Waste flowing at 15 percent of design flow through a 60-inch pipe interceptor may allow

• A. Photosynthesis to take place
• B. Combined sewers to relieve the waste load
• C. The waste to scour the collection system
• D. The waste to turn septic

Answer: (D)

Low flow in a large interceptor reduces the velocity of the wastewater, which allows solids to settle and the mixture to stay in the pipe longer. With less oxygen and more contact time, bacteria decompose the organic matter anaerobically, producing foul gases and odors and creating septic conditions inside the pipe. This is why a flow at only 15 percent of design can cause the waste to turn septic. The other options don’t fit because photosynthesis doesn’t occur in sewer pipes, low flow wouldn’t relieve the load in a controlled way, and low velocity isn’t what promotes scouring.