After using water for domestic or industrial purpose, it is usually cleaned before it returns to the river or sea. In the 1980s it became clear that serious environmental problems were occurring due to pollution of river water. This was when large scale water treatment started to become the norm. Sewage treatment is the process that removes the majority of the contaminants from waste water, or sewage. In this process two products are produced: a liquid effluent suitable for disposal in the natural environment, and sludge. To be effective, sewage must be conveyed to a treatment plant by appropriate conduits and infrastructure and the process itself must be subject to regulation and controls. (Because the by-product are potentially harmful to humans.) Industrial wastewater often requires different and specialized treatment methods.
Sewage is the liquid waste from toilets, baths, showers, kitchens, etc., that is disposed of via sewers. In many areas, sewage also includes liquid waste from industry and commerce. In the UK, the waste from toilets is termed “blackwater” or “foul waste”, the waste from wash basins, baths, and kitchens is termed “greywater” or “sullage water”, and industrial and commercial waste is termed “trade waste.”
The division of household water into greywater and blackwater is becoming more common in the developed world. Greywater can be recycled for watering plants or flushing toilets. Blackwater contains pathogens which need to decompose before they can safely be disposed of. Much sewage also includes some surface water from roofs or hard-standing areas. The municipal wastewater that arrives at a treatment plant includes most kinds of wastewater: residential, commercial, and industrial liquid waste discharges, and extra runoff from storm water.
Sewage systems that transport liquid waste discharge and stormwater together to a common treatment facility are called “combined sewer systems.” The construction of combined sewers is a less common practice in the U.S. and Canada as in the past and is no longer accepted with building regulations in the U.K. and other European countries. Instead, liquid waste and stormwater are collected and conveyed in separate sewer systems, referred to as “sanitary sewers” and “storm sewers” in the U.S. and as “foul sewers” and “surface water sewers” in the U.K.. Overflows from foul sewers designed to relieve pressure from heavy rainfall are termed “storm sewers” or “combined sewer overflows.”
As rainwater runs over the surface of roofs and the ground, it transports various contaminants including soil particles (sediment), heavy metals, organic compounds, animal waste, and oil and grease. Some jurisdictions require stormwater to receive some level of treatment before being discharged to the environment. Examples of treatment processes used for stormwater include sedimentation basins, wetlands, and vortex separators (to remove coarse solids).
The conventional sewage treatment process typically involves the following three stages:
1. Primary treatment - to settle out solids
2. Secondary treatment - to remove the dissolved and emulsified components
3. Tertiary treatment - to make the effluent fit to be received in the environment