Which treatment step comes after primary treatment in sewage processing?

• A. Tertiary treatment
• B. Disinfection
• C. Primary treatment again
• D. Secondary treatment

Answer: (D)

After primary treatment, the next step is secondary treatment. The primary stage mainly removes large solids by letting them settle out, but a lot of organic material and finer particles remain in the water. Secondary treatment uses microorganisms to biologically break down this remaining organic matter, often in aerated systems like activated sludge or biofilters. This biological process reduces the biochemical oxygen demand and further lowers suspended solids, making the wastewater much cleaner before any disinfection or advanced polishing steps. Disinfection and tertiary treatment occur later in the process, after secondary, to address pathogens and provide additional purification if needed.