Mad about Science: Sewage and fluid dynamics

By Brenden Bobby
Reader Columnist

I was frequently lectured in elementary school for my bathroom humor.

“No one is supposed to think about poop!” I was often told.

It turns out that a lot of very important people think about poop quite a bit. If they didn’t, we’d all have to think about poop constantly because we’d always be knee-deep in it.

This issue was starkly apparent in London in the 1840s during a period referred to as “The Great Stink.” The city of two million people suffered from a grossly inadequate sewage system cobbled together over centuries of use. Flush toilets were en vogue, pushing a higher quantity of waste into the failing infrastructure than ever before and then effectively dumping it straight into the River Thames. The searing temperatures of the summer of 1858 exacerbated the stench and even ground legislation in Parliament to a halt.

Who wants to work when all you can smell is yesterday’s bangers and mash?

Worse yet, a cholera outbreak had begun killing Londoners by the thousands, with an estimated 10,000 casualties during 1858 alone. During this time, most people still believed in the theory of miasma, which argued that bad smells were the source of disease. It wasn’t entirely inaccurate.

The real sources of diseases were the sources of the bad smells, and in the case of cholera, that meant it was the river of effluent flowing into and mixing with the city’s drinking water.

Dr. John Snow — unrelated to the character from George R.R. Martin’s A Song of Ice and Fire — brought forth the idea of cholera being waterborne. His ideas were mocked, and he unfortunately died before he could experience vindication. Nevertheless, the sewage problem needed to be dealt with, so Parliament tasked Sir Joseph Bazalgette with creating a sewer system to support London’s growing population. The work took over twenty years and nearly doubled its initial budget, but it was so well designed that it’s still used to this day.

The difficulty of dealing with human waste is that it’s a mix of liquids and solids. A pipe with a pitch that is too shallow will cause solids to settle and collect, eventually obstructing the pipe. Having a pipe with too great of a pitch causes the same problem, as liquids race down the pipe and leave the solids behind. Clearing pipes in elaborate sewer systems is a laborious and difficult process that no one wants to do if it can be helped.

The optimal speed for fluid to travel while carrying waste is about one meter per second. This keeps your poop in a group, one could say.

It seems like it would be easy to just slant a pipe and call it a day, but there is far more at play here when thinking about poo disposal. Eventually, the pipe will get too deep underground to properly service, which could be catastrophic if something solid gets lodged in there or some kind of catastrophe happens that damages the pipe and causes it to leach into the groundwater we drink from.

Additionally, engineers need to account for things like natural waterways when building water systems because external water can corrode the pipes from the outside. One solution to this is to build something called an inverted siphon, which is U-shaped and remains full at all times in order to work efficiently. The issue with a large U-shaped pipe is that it can collect solid waste, which builds up near the outflow side of the pipe and clogs it.

Another solution is to use many smaller pipes instead to maintain the same amount of pressure, collecting a minimal but steady amount of solids to prevent buildup.

Sewers are carefully designed to deal with a very specific subset of human waste — urine, feces, vomit and toilet paper. Unfortunately, humans like to flush other things down the toilet, like so-called “flushable” wet wipes that aren’t actually designed to break down in the same way as toilet paper. Toilet paper breaks down in minutes with agitation as it travels through sewer pipes, while flushable wipes do not.

Wipes will eventually decompose, but the real issue occurs when they come into contact with grease or fats from cooking that have been poured down a sink drain. The internet has illuminated some of the horrors faced by frustrated plumbers and municipal engineers, who have whipped their phones out to record things we’ve flushed down drains.

“Fatbergs” are a mass of congealed fat from cooking mixed with wet wipes and waste that jam up pipes like a cork in a wine bottle. This totally stops up a pipe, which leaves the waste with only one direction to go: back to the source.

If only my elementary school teachers could see me now, writing about poop for thousands of people to ruminate on and enjoy.

Stay curious, 7B.