percent methane and 50 percent carbon dioxide,” Carson said. “A transmission line was installed to convey the gas to MSD’s Water Reclamation Facility where it was used to power two 450 kilowatt Waukesha generators, which in turn provided electrical power to portions of the WRF.”
After about 10 years of gas extraction at the old landfill, “the methane content reduced to a level that could no longer support the generator requirements and the system was decommissioned,” Carson noted.
Whew! That was a lot of information to digest. I’m pooped.
Sorry about that. Couldn’t resist.
Got a question? Send it to John Boyle at firstname.lastname@example.org or 828-337-0941.
I’m not a “true believer” in the religion of AGCC (nee AGW), but the explanation of the disposal of methane CH4 (one Carbon atom + 4 Hydrogen) was at best incomplete. Water vapor (gaseous H2O) is also a more potent greenhouse gas than CO2, but it is generally self-regulating. When the air is saturated, condensation produces clouds and eventually precipitation of liquid or frozen water.
When methane is burned to produce heat energy the combustion reaction of Methane is: CH4 + 2 O2 → CO2 + 2 H2O + 890 KJ
So burning one mole of CH4 produces one mole of CO2 (OMG!) + 2 H20 water vapor moles and some energy. (Molecular weight of CO2 = 12 + 1 AMU’s. So a mole of CH4 is 13 grams or 6.02 (Avogadro’s number ) x 10^23 CH4 molecules. Disclaimer: Its been 56 years since my last Chem class.
Correction (I can’t see any way to edit a post).
Molecular weight of CO2 = 12 + 1 AMU’s……
Molecular weight of CH4 = 12 + 4 AMU’s. So a mole of CH4 is 16 grams or 6.02 (Avogadro’s number ) x 10^23 CH4 molecules
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