Dredger

A dredger was a worker who excavated sediment from the beds of rivers and similar bodies of water.

In 1814, Lord Sutcliffe used dredgers, hired and watched over by an overseer to collect the faeces of a serpent chained to the bed of the River Thames. This was moulded into blocks and sent to the steel mill, where it was capable of burning a thousand times hotter than coal. (TV: Thin Ice)