Climate Matters: Climate activism in the toilet


What is the carbon footprint of human defecation? This oddball question comes to me as I continue this journey of exploring what I as an individual and what we as a community can do to combat the climate crisis. Unlike choices of what to eat, how to travel, how to heat and cool our homes, what choices do we have in this most basic of human needs?

And how did I even come to consider this question? For most of my life, except in unpleasant moments of septic or plumbing failure, I have been blissfully unaware of what happens to the waste that I flush down the toilet every day.

The question came to me because Middlebury’s Town Energy Committee, of which I am a member, is analyzing the climate impact of various options as the town considers how to replace its current aged wastewater treatment plant. 

The current wastewater treatment system breaks down organic material using an aerobic process. On the surface it is a pretty solid choice from a climate point of view. Waste comes in via sewers, or via trucks for septic and industrial effluent. It is spun around, acted on by bacteria, sowed with lime, heated and dried, and out the other end emerges water clean enough to go into the Otter Creek, and bales ready to be applied to farm fields as fertilizer. While it requires almost a quarter-million dollars a year in electricity to operate, it contributes relatively little to greenhouse gas releases. That is because Vermont’s Green Mountain Power supplied electricity has very low carbon content. Sticking with an aerobic process seems on the surface to be the right choice given the greenhouse gas emissions associated with the other options being considered.

Except — for embedded carbon! 

Maybe you already know about embedded carbon, in which case skip this paragraph. Embedded carbon, I just learned, is the amount of emissions associated with the production of a material product. Cement and steel are the examples most often cited to explain embedded carbon. Making steel currently is a hugely energy intensive process. Large amounts of fossil fuels are burned to extract and produce it. Cement has a lot of embedded carbon both because it takes energy to make it, and because the chemistry of converting limestone to lime used to make cement, involves releasing lots of carbon dioxide. 

Back to Middlebury’s wastewater plant, it turns out that the key ingredient in processing our waste is lime. Middlebury’s plant uses almost 700 tons of lime each year. If you take into account the embedded carbon associated with the aerobic process of managing our waste, it no longer looks that great from a carbon footprint perspective.

An alternative process based on a machine called an anaerobic digester uses far less electricity, and does not have an embedded carbon problem. It does however have a methane problem. The anaerobic process produces a great deal of methane — methane that is not produced in the current set-up. Methane is even worse as a greenhouse gas than CO2, trapping heat in the atmosphere at a rate at least 28 times greater than carbon dioxide during the 100 years after it is released. The options under consideration for dealing with the undesired methane include: burning it to generate electricity, using some to operate the plant and sending the rest into the electrical grid; using it to operate the treatment plant and heat its buildings; feeding it to the natural gas grid to replace dirtier fossil natural gas, or even just burning it off. As with so many things, if one sticks to the current centralized model of treating our waste, there are no obvious carbon neutral solutions.

There is a promising new technology called the living machine that uses algae to eat the waste and is carbon negative. It is a technology designed not for industrial-scale waste treatment, but for more localized applications that would obviate the need for a central waste treatment plant altogether. Some think that a decentralized model like this is the future of treating our waste in a way that reduces our greenhouse gas emissions. And yet it is hard to imagine taking this radical step in rethinking how we approach processing our wastewater, in the same way that it is hard to rebuild our housing, transportation, agricultural and manufacturing systems to align with what we need to do to quickly to wean ourselves from fossil fuels. 

Deciding about how to treat our waste is only one of the many choices we as a town will confront in the coming years as we decide about how to maintain and build our public infrastructure in a climate emergency. These same challenges surface as we decide about replacing aging heating systems, building or rebuilding roads, and making room for renewable energy production facilities and electrical transmission capacity. 

Stepping back from the details of which approach is the best approach from financial, technical and environmental perspectives, I wonder what are the values that will inform this decision. Do we as a town just do what is least expensive financially in the short run? Are we willing to pay more to make good on our commitment to achieving carbon neutrality quickly? As someone who lacks the scientific and technical knowledge required to fully analyze the feasibility of various options, I am also struck by how dependent we are on having access to trustworthy people who can do the math, help frame the options in terms that lay people can grasp, and who are also thinking about the big picture and the long term. 

I wish that C02 were more like poop. When the automobile displaced the horse as our main mode of transportation, one of the many benefits cited was that our city streets would no longer be full of horse manure. If only we knew then that the odorless gas coming out of our tailpipes and our chimneys would put us in our current existential crisis. Modern sewage treatment may be as important to our public health as was the invention of penicillin. There is no turning back from this as a critical piece of our common infrastructure. But we need as a community to be open to new approaches to handling our waste. More broadly, we must consider the costs to our society of not making good on our commitment to radically reduce our reliance on fossil fuels and our production of greenhouse gasses. And to make good on that commitment will require courage, a willingness to take risks, and a collective re-imagining of how we do everything, including, yes, how we approach meeting our most basic of human needs. 


Mike Roy is a member of the Town of Middlebury Energy Committee, serves on the board of the Climate Action Economy Center of Addison County, and this year is serving as one of 16 Climate Catalysts with the Vermont Council on Rural Development. He added this footnote:

“Thanks to my colleague from the Middlebury Town Energy Committee Richard Hopkins for his meticulous research on the carbon intensity of these choices. Any details that I got wrong are entirely my fault.”

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