Suspended solids in settling tank sludge removed. A lot of organic material remains that encourages algae growth, which chokes out sunlight and eventually oxygen from bodies of water. Biological process to remove organic material aerobic bacteria are used to remove organic wastes. Aeration tanks mix sludge, water, air and bacteria to speed process. Wastewater then goes to a settling tank where microorganisms and sludge sink to the bottom.
Water is treated with chlorine before it is released.
Ultraviolet radiation is also being used as an alternative to chlorination. Secondary is better than just primary because it removes more of the organic material. Tertiary Treatment Chemical and physical processes to remove pesticides, phosphates, nitrates and other substances that the secondary stage does not fully remove. Few treatment plants do this they cost twice as much to construct and about 4 times as much to operate. The sludge must then be disposed.