Here it states,

Prior to the introduction of sterilization technologies most swimming pools were filtered to keep them somewhat clean and the water was changed frequently.

The above article references 'BULLETIN FOR THE HISTORY OF CHEMISTRY Volume 32, Number 2, 2007' (page 130) by the American Chemical Society Division for the History of Chemistry. This article clairifies,

Prior to the introduction of sterilization chlorine, bromine, ozone, or ultravioletlight, most swimming pools were filtered to keep them somewhat clean, and the water was changed frequently.