Few movies have left deeper marks on me than the tale of Mad Max. The lone warrior roaming the wasteland of a post-apocalyptic civilization – or at least what passes for it in the trilogy.
But as much as I liked the action, the third part had impressed me most for its hog lagoons. In Beyond the Thunderdome, an army of pigs swarms beneath Bartertown, one of the last semi-civilized outposts. The pigshit produces methane which keeps the lights of the town going and vehicles fuelled.
Most remember the movie for its chant “Two men enter, one man comes out”, but that huge tapestry of pigs is the films true moment to be remembered as this is innovation at its purest. Forget wind turbines and solar panels. In the future, we will get our fuel from hog-poo.
Let’s come down to earth. How real is the production of clean-burning methane gas from swine manure? In the end, its pretty liquid stuff and we all know from bioMethane production that the thicker the morass is, the better the methane yield.
In reality, swine manure produces just as much methane as any other manure. The specific methane yield from swine manure runs anywhere from 0,1 to 0,4 m3 CH4/VS depending on how fresh the manure is and what the pigs have been fed. Beef manure runs generally a bit lower. It all depends on what the animals have been fed but in general, pigs are incredibly efficient feed converters.
The real problem is that because of the extremely high wet content of swine manure, it is very difficult to digest economically. The wet mass potential is the volume of methane you can expect to produce from a given mass of manure as it is fed to the digester. Some swine manure has a solid content of around 2.5% – not really the stuff you want to shovel. Cattle manure typically runs to about 12% solids.
So our real problem is not that swine manure would be bad quality for methane potential but rather that because we love to handle swine manure as a liquid (it’s easier to handle and to transport) we trade ease of handling for methane potential.
It’s no surprise that most swine manure digesters at farms are in fact nothing but covered lagoons. One can always go for much more technically potent digesters but they are always expensive and are also quite a challenge to operate.
And here, co-digestion products come into the picture. Because one only needs to add other manure or products to the digester to thicken the brew and voila, up goes the volumetric yield. They are an incredibly potent way to increase methane yield from digesters. A co-digestion product is something you add to the digester to boost gas production. You can use beef manure, slaughter facility sludge, poultry litter, potatoes peel or food scraps, oat hulls or even wood shavings. The thick mixture added to the digester would produce a lot of gas.
Restaurant grease also is an extremely potent co-digestion product. Just adding 10% food grease to swine manure increases methane production by six-fold.
But as with everything in life, the dosage is the secret. A larger amount of a co-digestion product may become toxic. Remember, that’s bacteria living in there and as any living organism their efficacy and even their livelihood depend on a number of factors.
Some bacteria thrive in extremely acidic conditions that would be impossible to survive for humans. Other feed on sulfur sludge, again not really man’s preferred chow.
Just like all the rest of the bioMethane business, we are just starting to understand the complex mechanisms inside the digester. And we are continuously getting better at it. Let’s not forget. Todays Natural Gas was originally formed by bacteria that decomposed organic matter and produced hydrocarbons in the process. Nothing different from what the digester does.
Nature needed millions of years to get the job done. Man and swine are much quicker and I bet there are some swine farmers that would just love the opportunity to produce the cleanest fuel you can put into a vehicle from something that has classically been a problem for them.
Swine manure was a waste and getting rid of it was costly. Now it may just become the stuff heating your home and propel your car.
It all comes 100% CO2 neutral and on otherwise extremely low emissions.
In Mad Max 3 it was almost a capital offense to kill a pig. Doing so got you a ticket to the hog lagoon. We will certainly not go that far but we might come to see the byproducts of porcine digestion with different eyes.