Wastewater treatment, also called wastewater treatment, the removal of impurities from wastewater or sewage before it reaches groundwater or natural water bodies such as rivers, lakes, estuaries and oceans. Since pure water is not found in nature (i.e. outside of chemical laboratories), any distinction between clean water and polluted water depends on the type and concentration of impurities present in the water as well as its intended use. In general terms, water is said to be polluted when it contains enough impurities to make it unsuitable for a particular use, such as drinking, swimming or fishing. Although water quality is affected by natural conditions, the word pollution usually implies human activity as a source of contamination. Water pollution, therefore, is mainly caused by the drainage of contaminated wastewater into surface or groundwater, and wastewater treatment is an important element of water pollution control.
Direct waste water discharge
Many ancient cities had drainage systems, but they were primarily intended to carry rainwater away from roofs and sidewalks. A notable example is the drainage system of ancient Rome. It included many surface conduits which were connected to a large vaulted canal called the Cloaca Maxima (“Great Sewer”), which carried the drainage water to the River Tiber. Built in stone and on a large scale, the Cloaca Maxima is one of the oldest extant monuments of Roman engineering.
There was little progress in urban drainage or sewerage during the Middle Ages. Vaults and cesspools were used, but most of the waste was simply dumped into the gutters to be discharged through the flood drains. Baths (toilets) were installed in homes in the early 19th century, but were usually connected to cesspools, not sewers. In densely populated areas, local conditions soon became intolerable because cesspools were rarely emptied and often overflowed. The threat to public health has become evident. In England in the mid-19th century, cholera epidemics were traced directly to well water reserves contaminated with human waste from private vaults and cesspools. It soon became necessary that all toilets in the larger cities be connected directly to the sewers. This transferred sewage from the ground near the houses to nearby water bodies. Thus a new problem emerged: the pollution of surface water.
Developments in wastewater treatment
It was said that “the solution to pollution is dilution”. When small amounts of sewage are discharged into a flowing body of water, a natural process of self-purification of the flow occurs. However, densely populated communities generate so large amounts of wastewater that dilution alone does not prevent pollution. This makes it necessary to treat or purify the waste water to some extent before disposal.
The construction of centralized purification plants began in the late 19th and early 20th centuries, mainly in the UK and the United States. Instead of discharging the sewage directly into a nearby body of water, it was first passed through a combination of physical, biological and chemical processes that removed some or most of the pollutants. Also starting in 1900, new wastewater collection systems were designed to separate rainwater from domestic wastewater, so that treatment plants did not overload during rainy periods.
After the mid-twentieth century, growing public concern for environmental quality has led to wider and stricter regulation of wastewater disposal practices. Higher levels of treatment were needed. For example, pre-treatment of industrial wastewater, with the aim of preventing toxic chemicals from interfering with the biological processes used in wastewater treatment plants, has often become a necessity. In fact, wastewater treatment technology has advanced to the point where it has become possible to remove virtually all pollutants from wastewater. This was so expensive, however, that such high levels of treatment usually weren’t justified.