In this paper, a system to stimulate in extremely low frequency (ELF) electric field the microbiological activity of the microbial flora in the active sludge of a wastewater treatment plant is presented. The designed and realized system was tested in industrial environment with and without ELF electric field stimulation. By comparing the two cases for the same degree of pollutants elimination from the wastewater, it resulted that the ELF electric field application significantly reduces the processing times in the biological treatment stages (to approximately half). Thus, the energy consumption in the biological treatment stage is reduced by approx. 50%, representing approx. 30% of the wastewater treatment plant energy consumption.