WASTEWATER
By Rick Lallish
The city of Anytown plans to add a new subdivision. The subdivision is expected to be 324 residential multifamily units with an estimated 1,296 people. There is no planned commercial or industrial development. The collection system will have one lift station comprising of 8-inch sewer mains throughout. What is the expected population equivalent impact on the BOD levels in pounds added to the influent flow?
A. 12.96 pounds
B. 220.3 pounds
C. 259.2 pounds
D. 7,623 pounds
Answer: B. This problem can be solved using the population equivalents listed below. To solve, you need to take the expected population increase (1,296 people) and multiply it by 0.17 pounds of BOD per capita per day. This will give you an increase of 220.3 pounds of BOD added to the influent BOD level. Be aware of superfluous information in the question. This is a pretty simple problem found on many certification exams but is an essential factor in wastewater treatment.
The population equivalents and pounds formula are:
- 0.17 pounds BOD per capita per day
- 0.20 pounds TSS per capita per day
DRINKING WATER
By Drew Hoelscher
When should a filter have the lowest headloss?
A. Several hours after the ripening stage
B. When the filter is experiencing air binding
C. Immediately after the filter backwash process
D. When a coagulant aid is overfed to enhance turbidity removal
Answer: C. Headloss should be at the lowest reading immediately after filter backwash because the frictional pressure from the accumulated particulate matter on the media has been removed. Headloss should always be monitored, as it is an indicator for when a filter should be taken out of service and backwashed. Filter breakthrough can occur and cause a turbidity spike if the head loss becomes too great.
About the author(s)
Rick Lallish is water pollution control program director and Drew Hoelscher is program director of drinking water operations at the Environmental Resources Training Center of Southern Illinois University Edwardsville.
