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Wastewater is polluted water generated from rainwater runoff and human activities, also known as sewage. It can be categorized as domestic, industrial, or storm sewage. A sewerage system is a network of pipes and pumps designed to collect wastewater from a community and transport it to a treatment facility.12

There are three main levels of wastewater treatment:
* Primary treatment: Removes larger solids through screening, grit removal, and sedimentation.3
* Secondary treatment: Removes over 85% of suspended solids and biochemical oxygen demand (BOD).3
* Tertiary treatment: Advanced processes that can remove over 99% of impurities, producing high-quality effluent, and may include nutrient removal.34

Disinfection, often using chlorine or ultraviolet radiation, is the final step to destroy pathogens. Wastewater can also be reused for irrigation or other purposes after treatment. Alternative collection systems exist for areas where conventional gravity sewers are not feasible.356

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