a process and apparatus for biologically treating water . the inventive process preferably comprises the steps of : adding a sulfur - containing oxygen scavenger to the water in an amount effective for at least reducing the amount of dissolved free oxygen contained in the water ; removing at least a portion of a substance from the water by exposing the water to microorganisms effective for biologically removing the substance therefrom ; and feeding a biostimulant to the microorganisms effective for increasing the rate at which step occurs . the inventive apparatus preferably comprises a system for adding the sulfur - containing oxygen scavenger to the source water stream and a system for feeding the biostimulant to the microorganisms .