During a water treatment plant tour I shot video of the backwash process. This takes about an hour, so I shot several clips and put them together to show each step of the backwash process.

This filter is dual media, anthracite coal and sand. This is a very common setup for granular media filtration at drinking water treatment plants.

The backwash is done with water that has already been through the plant and is stored in a separate backwash water tank. Some plants use pumps to bring clean water back to the plant for backwash. This plant (and many others I've seen) uses an elevated tank so the backwash is gravity driven.

An interesting question for students is how the flow rate is controlled. That's something we can't see in the video. But if we were to go underneath the filter we would find a big butterfly valve. The automated controller moves that butterfly valve to adjust the flow rate for filtration. A similar valve (or a few, depending on the pipe network) would be used for backwash.

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