It sounds like a factory farm legend. One day, the manure collected under the floorboards of a pig barn starts foaming uncontrollably. Then, the foam releases gas that explodes. In one incident — all too real — over 1,500 pigs were killed, and one human seriously injured. What's going on?
As Sarah Zhang explains in a great essay over at Nautilus, the answer is complicated. Something is turning pig shit into explosives, and it's happened recently. Famers have been using the same barn designs for a long time, built with slatted floors that allow the pig manure to fall through cracks and form a pile under the structure. Only in the last five years had they ever seen the manure foam up. So what's changed, and why is it explosive? Zhang writes:
As a matter of physics, every foam needs three components: gas, stabilizer, and surfactant. Manure is normally teeming with microbes, which produce gases like methane and whose cells can act as stabilizers. Therefore the surfactant—any chemical such as soap that lowers the surface tension of liquids to allow bubbles to form—is a likely candidate for what's new.
Around the same time that foaming manure became a problem, farmers also started feeding their pigs more and more distiller's dried grains with solubles (DDGS), a cereal-like byproduct of ethanol production. DDGS is full of plant fibers and long-chain fatty acids—plausible surfactant candidates. Since the US provides huge subsidies for making ethanol from corn, DDGS was cheap and plentiful source of animal feed. It makes economic sense, at least.
DDGS has been a suspected culprit of foaming manure from the beginning (one magazine called it a "pig bang theory"), but experiments are still inconclusive.
So a new kind of pig feed may be responsible for the foaming, and two gasses in the foam — methane and hydrogen sulfide — are turning barns into gas explosions waiting to happen. But the problem, as Zhang points out, is that it's very hard to determine whether DDGS is actually the foam-creating culprit. There are too many other factors at play, and the barns themselves are usually piled with manure that's over a year old. That makes it hard to test for novel chemicals.
Currently, farmers are solving the problem by feeding pigs an antibiotic that seems to break down some of the microbes that cause foaming manure. Which means we're once again pumping antiobiotics into the environment at a massive scale, which breeds more antibiotic resistant bacteria.