What is the difference between primary and secondary treatment in septic systems?

• A. Primary Is Sludge Removal; Secondary Is Taste Improvement.
• B. Primary and Secondary Are the Same.
• C. Secondary Treatment Occurs in the Soil or Additional Treatment Units to Further Reduce Pollutants.
• D. Primary Treatment Occurs in the Septic Tank Separating Solids; Secondary Treatment Occurs in the Soil or Additional Treatment Units to Further Reduce Pollutants.

Answer: (D)

In septic systems, the first stage is primary treatment, which happens in the septic tank. Wastewater enters the tank and solids settle to the bottom as sludge while fats and oils rise to the top as scum. The liquid that leaves the tank is clarified and has a reduced solid load, but still contains dissolved and suspended matter.

Secondary treatment then takes place after the tank, in the soil (or in an additional treatment unit). Here, microorganisms in the soil biologically break down remaining organic material, and the soil provides physical filtration and adsorption to remove more pollutants. This stage further reduces contaminants such as nutrients and pathogens before the effluent reaches groundwater or a drain field.

So, primary treatment is solids separation in the septic tank, and secondary treatment is the further purification that occurs in the soil or in a treatment unit to reduce pollutants.