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Tuesday, 22 February 2011

Engineering for Writers

As I am a professional engineer, I thought I would share some knowledge with you in a series of engineering related articles. If I can help to add some authenticity, or at least help you to avoid some of the engineering fallacies that I hear in popular culture, then I will have achieved my aim. How's that for a bit of bloviating?!

The first topic on the agenda is SEWERAGE.

Don't you love it when I talk dirty?

First let's get some terms sorted.

A drain serves a single source, such as a house, or a factory or a highway. A sewer serves more than one property, or source. The network of sewers is called sewerage - not to be confused with sewage which is the smelly stuff that flows in a sewer.

A sewer can either carry just rainwater, in which case it is called a surface water or stormwater sewer or it can carry dirty water in which case it is called a foul or sanitary sewer. Older sewers, particularly those in cities, carry both rainwater and dirty water and are called combined sewers. It is these combined sewers which can be very large - sufficiently large for a person to walk in.

On the subject of size. 1000 houses would produce, on average, a FOUL WATER design flow of 45 litres per sec, and would need a pipe 225mm or 9 inches in diameter. Not something your hero would fit into. On the other hand 1000 houses would produce 1400 l/s in STORMWATER runoff in the sort of storm that you might see once a year. (This depends on where in the world you are - the figure quoted is relevant to the UK). This would need a pipe 900mm or 3 feet in diameter. Large enough to crawl in. However, your hero should beware - it takes 4 minutes for rainwater to work it's way into the sewer, with the flows building to a peak in 10 to 30 minutes depending on the network. I mention all of this just in case your hero should ever get trapped in a sewer during a rainstorm! Sadly the truth is that every year people are drowned in sewers, all too often children who think it is cool to hang out in their "den" which might have been cosy and dry for weeks.

Assuming you haven't zoned out by now here is a MYTH-BUSTER. The idea of a fish being flushed down a toilet finding its way to the sea is, alas, nonsense. (Even for Nemo!). Even if you live in a seaside town where flows can sometimes discharge to the sea, sewage has to pass through a screen with only 6mm gaps (1/4"). That is enough to stop the solids, sanitary towels and piscine escape artists. Inland, the sewers run to Treatment Works, where the sewage is screened, solids are removed in flocculation tanks, sedimentation tanks, and the resulting liquid is sprayed onto gravel filtration beds. Not a good place for a fish.

One last note. The large sewers have only been around since the 19th century. Not strictly accurate to have your medieval protagonist escaping through the sewers. In medieval times, householders disposed of their waste into a cesspool which was emptied daily by the night soil men. (A real lowest of the low job). Other waste ran in open channels in the road.

I hope this article has given you something to think on, and you never know, perhaps one day it might come in handy!


  1. Wow. Kinda cool....kinda icky. But you know, people always insinuate that to be a ditch digger or a garbage collector is a less than noble or worthy occupation. If we didn't have them, imagine the state our environment would become rather quickly.

    The same with people who design, maintain, and oversee one of the most vital elements in our modern world. Most of us don't give a second thought to any of those things....until they don't work!

    Very thoughtful and informative post!

  2. This is really interesting. Next time some author has her hero dashing through the sewer, I'll be fact checking!

  3. Thanks for that great information! It might come in handy for my writing some day.

  4. That was fantastic. By all means, please make this a series of posts about your expertise. All I knew about sewers before this I learned from watching Dirty Jobs.

    I generally consider what happens when I flush my toilet to be magic.

  5. Wow, interesting stuff! I know where to come when I have an engineering question.

  6. I'll admit, I wasn't sure where you were going with this. But after the first read through I pulled out an index card and took notes. Eventually, this information will authenticate a fictional short story or other creative endeavor. So, thanks!

  7. I definitely feel educated about sewers now. I didn't realise there was so much I didn't know about them! Thanks!


  8. Thanks, Dom, I know more than I did a few minutes ago. I'm not sure how much of it I'll retain, but if I flush a fish down the toilet, it won't be more than 1/4 inch in diameter, okay? So it still has a chance to get to the sea. Not so the baby alligators with which fiction is fraught.
    -- K

    Kay, Alberta, Canada
    An Unfittie's Guide to Adventurous Travel

  9. Talk dirty, go on! LOL. Seriously, the road I live in originally had just six houses, built for people returning from the colonies in the late 1800s. Ours is one of the orginals, divided into four flats. Now there are three times as many houses and nearly all of them contain flats. There are also several old trees (200 years plus), so you can imagine the problems we regularly have with the sewer system!

  10. Wow this is great stuff. Thanks. It's all being aborbed. Can't wait for the next one.

  11. Speaking of all things sewer - I absoloutely loved a documentary I watched a few years back about Balzagette and his london sewer system - a fantastic man, a visionary and a great dreamer!!!!!So yay for the sewerage system!! Take care

  12. Oh, make my day! I'm from a family of builders and I have fun laughing at the missuses of common tools in books. Contrary to common belief, screwdrivers do have limits. ; ) Great post!

  13. Loved your post! Don't stop! Ever since Nemo traveled the world, too many characters have wandered around in sewers. Time for a change of scenic escape.

  14. Thanks for the info! I have a dystopian/post-apocalyptic novel I just finished that I may need parts of this post for. Thanks! :)

  15. Hi Dom. As a fellow engineer I can say that the different disciplines in engineering are really, really *different*. While other kinds of engineering fascinate me - structural engineering, civil engineering, etc. my discipline is systems engineering and software engineering. Not too much sewage flowing in my work on a good day.

    By the way, we're in North Devon, and I have family in West Malling and Cuxton, so some time when we're in Kent, how about meeting up?

  16. Hi Tony. That sounds great - let me know when you are in this neck of the woods. :) My daughter is coming your way in July with her friends (somewhere between Bideford and Bude). It's a small country!!

  17. Hi Dom, Not sure when we'll next be there, but we'll definitely drop you a line when we're coming. Bideford's a lovely town - we do our morris dancing there, but I don't know Bude so well.


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