A downstream DO drop with no upstream drop indicates decreased plant removal efficiency of biochemical oxygen demand.

A downstream DO drop with no upstream drop indicates decreased plant removal efficiency of biochemical oxygen demand.

• A. False
• B. True
• C. Not enough data
• D. Both

Answer: (B)

A key idea here is that dissolved oxygen (DO) in a stream is consumed when microbes oxidize biodegradable organic matter (BOD). If the wastewater plant isn’t removing as much BOD in the treatment process, the effluent leaving the plant has a higher BOD load. Once this higher-BOD water enters the stream, downstream microbes use oxygen to break down that organic matter, pulling DO down as it flows. If upstream DO is not already dropping, the observed DO sag occurs downstream specifically because more BOD is entering the stream from the plant effluent. That pattern points to decreased BOD removal efficiency by the plant. While other factors can influence DO (temperature, flow, reaeration), the direct link between a downstream DO drop and higher downstream BOD due to poorer plant removal makes this interpretation the best fit.

A key idea here is that dissolved oxygen (DO) in a stream is consumed when microbes oxidize biodegradable organic matter (BOD). If the wastewater plant isn’t removing as much BOD in the treatment process, the effluent leaving the plant has a higher BOD load. Once this higher-BOD water enters the stream, downstream microbes use oxygen to break down that organic matter, pulling DO down as it flows. If upstream DO is not already dropping, the observed DO sag occurs downstream specifically because more BOD is entering the stream from the plant effluent. That pattern points to decreased BOD removal efficiency by the plant. While other factors can influence DO (temperature, flow, reaeration), the direct link between a downstream DO drop and higher downstream BOD due to poorer plant removal makes this interpretation the best fit.