During the past ten years, industry has developed hundreds of new chemicals that are known to pass undegraded through conventional systems for sewage treatment; these substances constitute a serious pollution problem. They may be discharged directly into sewage systems as industrial waste, or after purchase by the commercial or domestic consumer.
Special attention has been given to detergents, which until recently were a serious threat to both underground and surface waters as they contained chemical substances that could not be broken down by biological means. More persistent classes of substances which appear in domestic sewage systems are pharmaceutical drugs, silver, mercury and other toxic metals from photographic processing, solvents, pesticides, dyes and other substances used in hobbies and small business practices.