Free WW Collection Systems Questions and Answers — Questions and Answers
Question 1: What is the generally accepted minimum self-cleansing velocity in a sanitary sewer to prevent the deposition of solids?
- 0.5 fps
- 1.0 fps
- 2.0 fps (Correct answer)
- 5.0 fps
Correct answer: 2.0 fps
A minimum velocity of 2.0 feet per second (fps) is the industry standard for designing sanitary sewers. This velocity is typically sufficient to keep solids in suspension and prevent them from settling and accumulating in the pipe, which could lead to blockages and reduced capacity.
Question 2: Which piece of equipment is most effective for removing heavy grease blockages from a sewer line?
- Mechanical Rodder
- High-Pressure Water Jetter (Correct answer)
- Sewer Bucket Machine
- Hand Rods
Correct answer: High-Pressure Water Jetter
A high-pressure water jetter is the most effective tool for clearing grease blockages. The powerful stream of water can cut through the hardened grease, emulsify it, and flush it downstream. Mechanical rodders may poke a hole through the grease but often leave much of the blockage behind.
Question 3: In a wastewater lift station, what is the primary function of a check valve?
- To regulate the speed of the pump
- To prevent backflow into the wet well (Correct answer)
- To release air from the force main
- To measure the flow rate from the pump
Correct answer: To prevent backflow into the wet well
A check valve is a one-way valve installed on the discharge side of a pump. Its purpose is to prevent the water that has just been pumped up the force main from flowing back down into the wet well when the pump shuts off. This prevents the pump from having to re-pump the same water and protects it from backflow damage.
Question 4: The characteristic 'rotten egg' smell in a collection system is caused by the presence of which gas?
- Methane (CH₄)
- Ammonia (NH₃)
- Carbon Dioxide (CO₂)
- Hydrogen Sulfide (H₂S) (Correct answer)
Correct answer: Hydrogen Sulfide (H₂S)
Hydrogen sulfide (H₂S) is a gas produced by the anaerobic (without oxygen) decomposition of organic matter containing sulfur. It is well-known for its distinct and unpleasant rotten egg odor. In addition to being an odor nuisance, H₂S is toxic and highly corrosive to concrete and metal pipes.
Question 5: What is the most critical first step that must be taken before an employee enters a manhole?
- Set up traffic cones around the area
- Test the atmosphere with a calibrated gas meter (Correct answer)
- Notify the upstream industrial users
- Check the manhole lid for structural integrity
Correct answer: Test the atmosphere with a calibrated gas meter
Testing the atmosphere within the confined space is the most critical first step. Manholes can contain hazardous atmospheres, including oxygen deficiency, toxic gases (like H₂S), and explosive gases (like methane). A calibrated gas meter must be used to verify the space is safe for entry according to OSHA regulations.
Question 6: Unwanted groundwater entering the sewer system through cracks, faulty joints, or porous pipe walls is known as:
- Inflow
- Exfiltration
- Infiltration (Correct answer)
- Surcharge
Correct answer: Infiltration
Infiltration is the term for groundwater that enters a collection system through defects such as broken pipes, leaking joints, or cracked manholes. This is different from inflow, which is stormwater entering from direct connections like roof drains or catch basins. Both I&I (Infiltration and Inflow) increase flows to the treatment plant, raising operational costs and potentially causing overflows.
Question 7: Which type of sewer pipe material is a ceramic product known for its high resistance to chemical and acid corrosion, particularly from hydrogen sulfide?
- Polyvinyl Chloride (PVC)
- Ductile Iron Pipe (DIP)
- Vitrified Clay Pipe (VCP) (Correct answer)
- Reinforced Concrete Pipe (RCP)
Correct answer: Vitrified Clay Pipe (VCP)
Vitrified Clay Pipe (VCP) is a ceramic material that is fired at high temperatures, making it extremely hard, dense, and inert. This gives it superior resistance to a wide range of chemicals and acids found in wastewater, especially the sulfuric acid that forms from hydrogen sulfide gas. This corrosion resistance gives VCP a very long service life in sanitary sewer applications.
Free WW Collection Systems Questions and Answers
What is the generally accepted minimum self-cleansing velocity in a sanitary sewer to prevent the deposition of solids?