In the case of water for life (or urban public water supply), the raw water from a high quality water source (well water or well-protected water supply reservoir) needs to be sterilized as the finished water;
The original water from the general river or lake, first remove sediment and other turbidity impurities, then disinfect;
Pollution is more serious in the raw water, also need to remove the pollutants such as organic matter;
Raw water containing iron and manganese, such as some well water, needs to be removed from iron and manganese.
Domestic water can meet the water quality requirements of general industrial water, but industrial water needs to be further processed, such as softening and salt removal.
When the effluent discharge or reuse water quality requirements are low, only the coarse impurities and suspended matter (often referred to as primary treatment) are removed by screening and precipitation.
When it is required to remove organic matter, biological treatment (often referred to as secondary treatment) and disinfection are generally adopted after primary treatment.
After biological treatment of wastewater, the process referred to by tertiary treatment or deep processing, such as waste water discharged into the water to prevent eutrophication process conducted by the removal of nitrogen and phosphorus is belong to the tertiary treatment (see the physical and chemical treatment of water).
When waste water is used as a source of water, the water quality requirements of the finished product and the corresponding processing flow are determined according to its use.
In theory, modern water treatment technology can make any high-quality finished water from any inferior water.