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When it comes to renewable energy, solar, wind, and hydro keep coming up as number 1, but what about number 2? Literally. I’m talking about … well, poop. There’s a rich, carbon-neutral resource that we could tap into: Turning crap into energy instead of flushing it down the toilet. It could be used as a coal alternative, to sequester carbon, strengthen concrete and more. There’s an old technology that’s starting to make a splash and might change the way you think about number 2. Let’s see if we can come to a decision on turning waste into energy and if it’s worth it.

We’re always looking for clean energy in any source we can: solar, wind, crops, even the currents at the bottom of the sea. Trying to get more and more creative to find something that really stands out. A couple of months ago a fan of the channel put me into contact with a company that’s doing just that … and in a way I wasn’t expecting. What if we could turn good old number 2 into a resource for generating energy or producing products we use everyday, all while cleaning up the environment? Sounds like an idea that doesn’t stink. This was a topic right up my alley.

Usually we’re talking about new technologies, but this tech has been around since 1913! So what gives? Why are we still literally flushing away renewable energy, instead of taking “recycling” to a whole new level?

I’d argue that there are 3 unavoidable things in the world: death, taxes, and bathroom breaks. Doesn’t matter how much you hate creating waste: even the most hardcore recycler has to go sometime. Everybody poops.

What Usually Happens In Wastewater Treatment Processes

I’m going to go out on a limb and guess that the vast majority of you haven’t given much thought to what happens to your waste after you flush. I know I hadn’t. The trip to the porcelain throne is just the beginning. After our waste travels through the sewage system, to the wastewater treatment plant, they separate the solids from water. In rural areas, it travels to a septic tank where the water leeches out and the sludge is removed to be treated elsewhere. The water is treated through an intensive process (flocculation, chlorination, etc) before being discharged into local bodies of water.1 The leftover solids generally get taken care of in three different ways:

You can incinerate it, which is pretty much what it sounds like: you burn it. All of those materials—and its energy—literally go up in smoke (and into our atmosphere).

The second way is to landfill it. In the landfill those biosolids break down into methane and CO2, depending on whether oxygen is present. If there is no oxygen, anaerobic digestion takes place, generating CO2 and methane. If there is oxygen present, aerobic digestion occurs, generating CO2.2

The third way, and the most common, is to apply those biosolids to agricultural fields. About 50% of biosolids produced in the US are used for agriculture to grow products for animal feed.3 So human waste is used to grow products to feed farm animals like dairy cows.

The Current Problems

The problem with sewage management is that it never stops. Everybody poops about a pound per day, give or take4. Multiply that by several billion people, 365 days a year, and you can start to get a sense of the magnitude of the problem. In the US alone, we produce an estimated 5-1356 dry tons of sewage sludge (human waste, generally 25% solid). Add the water content, and that’s billions of gallons of waste to treat every year, and it’s never going to stop.7

Even after treatment, the final biosolids can still have contaminant issues. To put this politely … sometimes things come out in the same state they went in. This doesn’t just mean corn: it also includes things like detergents, microplastics, pharmaceuticals, and “forever” chemicals like PFAS.8 This becomes an even bigger problem when these solids are reused for fertilizer, because those contaminants are put right back into the crops, and the cycle continues. Multiple dairy farms in Maine have already had to close production because their milk had high levels of PFAS, leading to loss of income and public trust9.

As you can imagine, this type of waste isn’t always great for the environment either. The water content is usually treated and discharged to water bodies, but if the water isn’t treated properly, the solids, metals, and other contaminants can choke out wildlife habitats and fisheries.10 The waste is nutrient rich. If it’s dumped in a lake without treatment, it can lead to overgrowth of certain bacteria and algae, which also disrupt the natural habitat of aquatic life.

Then there’s the energy consumption part of this to consider. Wastewater treatment plants are estimated to consume more than 30TWh per year of electricity–that’s about $2 billion in annual electric costs, and likely not from very clean sources. Electricity usually takes up 25% to 40% of a wastewater treatment plant’s annual budget, and those needs are likely to rise with population growth and tightening water quality requirements.11

Biosolids also have a greenhouse gas problem. As I mentioned before, they break down in the landfill to methane and CO2. The problem there is that methane is 25X times potent than CO2 as a greenhouse gas (although it does have a shorter lifespan)12. Municipal solid waste landfills are the third-largest source of human-related methane emissions in the U.S.1314

Throughout history, humans have simply moved bowel movements out of sight and out of smell-range. Some of these sewage disposal methods have been in practice for nearly one hundred years, mostly because nobody has come up with a better way to deal with poop. There’s got to be a better way.

HTC Explained

That’s where this old, 1913 technology comes in.

Hydrothermal carbonization (HTC), also known as subcritical water or hot compressed water carbonization, was first identified in 1913 by the German Nobel prize chemist, Dr. Bergius.15 Right after this find, however, the 20th century had other plans for Germany, including a World War, the rise of Nazism, another World War, the breakup, a wall, and reunification of East and West Germany. With all this turmoil, it wasn’t until 2006 that it got its foothold at the Max Planck Institute for Colloidal Science in Potsdam Germany. This waste-management alternative has been seeing a surge in popularity in recent years. I had a chance to talk to Dan Spracklin from SoMax where they are commercializing the technology.

“What hydrothermal carbonization, or HTC, is, essentially most simple way of thinking about it is, an industrial size pressure cooker. But it runs continuously, so it’s not like you would have at home where you have maybe cooking some chicken in a pressure cooker or a crock pot where you do it in a batch. We do it consistently. What happens is, we apply heat and pressure to the material. Applying pressure puts water into a subcritical state.” -Dan Spracklin

When you put water into a subcritical state that means it starts to boil, but the pressure won’t let it turn into steam. This all happens with 10-50 bars of pressure at about 180-250C. This is the perfect temperature to have that water act as a reagent … meaning it sets off a whole chain of chemical reactions, which separate the larger molecules in the mix16.

“What we’re doing essentially is taking large molecules, first, we chop them up and then we pull off some oxygen atoms and some hydrogen atoms to make a more stable compound and smaller molecules. Then the last step is we recombine those molecules to make useful products and those products are essentially coal. We’re mimicking the way that mother nature formed coal. Typically earth does it over the period of hundreds of millions of years, so coal is typically about 250 to 360 million years in formation. We’re able to do this with sewage sludge in less than an hour.” -Dan Spracklin

The resulting product doesn’t just look like coal: it can be used as a coal alternative.17 The fuel value is similar to that of lignite coal18, and treatment plants have used these solids to co-fire in coal-handling infrastructure.19 And, in spite of its origins, the high temperature has killed all bacteria and other biological threats, making the new fuel sanitary.

As part of this process, the solid-phase of HTC produces a liquor-rich process water as a byproduct that is chock full of VOCs, formic acid, acetic acid, phenols and other derivatives (yum)18.

So what do we do with the HTC process liquid? This is where it gets exciting. That process liquid can be used alongside anaerobic digestion to produce biogas. That’s right: this liquor “byproduct” can be used as fuel in itself![^21][^21] This is why HTC has been suggested as an intermediate step for producing biodiesel. Certain governments, including Germany and Italy, heavily subsidize the practice of generating biogas from wastewater plants. Europe’s biogas production from industrial and urban sewage sludge increased by 40 percent between 2013 and 2017.20

So how much energy can you actually get from this, and does it balance out with the energy needed to make it happen? As with everything: it depends. When it comes to sewage in particular, HTC shows some strong energy potential. The energy requirement of sewage sludge’s drying process is reduced up to 62% from normal drying techniques21.22

Traditionally, wastewater treatment plants consume a tremendous amount of energy and that’s a huge chunk of their operating costs.23 Those savings can add up quickly. Some studies even say that the economic benefit from the hydrochar production for energy recovery has exceed the profits from biogas production per tonne of sewage sludge24. Translated: the plant can produce more biogas (possibly up to 5%) while saving on energy costs.24

The best part? That source of energy is ALWAYS plentiful. You don’t have to wait for the sun to shine or the wind to blow to collect that energy. You can treat waste 24/7, 365 days a year, and the material never stops flowing in.

You can also make some pretty cool things from the HTC process besides coal and biogas: there are diamonds to find in the roughage here.

One promising application is to replace the sand in concrete with the solid products of HTC. The construction industry is already infamous for their negative environmental impacts: they contribute a whopping 40% of the world’s energy-related CO2 emissions. Part of that is because they require a ton of virgin materials to produce things like brick and concrete25. By using the solid products of HTC, they no longer need to mine these materials.

In 2019, engineers from Australia’s Royal Melbourne Institute of Technology tested bricks made up of up to 25 percent biosolids and found that these hybrids cut down energy use by nearly half during the firing process.25 Companies like SoMax are working with sewage sludge to replace sand in the concrete mix. At just 10% replacement, you could offset all the emissions from cement production.26

We’re not just “recycling” here either: incorporating biosolids into construction materials is also a powerful carbon sequestration tool. These companies can use biosolids to offset sand consumption while also sequestering the carbon from the biosolids for thousands of years in the concrete. You can break it, reuse it, toss it: no matter what, that CO2 isn’t going to magically come out.

Also, remember that biosolid fertilizer problem earlier? With HTC in the mix, you can create a carbon-neutral fertilizer from the hydrochar, only this time, you can get rid of that pesky PFAS. This is no small feat. PFAS is called the “forever chemical” for a reason: they’re made up of carbon-fluorine bonds, which are one of the strongest bonds around. That makes it great for weather-resistant clothing and Teflon, but not so much for the environment. But for HTC? It doesn’t give a crap. You’re breaking those molecules apart and getting rid of those potentially harmful pollutants at the same time (with some energy generation to boot)2728

That pollutant-destroying aspect of HTC also makes it a good candidate for making another important resource: drinking water. (Ok, hear me out here.) HTC is just creating a carbon product, so you can activate it chemically or thermally, which opens the pores to filtration. That’s right – activated carbon, a crucial product to filtering drinking water29 30.

Remember, HTC is literally tearing these molecules apart and sticking them back together. On a chemical level, the end product is different from what you started with. It’s essentially a giant autoclave.

“That’s what we’re essentially doing is acting as a large autoclave. I would never do this and grab sewage sludge or biosolids, but I have no problem going and grabbing hydrochar, smelling it. I don’t recommend eating it. It won’t kill you, but I’m sure it doesn’t taste well. But yeah, it’s completely safe material.” -Dan Spracklin

Depending on what you use as feedstock, HTC can pretty much make everything from supercapacitor materials, to anode materials for fuel cells31, to energy, to syngas, to steel, to concrete and cement. The general process is the same: toss it in, cook it, and bam: at the end, you have carbon. (It’s like the world’s largest, and grossest, Easy Bake Oven.)

An economic study in Switzerland found that HTC-treatment per dry ton digested sewage sludge with industrial-size plant costs around $600-700 (compared to current sewage sludge treatment costs of $660-1000).32 Similar studies in Germany reported that HTC reduced the costs involved in sludge treatment and disposal.33 34 Treating biosolids with HTC can not only reduce energy costs, but at the end of it, you literally have less crap that you need to get rid of, which really puts a dent in those operational costs.35

SoMax has seen these energy savings first hand.

“In the case of our project in Phoenixville Pennsylvania, we’re able to meet 153% of the energy demand of that treatment plan. It goes from being the largest consumer of energy in that municipality to a net energy producer, and we’re going to produce 153% of the energy demand. That extra energy is going back out onto the grid to run things like City Hall, street lights, stop lights. It’ll run the police station, and the fire station, and provide them with electricity 24/7.” -Dan Spracklin

So if this is such a great system, why haven’t we been hearing more about it? Besides the fact we’re talking about poop. Google the studies on HTC, and you’ll soon see just how much it’s exploded across academia in the past few years. (You’ll even find a few that aren’t behind a paywall!) However, the lab-scale applications are still catching up to real-life applications (and with it, the real-life funding).

“It has not attracted the investment that say oil and gas gets, or other, solar. You’re not seeing hundreds of billions of dollars invested in this because A, the investors don’t know. B, even the so-called sustainability or carbon experts aren’t aware of this process, because it hasn’t really made its way out of academia into the real world. By building this plant in Pennsylvania, the first one in North America, we’re bringing it out into the world to make people aware of it. This was, to us, it’s really a coming out party. We’ve been operating in stealth mode until October of last year when there was a public announcement that our technology was selected by the US Department of Energy as the technology that should be implemented at small and medium wastewater treatment plants across the country.” -Dan Spracklin

While HTC has a lot of technical aspects to consider, the biggest challenge right now may actually be regulatory. Remember, this isn’t just a plumbing issue: it’s a public health issue. HTC requires biomass, and biomass comes with quite a bit of red tape. This red tape is due to the fact that poop is a biohazard and can harbor disease, but the legal paperwork can take years to process.

“So even we’ve got the stamp approval from the US Department of Energy to implement this process at wastewater treatment plants across the United States. In Pennsylvania it’s taken us three year to get the permit to even try this process out at a wastewater treatment plant.” -Dan Spracklin

If you tell people about the environmental value, back it up with the economic benefits, and have leaders lead by example, that’s when the magic happens. It may not be polite conversation, but talking about the literal power of crap is the first step.

Still surprised that biosolids can do so much? You’re not alone–just ask Dan!

“One of the final questions I do want to ask you is, “What is one of the most surprising things about this type of work in your experience?” What is the thing that’s caught you kind of off guard that you weren’t expecting? Is there anything? -Matt Ferrell

“The most surprising thing is that everything works. We have a little game we play in our lab here, on Wednesday we have a will it carbonize Wednesday. So, we just take off the wall things and say, “Will it carbonize?” So I’ve brought in some t-shirts that they had holes and them stuff, so we cut up the t-shirts and we put it through this process. Guess what? They carbonized. So it’s everything from macadamia nuts, to sunflower seeds, to flowers themselves. We’ve actually processed flowers whole, and even carcasses. The unique thing about that is the whole thing will carbonize. The bones will remain the shape but once you touch the bone it just goes into powder. Huge surprises for me were about the raw sludge converted into hydrachar. Actually, how well it acted as an activated carbon to filter out pollutants. That was surprising to me. I thought, “That will never work.” But yeah, it did.” -Dan Spracklin:

For me the decision seems pretty clear. HTC is a great, two birds-one stone solution. We have to deal with our shit … literally. Why not get some extra use out of it? Academia and industry leaders like Dan Spracklin agree: this tech has the potential to make a difference now. The question seems to be how to get over that gulf between the theoretical and full-scale application. Simply put, it may be time to carbonize, or get off the pot.


  1. Wastewater and Sewage Treatment ↩︎
  2. What is sewage sludge and what can be done with it? ↩︎
  3. EPA – “Basic Information about Biosolids” ↩︎
  4. eMedicine Health – “How Much Does Poop Weigh in Your Body?” ↩︎
  5. U.S. Wastewater Treatment Factsheet ↩︎
  6. EPA – “Basic Information about Biosolids” ↩︎
  7. EPS – “The Sources and Solutions: Wastewater” ↩︎
  8. United States National Sewage Sludge Repository at Arizona State University – A New Resource and Research Tool for Environmental Scientists, Engineers, and Epidemiologists ↩︎
  9. IATP – “With a second farm shuttered due to massive PFAS contamination, Maine legislators weigh easing access to the courts” ↩︎
  10. USGS – “Wastewater Treatment Water Use” ↩︎
  11. U.S. DOE – “Energy Data Management Manual for the Wastewater Treatment Sector” ↩︎
  12. EPA – “Importance of Methane” ↩︎
  13. EPA – “What are the trends in wastes and their effects on human health and the environment?” ↩︎
  14. EPA – “Basic Information about Landfill Gas” ↩︎
  15. Recent advances in hydrothermal carbonisation: from tailored carbon materials and biochemicals to applications and bioenergy ↩︎
  16. Hydrothermal Carbonization of Organic Waste and Biomass: A Review on Process, Reactor, and Plant Modeling ↩︎
  17. Hydrothermal carbonization for energy-efficient processing of sewage sludge: A review ↩︎
  18. Review Article: Hydrothermal Carbonization of Biomass for Energy and Crop Production ↩︎
  19. Biochar production through hydrothermal carbonization: Energy efficiency and cost analysis of an industrial-scale plant ↩︎
  20. Discover – “Why Scientists Don’t Want Our Poop to Go to Waste” ↩︎
  21. Hydrothermal carbonization of sewage sludge on industrial scale: energy efficiency, environmental effects and combustion ↩︎
  22. Hydrothermal Carbonization as an Energy-Efficient Alternative to Established Drying Technologies for Sewage Sludge: A Feasibility Study on a Laboratory Scale ↩︎
  23. Energy Data Management Manual for the Wastewater Treatment Sector ↩︎
  24. Hydrothermal carbonisation of mechanically dewatered digested sewage sludge—Energy and nutrient recovery in centralised biogas plant ↩︎
  25. Discover – “Why Scientists Don’t Want Our Poop to Go to Waste” ↩︎
  26. A Comprehensive Review on Hydrothermal Carbonization of Biomass and its Applications ↩︎
  27. Chapter 8 – Emerging extraction techniques: Hydrothermal processing ↩︎
  28. Effects of hydrothermal treatments on destruction of per- and polyfluoroalkyl substances in sewage sludge ↩︎
  29. Potential Use of Waste Activated Sludge Hydrothermally Treated as a Renewable Fuel or Activated Carbon Precursor ↩︎
  30. Bloomberg – “The Future of Water Is Recycled Sewage, And We’ll All Be Drinking It” ↩︎
  31. Review Article: Hydrothermal Carbonization of Biomass for Energy and Crop Production ↩︎
  32. Sewage Sludge Treatment by Hydrothermal Carbonization: Feasibility Study for Sustainable Nutrient Recovery and Fuel Production ↩︎
  33. Hydrothermal Carbonization as an Efficient Tool for Sewage Sludge Valorization and Phosphorous Recovery ↩︎
  34. Hydrothermal carbonization for sludge disposal in Germany: A comparative assessment for industrial-scale scenarios in 2030 ↩︎
  35. Municipal wastewater sludge as a renewable, cost-effective feedstock for transportation biofuels using hydrothermal liquefaction ↩︎
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Matt Ferrell
Matt Ferrell lives in the Boston area and is a UI/UX designer by trade, but has always been obsessed by technology and how it works. In 2018 he started his YouTube channel, Undecided with Matt Ferrell, where he explores sustainable and smart technologies like EVs, solar panels, and smart homes.

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