Secondary Treatment
Overview Microorganisms Eat WHAT?! The Process References

What is it?

After primary treatment, the goal of secondary treatment is to remove the soluble Biochemical Oxygen Demand (BOD) and to provide additional removal of suspended solids from the wastewater.

Wastewater treatment is not just done with pumps, settling, and chlorine. The removal of the excess organic matter is done by soliciting the help of microorganisms. These microorganisms convert colloidal and carbonaceous organic matter into various gases and protoplasm. Protoplasm is organic matter that contributes to the BOD, and secondary treatment is not complete until the protoplasm is removed as well. However, cellular protoplasm has a slightly higher specific gravity than water and therefore settles to the bottom of tanks where it can be removed from the effluent.

Why is it done?

The Clean Water Act of 1972 requires all wastewater to go through secondary treatment. Oxygen demanding pollutants, such as BOD are included in that list of items to be removed. A secondary treatment process is used to get rid of a portion of that organic matter.