What is the incremental annual BOD mass loading to a wastewater treatment plant when conventional plumbing fixtures are replaced with water-conserving fixtures?

What is the incremental annual BOD mass loading to a wastewater treatment plant when conventional plumbing fixtures are replaced with water-conserving fixtures?

• A. increases because the concentration will increase
• B. decreases because the volume and concentration will decrease
• C. no change because the concentration will increase
• D. decreases because the volume will decrease

Answer: (C)

BOD mass loading to a wastewater plant is determined by the product of the influent BOD concentration and the wastewater flow entering the plant over time. When conventional plumbing fixtures are replaced with water-conserving ones, the total flow into the sewer system drops. At the same time, with less dilution, the BOD concentration in the remaining wastewater tends to increase. If these two effects offset, the overall mass of BOD entering the plant per year stays roughly the same. So, the incremental annual BOD mass loading would not change significantly, which is why the reasoning points to a result where the concentration rises as volume falls, keeping the mass loading essentially constant. The other options imply a net change in mass loading by assuming a one-sided effect (only concentration or only volume changing in a way that increases or decreases mass), which doesn’t capture the balancing act between concentration and flow.

BOD mass loading to a wastewater plant is determined by the product of the influent BOD concentration and the wastewater flow entering the plant over time. When conventional plumbing fixtures are replaced with water-conserving ones, the total flow into the sewer system drops. At the same time, with less dilution, the BOD concentration in the remaining wastewater tends to increase. If these two effects offset, the overall mass of BOD entering the plant per year stays roughly the same. So, the incremental annual BOD mass loading would not change significantly, which is why the reasoning points to a result where the concentration rises as volume falls, keeping the mass loading essentially constant. The other options imply a net change in mass loading by assuming a one-sided effect (only concentration or only volume changing in a way that increases or decreases mass), which doesn’t capture the balancing act between concentration and flow.