Boltons's outfall sewer is usually around 1.3m due to it frequently crossing rivers on it's way to the treatment works at Ringley. I was interested to read mention of 2.1m sewer separate to the outfall sewer near town. It wouldn't make sense as the big pipe would need to join the smaller main one, unless there was a large river overflow nearby (which there's not). I went with TheVicar to check it out.
It turned out to be a online storage pipe just under 300m long which equates to around 3800m³. A small dropshaft from an incoming sewer feeds in next to the overflow which is higher up. When the pipe backfills the flow goes down the overflow straight to the outfall sewer. The whole process takes a while and hopefully the storm will have finished before the full flow is put on the main sewer, making it less likely for CSOs downstream to overflow.
At the downstream end the flow disappeared down a tiny pipe to the outfall sewer. This section was thick with the crap that washed down when the pipe was full, every surface was covered and it was pretty bad even by sewer standards.