“What’s the Difference between a Liquid and Slurry?” Something only an Engineer would Argue About

I have an uncle who is a retired professor of philosophy-- a logician -- if that means anything to you.  Once, during a family discussion about something incredibly mundane like the difference between “giving thanks” and “Thanksgiving”, he asked me, “What do engineers talk about at lunchtime?” I didn’t have a good answer then. Having spent the last year chairing a national panel on manure treatment technologies, I have one – “the difference between liquids and slurries”.

There are four states of consistency for mixtures of solids and liquids: solid, semi-solid, slurry, and liquid.  Figure 1 helps explain the differences in consistency.  Solid materials stack when poured out of a container, and they deform as a uniform body when pressure is applied to them.  Semi-solids stack like solids, but squirt like liquids when you squeeze them (think of stepping on a cow patty). Liquids and slurries spread-out in a puddle when poured from a container.  The difference between liquids and slurries falls down to, “slurries contain suspended solids”.  But, how many suspended solids do you need to add to a liquid for it to become a slurry?   That is what the engineers on the panel have been arguing about for more than a year.

One side of the argument says slurries contain a sufficient amount of solids so that particles do not separate from the liquid.  For swine manure, this is about 3% total solids.  So, following their logic from 0 to 3% solids, pig poo is a liquid; between 3 and 15% solids, swine manure is slurry; and above 15% solids hog feces act like a semi-solid.

The break at 15% makes some sense. You can pump slurry, but you have to scrape semi-solids (Figure 2). The 3% break might mean the difference between whether solids settle out of a pipe line or not when pumping the liquid-like manure.

My answer is any amount suspended particles turns a liquid into slurry. The material flushed out of a pull-plug hog barn is less than 1% solids. Have you ever used a regular irrigation pump to move flush water? The pump didn’t last too long did it? Water handling pumps are made with very tight tolerances.  Trying to pass any solid particle through an irrigation pump will ruin it. So my logic is the difference between slurry and liquid is the special equipment needed to handle suspended solids.

You can read more about manure handling in OSU Factsheet BAE 1751, Consistency of Manure/Water Mixtures.  I cannot promise it will lead to stimulating conversation at the family reunion, however.

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