conversion of the treatment process from chemical precipitation to septic tanks and sedimentation tanks began in december 1900, and was completed on 4 august 1904. the tanks covered an area of . air was excluded from them by the formation of a scum on the surface, and anaerobic microbes broke down the sewage. the 'first contact bacteria beds' covered , with a similar area used for 'second contact bacteria beds'. there were also which were used for 'storm water filters'. the bacteria beds were filled with a media of coke or granite chippings, around which microbes formed a slimy film. the oxygen-loving microbes digested the organic matter in the sewage as it passed through the media.