What is the breakpoint range for secondary wastewater?

• A. 5-25 mg/L
• B. 0-10 mg/L
• C. 25-150 mg/L
• D. 150-300 mg/L

Answer: (C)

In secondary wastewater, the term breakpoint refers to the level of biodegradable material still present after secondary treatment and where removal becomes less rapid as treatment continues. The residual BOD that you typically see in secondary effluent falls roughly in the tens to low hundreds of mg/L range, and a general guideline often cited is about 25 to 150 mg/L. This captures plants that are functioning and handling the load but still have noticeable lingering biodegradable material. Values much lower (0–10 mg/L or 5–25 mg/L) would usually point to tertiary treatment or an unusually low-strength influent, while much higher (150–300 mg/L) would be more characteristic of influent or primary effluent rather than true secondary effluent.