In wastewater treatment, what does the term 'mixed liquor' refer to?

The term 'mixed liquor' in wastewater treatment specifically refers to a suspension of activated sludge and untreated or partially treated wastewater. This mixture is crucial in the biological treatment phase, particularly in the activated sludge process. The activated sludge contains microorganisms that are vital for the breakdown of organic matter in the wastewater.

During this process, the microorganisms consume the organic material present in the wastewater, which facilitates the removal of contaminants. The mixing of activated sludge with the incoming wastewater creates the 'mixed liquor' that allows for efficient biological treatment to occur. This helps to ensure that the system effectively reduces biochemical oxygen demand (BOD), removes pathogens, and clarifies the water before it progresses to further treatment or discharge.

The other possible choices do not accurately represent the term 'mixed liquor'. For instance, sludge with high water content refers to the thickened solids but does not involve the active biological process that occurs in mixed liquor. A mixture of treated effluent and solids would imply that the water has already undergone significant treatment, which does not encompass the definition of mixed liquor. Settled solids in a basin are the remnants after sedimentation has occurred, not the active mixture used in biological treatment.