BOD values for the influent to a domestic wastewater plant are usually this fraction of the COD value?

BOD values for the influent to a domestic wastewater plant are usually this fraction of the COD value?

• A. 0.67
• B. 0.50
• C. 0.75
• D. 0.80

Answer: (A)

Understanding the relationship between BOD and COD helps explain the typical fraction. COD measures all oxidizable organics, including substances that microbes can’t readily biodegrade, while BOD5 measures only the portion that microbes can biologically oxidize in five days. In domestic influent, a portion of the organic matter isn’t readily biodegradable, so the BOD is lower than the COD. Empirically, BOD5 is usually about 0.5 to 0.7 times COD for municipal wastewater, with 0.67 (two-thirds) being a common representative value. So BOD values are typically around 67% of the COD. For example, a COD of 600 mg/L would yield a BOD around 400 mg/L. The other fractions shown (0.50, 0.75, 0.80) don’t align as closely with the typical range for domestic influent.

Understanding the relationship between BOD and COD helps explain the typical fraction. COD measures all oxidizable organics, including substances that microbes can’t readily biodegrade, while BOD5 measures only the portion that microbes can biologically oxidize in five days. In domestic influent, a portion of the organic matter isn’t readily biodegradable, so the BOD is lower than the COD. Empirically, BOD5 is usually about 0.5 to 0.7 times COD for municipal wastewater, with 0.67 (two-thirds) being a common representative value. So BOD values are typically around 67% of the COD. For example, a COD of 600 mg/L would yield a BOD around 400 mg/L. The other fractions shown (0.50, 0.75, 0.80) don’t align as closely with the typical range for domestic influent.