The aerobic bio-treatment process with the activated sludge method is a complex system which has the properties of essential nonlinearity and strong disturbance to treat city sewage.

Triclosan has beenshown to interfere with the basic biological systems of a number of animals, including rats anda range of amphibians.

Methoxychlor is a DDT substitute and one of endocrine-disrupting chemicals which heavily pollutes the environment.