Increased control and efficiency over the wastewater purification can be achieved through creating conditions that allow to selectively prioritize the digestive function of microorganism in the activated sludge. The gas-dispersion return sludge is created using pure oxygen or oxygen containing trace amounts of ozone as a reactive gas, which is blended with return sludge to create a mixture of gas and liquid, which is passed through an atomizer or a cavitation pump to instantly render the reactive gas in the mixture to an ultra-fine bubble state. At least a portion of the ultra-fine bubbles dissolve within the gas-dispersion return sludge, activating the dormant microorganisms. Due to a complete or an almost complete absence of digestable organic material in the gas-dispersion return sludge, the microorganism prioritize their digestive function, and when exposed to organic pollutants present in wastewater, digest the pollutants into water and carbon dioxide at an increased rate.