The wastewater treatment (mainly including the urban sewage and some industrial wastewater) can be divided into 3 grades based on the treatment degree.

Primary treatment is to remove the solid containments with the suspended state in the wastewater. So, the physical treatment method is often adopted. After primary treatment, the removal rate of suspended solid is 70%-80%. The removal rate of BOD is only about 25%-40%. The purified degree of wastewater is not high.

The secondary treatment is to sharply remove the organic containments in the wastewater. Taking BOD as example, after the secondary treatment, the removal rate of BOD in the wastewater is 80%-90%. For example, BOD contents in the water after urban sewage treatment can be lower than 30mg/L. Various treatment units with aerobic treatment method can almost meet this requirement.

The tertiary treatment is to further remove the containment not removed in the secondary treatment, which includes the non-biodegradable organic matter, phosphorus, nitrogen and soluble inorganic matter in the microorganism.

The tertiary treatment is the synonym of advanced treatment. But, both are not completely same. The tertiary treatment is a or several treatment unit(s) added to remove some special containment such as phosphorus, nitrogen from the wastewater after secondary treatment. The advanced treatment is the treatment unit or system added after secondary treatment in order to focus on the recycling of wastewater. With big cost and complex management, the tertiary treatment can make the best of water resource. Few countries established some sewage tertiary treatment factory. 

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