Science AMA Series: I am Justin Billing, a scientist at Pacific Northwest National Laboratory (PNNL) where I research the production of biofuels. Our recent work – the conversion of human waste to biofuels – has recently garnered a lot of attention – and I’m here to talk about it. AMA!



Thanks for doing the ama. This is a neat idea - I'm wondering about the energy yield. Presumably achieving those temperatures and pressures takes a fair bit of energy - how many megawatt hours to you net from a ton of sewage relative to a ton of, say, coal?

Or maybe a better comparison is to something like a bioreactor where microbes are doing the work.


I will link our preliminary technoeconomic analysis in a moment. This is a common question. The metric we use is the Energy Return on Investment (EROI). This is the energy content of the biocrude divided by the process energy required to make the fuel, the heat inputs to the reactor. We have estimated an EROI of 3 or 4 for the sludge conversion process, and this squares with literature values in that range for HTL. Bio-ethanol or traditional bio-diesel have EROI slightly above 1, so this process is much more energy efficient.

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Fascinating, we'll look for it. Thanks.

What happens to the nitrogen-bearing compounds present in the sewage?


Great question. They are found in the biocrude at a 0.8 ratio compared to feedstock, on average. These need to be reacted out by hydrotreatment (catalyst, heat, H2). It is an issue for refining. Some nitrogen enters the aqueous phase from the reactor and can be recycled for nutrient value.

I have heard about cases where biogas made from sewage contained compounds of flourine. This had a strong corrosive effect on the combustion engines using this gas. The engines were more or less destroyed after a short time running on this gas. Do you see that any similar problems can happen with the fuel produced by hydrothermal liquefaction?


Always an issue. Fortunately with the aqueous reaction medium, most of the inorganic load stays soluble in the aq phase or precipitates with the solids. Organic fluorine would be in the biocrude for sure. We are analyzing for perfluoro compounds like flame retardants, etc. that we know end up in sludge. We are scaling up and as we start to make gallon vs liter quantities we can get real engine tests and better answers to this question.

I'm interested in the emissions produced by this fuel. You mentioned its similar properties to petroleum, but hopefully there is a net reduction in emissions relative to fossil fuels because the feedstock is a waste product.

Can you say how this compares to the net reduction in emissions from a cellulosic ethanol produced from agricultural by-product? I imagine that the ethanol burns more cleanly that this fuel.

Can you tell us about the amount and type of energy needed to produce the fuel? It seems as if it would require a lot of energy which would lower the net reductions.

Thanks so much for your work and this AMA! It's such an interesting project!


Net CO2 emisson per our LCA: 50-75% reduction vs. petroleum. There is some energy/carbon required to do the conversion, but the net result is a carbon offset >50% (which is our target).

Hi Justin, it seems like hydrothermal liquification is showing a lot of promise lately and I have done some small experiments with it myself on organic waste (although staying away from human waste for now).

1) In terms of energy production, how feasible is this approach - is there a significant energy gain when factoring in the costs of the pressure reaction and drying the biocoal after production?

2) What do you see as the greatest barrier to getting this method of energy production adopted on a large scale.

3) On the technical side, is there a strong correlation between the pressures used and the end product - does it require the highest pressures to produce more pure carbon (and if possible, do you have some good sources for what products are given off at each stage)?



See replies on EROI above. 3-4 units of biocrude energy for every unit of process energy to run the reactor.

Greatest barrier? Technically, scaling up all unit operations is not trivial. Pumping wet solids to high pressure is not trivial. Continuous ash removal is not trivial. Commercially, establishing real off-take contracts is valley of death for biofuels. One must produce a certain amount to attract refiners' attention, but at the same time one has to produce something useful/compatible, which one may not know absolutely until you produce enough of it to find out! Process scaling is what we do, so we're active on all these fronts.

Hi Justin.

I'm an undergraduate physics student and I've been interested in energy. What I'd like to know is, what role could physicists play in the research of biofuels?

Thank you!


Complex reactor modeling. We know that the biopolymers decompose at temperature and pressure and recombine in thermodynamically favorable ways. But we don't pretend to understand the cascade of hundreds of reactions and how one might influence them to an optimal output (if at all). 300-350C seems optimal, condensed phase (high pressure) is essential to keep things running smoothly.

Do you think this process will be used in establishing a colony on Mars (or elsewhere)?


Possibly, but we'll work on earth's feces first.

Hi Justin. I'm working with a group that has developed a similar sludge to bio-oil/bio-gas process - so I'm extremely excited about this AMA! My question is, how do you plan on utilizing the residual bio-char (the leftover solids)? Will this be fed back into the system as a fuel for heat? Or do you have other plans?


We produce biocrude, not biochar, though other research groups are exploring biochar.

1) The article you linked states "The pilot project will cost between $8 to $9 million (Canadian) with Metro Vancouver providing nearly one-half of the cost directly and the remaining balance subject to external funding." Will the final cost for installation be less than this figure? 2) When will the technology break even and start generating revenue for each city? After all costs are factored in, how much will the end-product sell for? *Edit: Thanks to u/yacht_boy for the correction. 3) Is any other city besides Vancouver taking part in the trials?


Strangeattractors, these are a good series of questions. I am primarily a process engineer that does some business development and some analytical chemistry. So I can't speculate too much on other's business plans. A good start for answers is the techno-economic analysis report that I linked (PNNL, Snowden-Swan). Fully built out, we are targeting a minimum fuel selling price starting at $5/gallon (gas equivalent) with a reasonable path to $3/gal, the DOE target that we work towards. TEA is an art and I don't understand all of the pieces, but I do know that we do it according to the DOE framework, so we're apples to apples with other labs and research groups.

Has the fuel produced from this process been tested and what was it tested as a fuel source for? Does the biocrude have the same byproducts crude oil gives when refined and then burned for fuel? If so are there any differences in the chemical composition (ex cars produce smoke from gas, is the smoke chemically different when using biofuel)?



Excellent, thanks for the question. We upgrade the crude in lab-scale hydrotreaters to produce a mostly hydrocarbon product. We fractionate and find about 50% of the product ends up in the distillate range (jet/diesel). The we do Simulated Distillation (SimDis, GC method) and a whole range of ASTM fuel tests to see what it looks like. In most respects, it is comparable to petro-fuels, and will certainly qualify as a blend. As we make bigger quantities, we can do more engine tests. We have done limited engine testing, all successful. Qualifying fuels is a process. Algae and sludge both have good lipid content, resulting in alkanes in the final product, boosting cetane. Wood HTL has more naphthenes and aromatics which are less desirable for the target fuel pool.

I am always excited to read about wastewater being used in energy recapturing studies, is your hydrothermal process a more intensive version of the thermal hydrolysis processes such as the Cambi THP? Is the system in design reasonable enough for small or mid size WWTP's to have their own or would a region opt for a centralized location to ship sludge byproducts to? What limitations in sludge quality do you find the process has with the large variances of raw waste different systems receive (i.e. Heavy BOD industry, large residential zones, fluoridating WTP's, etc.) thanks for the AMA! I really believe this stuff is the future of green energy options!


The low feedstock cost allowed us to scale down to 100 ton/day for an economical operation, putting us in range of actual feedstock availability in metro regions of, say, 1 million people. That's general, the TEA report has more specifics.

Thanks for this awesome AMA! My question is:

Given the extremely high dilution of sewage in existing wastewater systems due to a lack of water conservation technology, what is the energy trade off to pressurizing and heating up this waste product that is virtually water?


Our process deals only with concentrated solids at 15-20 wt% solids, up to the practical pumpability limit, and practical dewatering limit. Higher solids content shrinks the reactor size and boosts yields. But one still must be able to pump it.

My question would be based on the fact that over here in the UK, human waste is used as a fertiliser after going through tertiary anaerobic digestion to kill off most/if not all pathogens and then 77% of the UK's waste is recycled onto farmland to fertilise the soil. (source:

What impact would using human waste to produce fuel have on the resultant need to use artificial fertiliser, which, for the most part, are made using non-renewable sources of energy?

Would the trade off be worth it in such a case?


The good news is that HTL produces a solid co-product that contains most of the phosphorous, so it is not lost to fuel or the water. Also keeps excess P out of the environment (run-off) producing toxic algae blooms, etc. Regulations on land-application may become more stringent, so treatment plants are looking at alternatives to AD/land application.

Hi, I'm an undergraduate researcher working on HTL with algae and manure feedstocks. I recently read another PNNL report (PNNL-23227) which used an Aspen Plus model based on, among many other things, elemental analysis of the aqueous product. 1) How do you guys at PNNL characterize the aqueous product? Specifically can you offer any insights on conducting an elemental analysis of organic compoments of the aqueous phase? 2) In your new pilot plant, will you be refining your biocrude with hydrotreatment?


We have several papers on aqueous product characterization. There is much carbon yet to be recovered from HTL aqueous. We typically use IC, ICP, HPLC, TOC, COD, GC-MS, ammonia. Now, we're looking at TKN, BOD, and even FT-ICR MS. Authors to look for would be Panisko, Howe, Maddi, Albrecht (all PNNL).

Do you have plans to make smallish versions that could be beside feedlots or dense farms and use bio-waste from livestock?


Aha, yes, the next frontier! We are looking into it. Other groups have had success at batch conversions but no one has done a larger scale continuous liquefaction of animal manure to my knowledge. It's good to point out that HTL has general utility. We've done wood, straw, stover, grape pomace, beer grains, sugar beet culls, a large program on microalgae, food waste (cheesy fries), and the general result is that the process seems feedstock agnostic. Of course, yields vary based on feedstock quality.

Does it work differently when burning depending on what kind of diet I eat? For instance, can you get better quality fuel if you use high quality poo? That sounds so dumb but it honestly what's on my mind.


Biocrude yield and quality is a function of the feedstock, most certainly. We are now on our second batch of sludge, so we are tracking the various properties. From algae, we learned that lipid content is king (stands to reason), and lipids in equals lipids/fuel out. Protein content was a net positive, the amino acids unravel into fuel range compounds under these conditions. Carbohydrate content is still somewhat indeterminate. Sometimes carb content seems positively correlated to yield, sometimes negatively. Ash (sodium, calcium, magnesium, iron, etc.) is always a loser, less is better, ash in equals ash out, but we still don't know whether and how the inorganic species interact in the reaction environment. Buffers? Low-grade catalysts? Energy/momentum transfer? Still much room for better chemists than I to make contributions to the fundamental aspects. Our group is process engineering, primarily focused on scale-up and overcoming barriers to commercialization.

Hi and thanks for the AMA.

Money / funding aside, what do you see as the biggest hurdle to bring this system to main stream use?


Process engineering scale-up of all unit operations. Pumping and heat integration are two biggies for the conversion process, and refinery integration and off-take decisions will be necessary on the back end. Deciding how to upgrade the crude, where and when to convert to fuels, a stand-alone upgrader, a regional upgrader, a convenient insertion point in an existing refinery. To start asking these questions, we need to achieve the next scale, which we are actively pursuing.

If I burn biofuel, Am I then really more eco-friendly because I'm still blowing CO2 in the athmosphere cause there still needs to be combustion right?


That's a good question, and worth pondering. Limiting consumption is obviously a 1-for-1 win for emissions and our wallets. The carbons in the biofuel don't know that they're renewable, especially once combusted it's all the same CO2. This much is true. Life-cycle analysis compares the whole process from sludge to fuel vs. more extraction from the ground. Using renewable feedstocks allows for the original plant matter to have consumed some CO2 during its life. With sludge, it's a bit different, almost think like a sunk cost. The carbon in the sludge will either decompose into methane and CO2 (land-apply or landfill) or require even more carbon (methane) to combust it in an incinerator. And using it as a fuel offsets the same volume of petroleum (which did uptake CO2 some time in the distant past, but we don't count that). Adding all these types of considerations together, we find a 50-75% reduction in net CO2 emissions with sludge biofuel compared with petroleum. See TEA report linked in intro, Appendix B.

Thanks for your work and doing this AMA. My question is hardly work related; do you like living in Richland?


Actually, I don't like living in Richland. I grew up in rural Montana, and really nowhere else is home. It's a dilemma. Love PNNL, and love the Chemical and Biological Process Development Group. Like the Columbia River, fresh asparagus (well, almost fresh anything, with irrigation help), like dry summer heat. But still, it's not Montana. What can I say?

The efficiency of biofuels seems dismal compared to other renewable sources like solar (15-20% commercial efficiency, with direct energy conversion). For example, assuming photosynthesis is only 10% efficient to begin with, any further digestion of plant/animal matter can only serve to further reduce the efficiency of the energy conversion process.

Why is it we're investing in technologies that can only produce minimal amounts of energy at best that ultimately rely on the sun? (e.x. EROI = 3 for poop biofuel compared to EROI > 30 for CdTe solar cells) Biofuels will always be guaranteed to be less efficient than solar because of photosynthesis' inefficiency; why should we be excited about them?


Heavy transportation (trucks, trains, boats, planes) needs liquid fuels, not a ready electric replacement for their function. But I agree with respect to overall efficiency. Light transport should break towards electric, that's not hard to imagine with all the advances in battery science. The infrastructure will catch up. Liquid fuels have high energy density and are easily transportable, so I think they will always be some part of the mix.

What happens if I do a poo in the petrol tank?


Deep mischief.

Have you calculated the EROEI? Sounds like a lot of energy input is required for the process. If you have can you also detail how it was calculated (i.e. what factors did you include for energy invested). I am intetrsted to know how this compares to the production of biomethane, which can be easily done at small scales and often requires little in the way of energy inputs.


Please see the TEA report linked in the intro. Our EROI is 3-4, so we spend some energy, but the biocrude is a much-concentrated product with similar HHV to conventional crude oil. Actually, I don't think the EROI is in the report, but it is in the literature for HTL of algae. The value that I gave you was from Lesley Snowden-Swan, the author of the TEA report. She is the master of such assessments and could help you more than I can!

Hi Justin, thanks for doing this AMA. Can you comment on whether enzymes would be useful for this process? I know that for lignocellulosic biofuels, fungal cellulases are often used to help degrade biomass.


Thanks for the question, sludge HTL benefits from biological pretreatment. We get wonderful enzymatic activity in the human colon! In addition, the secondary sludge (aerobic, waste-activated) is mostly surplus or dead biological cell mass.

How did you get trade secrets from Changing World Technologies?


I put on a turkey suit and snuck behind the barricade! But in all seriousness, PNNL has been developing thermochemical conversion for 30 years, so we didn't need secrets from CWT but we are aware of and intrigued by their operations. We aren't doing quite the same thing.

Your site says that a "biocrude" phase is created in the reactor - What is done with this? Is it ready for refining at this point or does further processing need to be done before it is treated like any other crude oil source?


Biocrude has higher oxygen and (with algae and sewage) higher nitrogen than most refiners are accustomed to seeing and creates a resistance. Catalytic hydrotreatment addresses both of these concerns and makes a hydrocarbon product that is readily integrated into the refining infrastructure, yet at some cost. Building greenfield, stand-alone hydrotreaters at smaller scales isn't impossible, but it does add some cost. Better to co-process at a refinery if possible. There may also be potential that we've explored for other fuel grades (bunker oil, BTUs for combustion) but these are not quite the prize that infrastructure-compatible, low carbon liquid transportation fuels provide. It's continuing research and we are open to input.

Hi. How much biofuel can be produced using this technique considering the amount of waste available?

Would it be enough to sustain let's say, a whole city and remove, or minimize, the dependency on fossil fuels? Thanks


A back-of-the-envelope suggests that the average city could run it's fleet on the fuel from sludge solids. About that scale.

Sounds fascinating! What type of cultural road blocks are you guys expecting/ had to face?


So far, we have had nothing but positive feedback from wastewater treatment plants. There are several interested cities. If we maintain the dialog with all stakeholders and scale responsibly and don't over-promise, I think we can have a successful roll-out to the next size. Then we'll have much better data to decide what to do next. The WE&RF is an excellent example of DOE, utilities, EPA, consultants, and national labs working together. Just read the title pages in the WE&RF report (linked in intro) to see the team. Involving everyone in the discussion is so crucial.

Do you think we should be focusing our efforts on creating biofuels from abundant sources and then re-engineering combustion engines?

Or should we be focusing on creating biofuels with energy densities capable of replacing fossil fuels in traditional combustion engines?

Which do you personally think is more promising and why?


The current thinking is an "all of the above" strategy. There is a multi-lab consortium called "Co-Optima" that is looking at new fuels and engines (co-optimizing them, thus the name). I think we will see rapid adoption of electric vehicles for light transport (cars, light trucks). But we'll need liquid fuels for heavy transport for a much longer time, perhaps indefinitely (think OTR trucks, trains, ships, airplanes, earth-moving equipment, etc. - whatever cannot be practically electrified). So biofuels should be targeting jet/diesel range replacements, for both now and long-term. And this is where technologies like HTL fit in. Our crude naturally contains a fairly generous distillate (diesel/jet) fraction (>50%).

I was excited to see the reveal of this technology. My question is how does the process breakdown (or not) the hard-to-eliminate pollutants in solid waste like heavy metals, hormones (and mimickers) and other pharmaceuticals? Is this a better way to destroy or concentrate pollutants like these?


We are actively analyzing these pollutants, both at the inlet and outlet of the process, we have only started this work. Heavy metals go to reactor solids. Most organics are broken down and are incorporated in the biocrude and will be combusted. We want to understand all of our streams and how they impact the operation of WWTPs.

Is this process optimized for human sewage systems or would it work approximately as well for livestock waste? As I'm sure you know the waste from Concentrated Animal Feed Operations (CAFOs) is a significant problem, with livestock producing significantly more than total human waste.


I left a comment on CAFOs somewhere up the thread. Manure should work. There are challenges but hey, what is research but encountering and overcoming challenges!?!

I hope this isn't a foolish question, but I'm curious to know - what is the likelihood that such a conversion process becomes widespread, and how long would implementing the process at waste treatment facilities across the US take?


Supposing the process continues to perform well at increasing scale, I would expect a roll-out over decades as aging infrastructure is upgraded and/or replaced. However, there are many public facilities that are already at that juncture and are looking for a better way forward.

Can tree litter be treated with this or a similar method? What are the barriers for that?


Yes, we've done HTL of woody feedstocks and can create useful biocrude. But they start out essentially dry, so we are at parity with pyrolysis technologies because we don't get the 'evaporation of water' boost, and perhaps pyrolysis would be a good choice for tree litter. The main feedstock that we used was forest product residual from Georgia loblolly pine.

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