The invention includes a method for the optimization of the soluble CO2 concentration in the aeration basin of an activated sludge system, which significantly improves the specific growth rate of the nitrifying bacteria. The result is a reduction in capital and energy costs for municipalities. The rate of nitrification is a product of the nitrifying bacteria biomass concentration and the specific growth rate of the bacteria. In the activated sludge system, the biomass concentration is maintained at high concentrations by reducing the wasting rate. The specific growth rate is a function of the ammonium concentration and the environmental conditions. Here, the inventors show that growth of nitrifying bacteria is inhibited when the soluble CO2 concentration is elevated beyond certain parameters. Elevated soluble CO2 concentration also reduces the pH, which also impacts the rate of growth of nitrifying bacteria.