Define primary treatment.

• A. The complete removal of all solids from the wastewater.
• B. Thermal treatment to sterilize the wastewater.
• C. Chemical coagulation to remove dissolved minerals.
• D. Clarification by solid and liquid separation that removes a substantial amount of suspended and floating matter.

Answer: (D)

Primary treatment is a physical process that uses gravity to separate solids from wastewater. In a primary clarifier, wastewater sits long enough for solids to settle to the bottom and for floatable materials to be skimmed from the surface. This clarification by solid and liquid separation removes a substantial amount of suspended and floating matter, reducing the contaminant load before the water moves on to secondary treatment. It isn’t about removing every solid, nor does it involve heating to sterilize or chemically treating dissolved minerals.