<P> At the end of the 19th century, since primary treatment still left odor problems, it was discovered that bad odors could be prevented by introducing oxygen into the decomposing sewage . This was the beginning of the biological aerobic and anaerobic treatments which are fundamental to waste water processes . </P> <P> As pollution of water bodies became a concern, cities attempted to treat the sewage before discharge . In the late 19th century some cities began to add chemical treatment and sedimentation systems to their sewers . In the United States, the first sewage treatment plant using chemical precipitation was built in Worcester, Massachusetts in 1890 . Most cities in the Western world added more expensive systems for sewage treatment in the early 20th century, after scientists at the University of Manchester discovered the sewage treatment process of activated sludge in 1912 . During the half - century around 1900, these public health interventions succeeded in drastically reducing the incidence of water - borne diseases among the urban population, and were an important cause in the increases of life expectancy experienced at the time . </P> <P> With the onset of the industrial revolution and related advances in technology, the flush toilet began to emerge into its modern form . It needs to be connected to a sewer system though . Where this is not feasible or desired, dry toilets are an alternative option . </P> <P> An ambitious engineering project to bring fresh water from Hertfordshire to London was undertaken by Hugh Myddleton who oversaw the construction of the New River between 1609 and 1613 . The New River Company became one of the largest private water companies of the time, supplying the City of London and other central areas . </P>

When did indoor plumbing become common in europe