Abstract: A method and apparatus is provided for optimizing the efficiency of a biological process of treating waste and sewage, and more particularly a process in which air is supplied to the waste to adjust the dissolved oxygen content thereof to a desired optimum level. The process includes controlling the amount of air supplied to the process by adjusting the air supply means in the process in accordance with the ambient temperature of the air into the air supply means in combination with other air volume control means which are dependent on other process variables. The invention enables attainment of optimum concentrations of dissolved oxygen with minimum consumption of power and energy.