Abstract: An example of an apparatus and method of operating the apparatus to treat produced water. The apparatus includes a cavitation chamber. In addition, the apparatus includes an inlet to receive produced water with a microorganism. The apparatus further includes a pump to pump the produced water from the inlet into the cavitation chamber at a predetermined pressure. The apparatus also includes an injector to inject a biocide to the produced water to control a population of the microorganism. Furthermore, the apparatus includes a micro-bubble generator disposed within the cavitation chamber. The micro-bubble generator reduces a pressure of the produced water below a fluid vapor pressure to create micro-bubbles which collapse to generate a micro shockwave to enhance the efficacy of the biocide at reducing the population of the microorganism. The apparatus further includes an outlet to release the produced water after the population of the microorganism is lowered.