Raw Influent (Sewage) is the liquid waste from toilets,baths,showers,kitchens,sinks etc.Household waste that is disposed of via sewers.In many areas sewage also includes some liquid waste from industry and commerce.In the UK,the waste from toilets is termed foul waste,the waste from items such as basins,baths,kitchens is termed sullage water,and the industrial and commercial waste is termed trade waste.
The division of household water drains into greywater and blackwater is becoming more common in the developed world,with greywater being permitted to be used for watering plants or recycled for flushing toilets.A lot of sewage also includes some surface water from roofs or hard-standing areas.Municipal wastewater therefore includes residential,commercial,and industrial liquid waste discharges,and may include stormwater runoff.Sewage systems capable of handling stormwater are known as combined systems.Such systems are usually avoided since they complicate and thereby reduce the efficiency of sewage treatment plants owing to their seasonality.In addition,heavy storms may overwhelm the sewage treatment system,causing a spill or overflow.It is preferable to have a separate storm drain system for stormwater.
The construction of combined sewers is a less common practice in the United States and Canada than in the past and is no longer accepted within building regulations in the UK 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 UK.Overflows from foul sewers designed to relieve pressure from heavy rainfall are termed storm sewers or combined sewer overflows.
As rainfall runs over the surface of roofs and the ground,it may pick up 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 directly into waterways.Examples of treatment processes used for stormwater include sedimentation basins,wetlands,and vortex separators (to remove coarse solids).
The site where the process is conducted is called a sewage treatment plant.The flow scheme of a sewage treatment plant is generally the same for all countries:
Mechanical treatment;
Influx (Influent)
Removal of large objects
Removal of sand and grit
Pre-precipitation
Biological treatment;
Oxidation bed (oxidizing bed) or aeration system
Post precipitation
Effluent
Chemical treatment (this step is usually combined with settling and other processes to remove solids,such as filtration.The combination is referred to in the US as physical-chemical treatment.).
