Science Questions

Can we use human excrement for fuel?

Sat, 8th Oct 2011

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Matt Fisher asked:

Hi Dr. Smith,


I've always wondered if human waste can, or is being used, to produce electricity.† I know that rotting cow manure and garbage can produce energy, but I've never heard of human waste used for this purpose. Seems like a great source of energy, and the infrastructure to collect it is certainly in place.† Can you shed some light on this?


Matt Fisher

Washington State


We put this to Dr. Piers Clark, Commercial Director at Thames Water...

Piers -   In Thames Water, weíve made energy from poo for many, many decades now and we do it through a process called anaerobic digestion.  Anaerobic digestion is a process that's very similar to what happens inside your stomach A toilet in a theater in Munichactually when you eat food.  It also happens when we ferment berries to make beer.  Itís basically the gradual breakdown of organic matter through a natural process and the molecules that make up the food that we eat and that ultimately come out as poo gradually get broken down into a form of a gas and the gas is called biogas.  Itís a mixture of methane and carbon dioxide.  And carbon dioxide is naturally in the air and methane is the gas that people most readily associate with being the gas that you burn on your cookers at home.  We run this natural process and have done so for many years.  We produce about 15 million pounds worth of energy every year which equates to about 15% of our total energy usage in Thames Water.


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Co-firing would seem the best compromise technology.

This pdf contains a description of using Sewage Sludge and other biomass sources for coal co-firing:
Evaluating the Sustainability of Co-firing in the UK

Using anaerobic digestion (oxygen-free biological breaking down of biomass) is another option, but I believe it is hard to scale up. peppercorn, Fri, 9th Sep 2011

And for the Scotts:

Kincardine peppercorn, Fri, 9th Sep 2011

That's Scots laddie 

Is this plan a load of WDF?

"Honey, can you put another log on the fire?" Geezer, Fri, 9th Sep 2011 RD, Fri, 9th Sep 2011

I've seen notes about using methane generated from cattle.  I've always wondered why it isn't done with human waste. 

It would be relatively easy to modify the plumbing in a new house...  At least for a rural house, you would create vent pipe loops from the septic tank to the place near the fixture/trap where the vent pipe is normally attached.  Then carefully regulate the pressures to maintain the fluid in the trap.

A municipal system would be similar, but might require a separate vent system that goes back all the way to the sewer treatment plant, or at least tying into the sewage line in multiple places.  Or, you could just generate the gas once it is collected at the treatment plant.

Dried waste could be burnt like is done with cow dung and buffalo chips.  There are good soil nutrients in the waste and sludge, but at least in western countries, there are concerns about diseases and contaminants in the waste so burning it might not be a bad idea. CliffordK, Fri, 9th Sep 2011

I wonder if Shakespeare had this in mind when he wrote Richard the Turd, or does it have more to do with Newton's Turd Law of Motions?
Geezer, Sat, 10th Sep 2011

The only problem I have with it is how unsanitary human feces can be. It takes a lot of energy to fire it enough just to turn it into soil. Which, to me is a much more practical use for any excrement. JargonJam, Thu, 22nd Sep 2011

One of the problems with digestion of human waste is that the volume of gas that could be generated from an individual household is very small. 
On a commercial basis, even a seperate sewer system to deal with commercial/ industrial effluent, contaminants  - specifically siloxane - within the human effluent will tend to increase running costs of generators.
Siloxane compounds are a constituent of many shampoo/ showering/ shaving products as a lubricant and when burnt in an internal combustion engine turn to SiO2 inside the cylinder/ combustion chamber.  This silica ash destroys the affected parts of the engine. 

On a landfill gas powered generator site I worked on (similar gas to a sewage digestor) that had been used to dispose of off spec shampoo; 70l of sump oil was changed every 170 running hours and the engines rebuilt every 3000 hours.  The neighbouring site the oil was changed every 1000 hours and the engine serviced/ inspected every 10000...  Mazurka, Mon, 26th Sep 2011

Why not put all the human excrement into an airtight building and let the waste breakdown into methane and put a gas valve into the room to let the gas escape into a tube leading to a big container where the methane is then it and heats up steam to move turbines and generate electricity there are enough humans on this earth to power the world and it would cost less because you will not be digging for fuels it could be directed from the sewage plants. The science enthusiast, Wed, 5th Oct 2011

"there are enough humans on this earth to power the world "
There are definitely not enough humans to produce enough waste to power the world that way.
On the other hand, it would recover a little energy and save a little fossil fuel. Also it would get rid of the waste.
the biggest problem is that waste is generated all over the world and transporting it to one big fermenter (or even quite a lot of smaller ones) would take more energy than you could recover from the methane. Bored chemist, Wed, 5th Oct 2011

There are definitely not enough humans to produce enough waste to power the world that way.

Certainly, that is a problem with a number of things.  For example, people don't eat enough French Fries to produce enough Waste Vegetable Oil to power everyone's cars.

But, every little bit helps. 

Transporting from rural septic tanks to a central processor would be inefficient.  However, most cities already have centralized waste processing.  Could they keep up with the waste flow while using a bioreactor?  Would running a centrifuge before the bioreactor take too much energy?

Even in a rural setting, perhaps one could run one's stove/oven on biogas.  And, that might make a big difference when designing an off-grid solar system.

Dehydrating human excrement and mixed waste to the point where it could be burnt would likely be energetically expensive, unless somehow you could add a steam generator & superheater to your dehydrator, or if solar energy was sufficient for the dehydration.

One advantage of using human generated methane.  No need to add scent to the gas.

(sorry, I had originally misread "aren't", fixed the quote to show who is who, and indicate the negative I thought I was replying to). CliffordK, Wed, 5th Oct 2011

You might want to attribute that quote correctly; it's plainly from The Science Enthusiast. Bored chemist, Thu, 6th Oct 2011

i saw a 50' long 2 bag setup for collecting methane. They excrementized into a bucket & dumped it into the system= 3rd world rocket science !
CZARCAR, Sun, 9th Oct 2011

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