Common practices of burying excreta in the ground (latrines), dumping it on open ground, or disposing of it as sewage - raw or treated - invariably pollute water sources, contaminate soil and infect food and other objects, which become the main vehicles of transfer of the pathogens back to humans. Thus, inadequate disposal or lack of sewage systems has an effect both at the individual and at the community level. With increasing mobility and international travel, people who visit unsanitary areas are also at risk and, if infected, may return to their homes as carries of the disease.
While modern sewage plants efficiently screen out bacteria and solid waste, many older facilities cannot break down certain chemicals and hormones in pharmaceuticals, including pain killers, caffeine, antibiotics and birth control pills, which, as they spread into wastewater, cause environmental harm. Viruses also pass through. Reports of measured pollutants in both the USA (the American Chemical Society meeting in 2000 included concern about perfumes) and in Europe's waterways (anti-depressants, seizure medication, cancer treatments and cholesterol-lowering compounds), also warn that certain species of fish are in jeopardy of extinction.
The Mediterranean Sea was described in 1993 as a diluted sewer. The Mediterranean Basin is home to 130 million people and is visited by 100 million tourists annually. Jointly these generate 2 billion tonnes of sewage, of which roughly 30% is treated. The remainder is discharged into the sea untreated and contaminates the area with little opportunity to escape. As a result, only 4% of shellfish from the area are considered fit for human consumption and periodic increases in algae are to be expected. In the Caribbean only about 10% of sewage is treated, although greater tidal activity in this area results in cleaner seas.
In 2001, human adenoviruses were detected at the mouths of four urban rivers in Southern California, pointing to urban runoff as a source of coastal viral contamination. Of the total 12 sites sampled, those contaminated with bacteria were not necessarily the same as those contaminated with viruses. Both bio-pollutants are regarded as an index for human faecal pollution and the possible presence of other human viruses.
In developing countries there are practically no treatment facilities available.
Sewage pollution is also common in groundwater in many developing countries. For example, groundwater in Merida, Mexico, has been severely affected by the influx of stormwater and sewage, and there is a risk that the contamination will spread to the wells which supply the city. Similar problems have occurred in Sri Lanka and many Indian cities, and are expected in Jakarta and Manila, which have 900,000 and 600,000 septic tanks respectively.
In developing countries, sewage creates a direct public health risk. In the rural areas and suburban slum districts, residents use open pit latrines. The carrying of wastewater in open drains is a characteristic feature of many developing countries. Except for a few exceptions, such as hospitals, educational institutions, government-reserved residential areas, some private housing developments and army barracks, most urban areas have no central sewerage system or sanitary sewage disposal system. Excreta, together with refuse, are indiscriminately discharged into open drains, and ultimately finds their way into storm water drains, streams and nearby rivers.