Secondary treatment Secondary treatment Secondary treatment, also known as biological treatment, further reduces the amount of solids by helping bacteria and other micro- organisms consume the organic material in the sewage. Oxygen is critical to this treatment stage.
Air activated sludge and biological filters are two of many methods of secondary treatment.
It is important to remember that when metals and other toxics are removed from the effluent, they usually end up in the sludge, which then needs disposal. All else being equal, the cleaner the effluent, the dirtier the sludge in a conventional secondary treatment system. At the same time, even effluent from a plant using secondary treatment can be harmful to aquatic life because it can remain high in ammonia, which is toxic to fish and other marine organisms. This report was published in February1995. All rights reserved.