A ping-pong ball travels from manhole A to manhole B 200 feet away in 1.5 minutes. The sewer line is a 6-inch sewer main. The line is running full. What is the velocity in this sewer main? Bonus: Is it within recommended speed?
The answer to this question is C, 2.22 feet per second. This can be solved using the Velocity formula below. The velocity of flow in collection systems is an important fact to know. If the flow is too slow (below 1.5 feet per second) the solids (organic and inorganic) may settle out and cause septicity or hydrogen sulfide conditions. If the velocity is too quick, erosion of the pipes may occur over time. To solve this particular question, you need to divide the distance (200 feet) by the time in takes to go from manhole A to manhole B. It is recommended to convert this into seconds 1.5 minutes = 90 seconds). This will give you a result of 2.22 feet per second which is slightly above the recommended velocity of 2 feet per second. Also, the size of the pipe in this case is not relevant and is included to attempt to complicate the problem.
An operator is instructed to prepare a total coliform analysis for process control. What temperature should the incubator be set at?
The correct answer is C, 35 degrees C (plus or minus 0.5 degrees). Collecting samples for total coliform bacteria analysis is common for water operators. Total coliform bacteria are used as an indicator species to help determine the quality of the water in the distribution system. In addition to compliance related samples, total coliform bacteria can be analyzed for process control related instances and operators should be aware of the incubation process. In short, the sample is prepared and incubated at 35 degrees C for approximately 24 hours. When coliform bacteria are present, they will ferment the added sugars and cause the sample to appear more yellow in color.
