Abstract: Systems and methods have been developed for treating waste water and produced water, so as to remove contaminants therefrom. The systems and methods allow specifically for the removal of contaminants from produced water from oil and gas wells, fracturing flow-back water, and water- or brine-based drilling fluids. The water is treated by contacting the contaminated fluid with ozone after contacting the fluid with a diffused air system to generate small bubbles for entrainment of some contaminants. Thereafter, the water is contacted with an electro-coagulation system in order to remove flocculants and adjust the pH and the total dissolved solids levels of the fluid.