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How do primary secondary and tertiary wastewater treatments differ?

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Diagram of a water treatment plant showing the process from water inlet through mixing, coagulation, flocculation, settling, filtering, chemical treatment with chlorine, storage, and high-lift pumping to the distribution system. — water purification Water from inlets located in the water supply, such as a lake, is sent to be mixed, coagulated, and flocculated and is then sent to the waterworks for purification by filtering and chemical treatment. After being treated, the water is pumped into water mains for storage or distribution.

Wastewater treatment is categorized into three levels based on the degree of purification required to protect public health and water quality.1

Primary Treatment

Primary treatment serves as the initial step, removing approximately 60 percent of suspended solids and 35 percent of biochemical oxygen demand (BOD). It does not remove dissolved impurities.1

Secondary Treatment

Secondary treatment focuses on removing soluble organic matter and additional suspended solids. This is typically achieved through biological processes where microbes consume organic impurities, converting them into carbon dioxide, water, and energy. This level removes over 85 percent of both suspended solids and BOD and is a standard requirement in many developed countries.21

Tertiary Treatment

Tertiary (or advanced) treatment is used when higher purity is required, such as removing more than 99 percent of impurities or reducing plant nutrients like phosphates and nitrates. These processes, which can include chemical precipitation or nitrification-denitrification, are often expensive and implemented under special circumstances.31

Following these stages, disinfection is the final step to destroy remaining pathogens.1