A fluid treatment system and method for dissolving a gas into a fluid and subsequently allowing microbial or chemical reactions with the fluid mixture to decontaminate the fluid. The system comprises a pressurized gas transfer reactor, a depressurization device, a means of rapid dilution, a reactor, and a means of secondary clarification. Gas is dissolved into a fluid within the pressurized gas transfer reactor. The fluid is then transferred to the depressurization device where it is depressurized and diluted. Thereafter, or concurrently, the fluid flows into the reactor where microbes consume contaminants and dissolved gas within the fluid. The decontaminated fluid then either flows into a secondary clarification tank or membrane system where the bacteria within the decontaminated fluid are removed, or the fluid is recycled through the system. The method of present invention utilizes the system of present invention and operates pumps and a regenerative turbine or throttling valve.