In a waste water treatment plant, when biological processes are used to purify water, it is called ___.

Secondary treatment is called the activated sludge process. Air pumped into large aeration tanks mixes the waste water and sludge, that stimulates the growth of oxygen-using bacteria and other tiny organisms, that are naturally present in the sewage. These beneficial microorganisms consume most of the remaining organic materials, that are polluting the water and this produces heavier particles, that will settle later in the treatment process.