May 25, 1806: Letter from Thomas Telford discussing design of the filtration works at Glasgow, Scotland. Glasgow was the third city in the world to receive filtered water (after Paisley, Scotland and Paris). Delivery of water by pipes to customers began in 1807.
Reference: Baker, Moses N. 1981. The Quest for Pure Water: the History of Water Purification from the Earliest Records to the Twentieth Century. 2nd Edition. Vol. 1. Denver, Co.: American Water Works Association, 80-1.