There will always be wars, and people like you and me will fight them...

One of the first jobs I had after dropping out of university was working as a labourer for the local council. And one of the major projects I worked on there was climbing down into the town's water filters and shovelling it out layer by layer, wheelbarrow load by wheelbarrow load, to be replaced with fresh stuff. It went exactly as this diagram has it, with the larger rocks at the bottom. I'd expected the sequence to be the reverse: coarser rocks at the top to capture the sticks etc., with progressively finer layers to get the finer particles. I never found out why this seeming inconsistency existed.

The other fellow I was working with on this project was a fifty-year-old German ex-soldier from the Second World War. He was the fittest bloke I've ever known and ran rings around me as a weedy little heavy 'smoking' hippy!

But he was very patient as he showed me how to survive a day of extremely sweaty, hard labour in such a confined environment during the summer. He told me how after the second world war, as a prisoner he was forced to unload the coal wagons that rolled into the local railway station. Each man had a full wagon to unload with a shovel, and they could go home as soon as their wagon had been unloaded. Apparently, he was always the first to get away for the day 🙂

I remember many conversations with him, including one where he said, unsolicited, "Wars? What can you do? There will always be wars, and people like you and me will fight them."

I'll never forget that job, even if I didn't learn why the big rocks were at the bottom, until one fine day surfing the web I found this:

What is the function of gravel in water filtration?

And for those who don't want to go offsite:

"I guess you are referring to the type of filter usually found at waterworks supplying the drinking water mains, and other industrial sized water filtration systems.

These can be either open gravity fed filter tanks or closed cylindrical pressure vessels

The filtration is actually performed through a deep bed of sand. This bed entraps particles of dirt from the water. This process is often enhanced by adding small quantities of certain chemicals such as alum or ferric sulphate to the water. These help the impurities clump together and to stick to the sand particles in the filter.

The bottom of the sand bed is supported on a layer of gravel. Small gravel under the sand, then progressively larger grades until at the bottom of the tank or vessel the gravel surrounds a network of perforated pipes which collect the filtered water.

When the filter is dirty, water is diverted into the bottom collector pipes and flows upwards through the gravel and sand. The sand is fluidised and the dirt particles are flushed out of the top into a waste drain. The flowrate of the water is designed so it is too slow to disturb the gravel underbed, fast enough to separate the sand grains and lift out the dirt, but not fast enough to wash the sand out with it.

Now the purpose of the gravel. It stops the sand entering the bottom collector pipework. And when backflushing ensures a uniform flow of water up through the sand bed."

cheers,

Erron