When raw sewage is or may become septic, which treatment step may be necessary?

• A. Pre-aeration
• B. Filtration
• C. Chlorination
• D. Coagulation

Answer: (A)

Introducing air into the wastewater to promote aerobic microbial activity is the key idea. When sewage is septic, oxygen is scarce and anaerobic microbes dominate, which leads to odors and slower stabilization. Pre-aeration provides oxygen early, jump-starting aerobic digestion, reducing odors and hydrogen sulfide formation, and helping downstream treatment proceed more efficiently. It also aids mixing and helps break up solids for later processing. The other steps don’t directly address the lack of oxygen: filtration removes solids, coagulation targets particle flocculation, and chlorination disinfects but doesn’t stabilize the organic load.