Secondary treatment consists of two steps: biological treatment and settling. Biological treatment is carried out by microorganisms in an aeration tank. The microorganisms require air to live; air diffusers in the bottom of the tank pump air into the tank. The mix of all these microorganisms, called activated sludge, eat the organic matter that was not removed in the previous processes. The more they eat, the more they reproduce and grow. As the microorganisms grow, they form together in clumps, called floc, which captures small particles that were not removed in previous steps. This floc will be removed in the second half of secondary treatment – settling.
Microscopic image of Activated Sludge:
By Frank Fox - http://www.mikro-foto.de, CC BY-SA 3.0 de, https://commons.wikimedia.org/w/index.php?curid=20322044
Secondary settling takes place in a clarifier, same as settling in primary treatment. The solids settle to the bottom of the tank; some of the solids are returned to the aeration tank, while the rest is processed elsewhere for disposal. The clean water continues on to the last stage – disinfection.
Fats, oils, and grease can interfere with this step of the process. Instead of settling to the bottom, they rise to top and leave the plant with the treated water. This can cause issues with the environment and the Department of Environmental Protection can fine the District. for allowing the fats, oils, and grease to exit the plant.