18 Pumps, Valves & Ancillary Equipment Practice Questions & Answers

1. 1. A plant is selecting a centrifugal pump for raw primary sludge that carries rags and grit. Which impeller design fits this service, and why?

   • A.A closed impeller with tight wear-ring clearances for lift
   • B.An open, non-clog impeller with wide, unobstructed passages✓ Answer
   • C.Multistage closed impeller for greater developed head
   • D.A closed impeller with narrow passages for higher efficiency

   Solids-handling service calls for an open (non-clog) impeller: it has few vanes and wide, unobstructed passages, so rags and grit pass through instead of wrapping the vane inlets. Closed impellers reach higher efficiency and head, which is why they serve clear water, effluent and utility duty, but their narrow passages and tight wear-ring clearances blind off quickly on raw sludge.

   Source: CSUS/Office of Water Programs, Operation of Wastewater Treatment Plants, Vol. 1, 8th ed. — pumping chapter, open (non-clog) vs. enclosed impeller selection; WPI 2025 Need-to-Know Criteria, Equipment Evaluation area, bullet 'Pumps (centrifugal)'Report a problem with this question

2. 2. An influent pump lifting from a wet well begins to rattle as if gravel were passing through the casing, the unit vibrates, and the suction gauge shows an unusually high vacuum. What is happening?

   • A.Bearing failure from over-greasing the housing
   • B.Cavitation from low pressure at the impeller eye✓ Answer
   • C.Air binding from air trapped in the pump casing
   • D.Water hammer from a check valve slamming shut

   When suction lift or a restricted suction line drops the pressure at the impeller eye below the vapor pressure of the liquid, vapor bubbles form and then collapse violently as they reach the higher-pressure side of the impeller. That implosion is the marbles-and-gravel rattle, and over time it pits the impeller vanes and shortens bearing and seal life; a high suction vacuum reading is the confirming clue.

   Source: CSUS/Office of Water Programs, Operation of Wastewater Treatment Plants, Vol. 1, 8th ed. — cavitation and net positive suction head; WPI 2025 Need-to-Know Criteria, Equipment Evaluation area, 'Pumps'Report a problem with this question

3. 3. A pump is cavitating because the wet-well level has been drawn down. Which action should the operator take?

   • A.Increase pump speed to push the vapor bubbles through
   • B.Throttle the discharge valve and raise the wet-well level✓ Answer
   • C.Throttle the suction valve to steady flow into the pump
   • D.Close the discharge valve and let the pump re-prime

   Cavitation is corrected by restoring suction pressure or reducing flow, so the operator raises the level over the suction and throttles on the discharge side. Throttling the suction only lowers the pressure at the impeller eye further and makes the cavitation worse, and higher speed raises the required net positive suction head, which also worsens it.

   Source: CSUS/Office of Water Programs, Operation of Wastewater Treatment Plants, Vol. 1, 8th ed. — correcting cavitation (never throttle the suction); PA DEP Wastewater Operator Training Module 9, Pumps and HydraulicsReport a problem with this question

4. 4. An operator starts a centrifugal pump against a nearly closed discharge valve, is called away, and returns half an hour later to find the valve still closed and the casing hot. What should be done?

   • A.Vent the casing and restart to restore prime
   • B.Open the discharge valve fully right away
   • C.Stop the pump and let it cool before opening✓ Answer
   • D.Leave it running; shutoff head does no harm

   A centrifugal pump may be started against a closed or nearly closed discharge to limit the starting load, but running dead-headed converts all of the input horsepower into heat in the trapped liquid. The water can flash to steam and pressurize the casing, and the mechanical seal or packing is damaged first, so the unit is shut down and allowed to cool before the casing or valve is opened.

   Source: CSUS/Office of Water Programs, Operation of Wastewater Treatment Plants, Vol. 1, 8th ed. — centrifugal pump starting and dead-heading precautionsReport a problem with this question

5. 5. A progressing cavity pump moves thickened sludge to the dewatering building. Which device must be provided on its discharge piping?

   • A.A vacuum breaker on the suction side of the pump
   • B.An air-and-vacuum valve at the piping high point
   • C.A swing check valve on the discharge piping
   • D.A pressure relief valve on the discharge piping✓ Answer

   A positive displacement pump delivers a fixed volume per revolution regardless of discharge head, so if a downstream valve is closed the pressure climbs until the pump, the piping, or a seal fails. A relief valve set below the piping rating, returning flow to the suction or the tank, is the required protection; a centrifugal pump does not need one because it simply rides up to shutoff head.

   Source: CSUS/Office of Water Programs, Operation of Wastewater Treatment Plants, Vol. 1, 8th ed. — positive displacement pumps, relief valve requirement; WEF Wastewater Treatment Fundamentals 2, support systemsReport a problem with this question

6. 6. A wet well was pumped down too far and a progressing cavity pump ran dry for several minutes before it was stopped. What damage is most likely?

   • A.The pump lost prime and must be refilled by hand
   • B.The shaft sleeve scored where the packing rides
   • C.The wear rings galled and efficiency dropped
   • D.The elastomer stator overheated and was damaged✓ Answer

   In a progressing cavity pump the single-helix metal rotor turns inside a double-helix elastomer stator, and the pumped liquid is the only thing lubricating and cooling that interference fit. Running dry generates friction heat that burns and tears the stator within seconds to a minute, which is why these pumps are interlocked with a low-level cutout or a dry-run protection device.

   Source: CSUS/Office of Water Programs, Operation of Wastewater Treatment Plants, Vol. 1, 8th ed. — progressing cavity (screw) pumps, dry-running protectionReport a problem with this question

7. 7. A packed raw-sewage pump drips slowly and steadily from the stuffing box while it runs. What should the operator do?

   • A.Tighten the gland until the leakage stops entirely
   • B.Raise the seal-water pressure until the drip stops
   • C.Replace the packing, because any leak means failure
   • D.Leave it; the drip cools and lubricates the packing✓ Answer

   Packing is designed to leak a controlled amount, commonly on the order of 30 to 60 drops per minute, because that flow carries away friction heat and lubricates the shaft sleeve. Tightening the gland until the leak stops runs the packing dry, glazes and burns it, and scores the sleeve, so a steady slow drip is the correct condition to maintain.

   Source: CSUS/Office of Water Programs, Operation of Wastewater Treatment Plants, Vol. 1, 8th ed. — stuffing box packing adjustment and normal leakage rateReport a problem with this question

8. 8. A pump that met its rated flow last week now delivers noticeably less. The suction gauge shows a higher vacuum than normal while the discharge pressure is lower. What is the most likely cause?

   • A.A worn impeller with excessive ring clearance
   • B.A partly closed valve in the discharge piping
   • C.A partly plugged suction line or intake screen✓ Answer
   • D.Extra back-pressure from a fouled force main

   A restriction upstream of the impeller forces the pump to pull harder to draw the same liquid, so the suction vacuum rises while the reduced flow leaves less energy for discharge pressure. A closed discharge valve or a fouled force main would instead raise the discharge pressure, and a worn impeller would cut discharge pressure without deepening the suction vacuum.

   Source: CSUS/Office of Water Programs, Operation of Wastewater Treatment Plants, Vol. 1, 8th ed. — pump troubleshooting using suction and discharge gauge readingsReport a problem with this question

9. 9. Over three weeks of rounds, a pump's outboard bearing housing has run hotter each week and vibration readings are trending upward. What is the appropriate response?

   • A.Throttle the discharge to lower the load on the bearing
   • B.Take the unit out of service and check bearings and alignment✓ Answer
   • C.Add grease each round until the housing temperature drops
   • D.Run it to failure, since the alarms will shut the pump down

   Rising bearing temperature and a rising vibration trend are early indications of misalignment, imbalance, or a bearing beginning to fail, and they appear well before the unit seizes. Predictive maintenance means acting on the trend by switching to the standby unit and inspecting the bearing and coupling alignment; adding grease is not a fix, since over-greasing itself churns and overheats a bearing.

   Source: CSUS/Office of Water Programs, Operation of Wastewater Treatment Plants, Vol. 1, 8th ed. — preventive/predictive maintenance, bearing temperature and vibration monitoring; PA DEP Module 11, MaintenanceReport a problem with this question

10. 10. After a larger submersible mixer was installed in an anoxic zone, a visible vortex appears at the surface and nitrate removal has fallen off. Why?

    • A.The extra mixing energy is entraining oxygen into the zone✓ Answer
    • B.The larger unit has cut the detention time in the zone
    • C.The extra turbulence is stripping nitrate out of the liquid
    • D.The stronger flow is washing MLSS out of the anoxic zone

    An anoxic mixer has one job: keep the mixed liquor suspended and in contact with substrate without breaking the surface. Surface turbulence and vortexing entrain dissolved oxygen, and the facultative bacteria will use that oxygen in preference to nitrate, so denitrification stops until the mixer is resized, lowered, or reaimed away from the surface.

    Source: WEF Wastewater Treatment Fundamentals — mixing in anoxic/anaerobic zones (avoid oxygen entrainment); WPI 2025 Need-to-Know Criteria, Equipment Evaluation area, bullet 'Mixers'Report a problem with this question

11. 11. A hypochlorite metering pump's dial has stayed at 60% but the chlorine residual is drifting down. How does the operator confirm what the pump is actually delivering?

    • A.Compare discharge pressure with the injection pressure
    • B.Count strokes per minute times the rated stroke volume
    • C.Time a measured draw-down from the calibration column✓ Answer
    • D.Compare the dial setting with the pump's rated output

    The dial setting and the nameplate stroke volume are nominal values; the volume a metering pump really delivers changes with back-pressure, viscosity, valve wear, and gas in the liquid end. A draw-down calibration isolates the suction to a graduated column and times how long the pump takes to remove a measured volume, which is the only direct measurement of actual feed rate at that setting.

    Source: PA DEP Wastewater Operator Training Module 7, Chemical Feed — draw-down (calibration column) calibration of metering pumps; CSUS/OWP Operation of Wastewater Treatment Plants, Vol. 1, 8th ed., chemical feedersReport a problem with this question

12. 12. A sodium hypochlorite diaphragm metering pump is stroking normally, the suction line and foot valve are clear and the tank is full, yet almost no chemical reaches the injection point. What is the most likely cause?

    • A.Off-gassing has vapor-locked the pump's liquid end✓ Answer
    • B.The stroke setting has drifted below its usable range
    • C.The back-pressure valve is holding too little pressure
    • D.The stored hypochlorite has degraded to a weak strength

    Sodium hypochlorite decomposes and releases oxygen, and those bubbles collect in the pump head so the diaphragm compresses gas instead of moving liquid — the pump strokes but delivers nothing. The standard corrections are a degassing or priming valve, a flooded suction with a short suction line, and keeping the chemical cool and out of sunlight to slow decomposition.

    Source: PA DEP Wastewater Operator Training Module 7, Chemical Feed — vapor (gas) locking of hypochlorite metering pumps and degassing valvesReport a problem with this question

13. 13. A plant feeds calcium hypochlorite of 65% available chlorine at a dose of 8.0 mg/L into a flow of 0.85 MGD (3,220 m3/day). How much product must be fed per day?

    • A.57 lb/day (26 kg/day)
    • B.8.7 lb/day (4.0 kg/day)
    • C.87 lb/day (40 kg/day)✓ Answer
    • D.37 lb/day (17 kg/day)

    The pounds formula gives pounds of pure chemical: 8.0 mg/L x 0.85 MGD x 8.34 = 56.7 lb/day of available chlorine. Because the product is only 65% available chlorine, that result is divided by 0.65 to get 87 lb/day of product (metric: 8.0 mg/L x 3,220 m3/day / 1,000 = 25.8 kg, divided by 0.65 = 40 kg/day). Forgetting to divide by the purity is the classic error.

    Source: WPI Formula/Conversion Table supplied with the exam — Chemical Feed Rate, lb/day = dose mg/L x flow MGD x 8.34 / purity (decimal); kg/day = dose x m3/day / (purity x 1,000)Report a problem with this question

14. 14. A peristaltic (hose) pump feeds a small, off-gassing chemical stream. What is the scheduled preventive maintenance item for this pump?

    • A.Replacing the pump tube before it wears through✓ Answer
    • B.Repacking the stuffing box around the drive shaft
    • C.Replacing the mechanical seal behind the impeller
    • D.Regrinding the suction and discharge check valves

    A peristaltic pump has no seals, no impeller and no check valves: rollers squeeze the liquid along inside a flexible tube, which is what makes it self-priming and tolerant of gas bubbles and abrasives. The tube is the only wear part, so it is replaced on a schedule before it ruptures and releases chemical into the pump case.

    Source: CSUS/Office of Water Programs, Operation of Wastewater Treatment Plants, Vol. 1, 8th ed. — chemical feed pumps, peristaltic (tube) pump maintenance; WPI 2025 NTK, 'Pumps (peristaltic)'Report a problem with this question

15. 15. Which valve is best suited to isolate a line carrying thickened sludge, and why?

    • A.A butterfly valve, whose disc sits in the flow stream
    • B.A globe valve, whose seat turns flow through the body
    • C.A gate valve held part open to throttle the sludge
    • D.An eccentric plug valve, full port when fully open✓ Answer

    An eccentric plug valve is the wastewater and sludge workhorse because the bore is clear and full port when open, so solids and rags pass without lodging, and the plug seats tightly against grit. A butterfly disc stays in the flow path and collects rags, a globe valve turns the flow and traps solids, and a gate valve is an isolation valve only — throttling it erodes the disc and seat.

    Source: CSUS/Office of Water Programs, Operation of Wastewater Treatment Plants, Vol. 1, 8th ed. — valve types and service selection (plug valves for sludge); WPI 2025 NTK, ancillary equipment 'valves'Report a problem with this question

16. 16. Why is a rising-stem gate or globe valve normally left about one turn back from fully open?

    • A.Backseated hard, it can seize and appear to be closed✓ Answer
    • B.Backseated hard, its disc erodes in the flow stream
    • C.Backseated hard, it can slam shut if pressure drops
    • D.Backseated hard, it will leak past the stem packing

    Driven hard against its backseat stop, the stem can seize in the fully open position. The next operator turns the handwheel, finds no movement, and concludes the valve is closed — then opens a line or a pump that is in fact still under full flow and pressure, which has caused floods and injuries. Backing off about one turn, and exercising valves on a routine schedule, prevents that.

    Source: CSUS/Office of Water Programs, Operation of Wastewater Treatment Plants, Vol. 1, 8th ed. — valve operation and exercising (do not backseat gate/globe valves hard open)Report a problem with this question

17. 17. A dewatered cake conveyor jams. What must be done before anyone reaches in to clear the jam?

    • A.Lock and tag every energy source, then verify zero energy✓ Answer
    • B.Press the emergency stop button at the local control panel
    • C.Bump the drive in reverse until the jammed material frees
    • D.Pull the emergency stop cord and clear the jam right away

    An emergency stop is a control-circuit device, not an energy-isolating device, so a conveyor stopped that way can be restarted from another station, by an automatic control, or by the stored gravity energy of loaded belt. Lockout/tagout requires isolating every energy source, applying a lock and tag for each worker, and then verifying zero energy by attempting a start before any body part enters the machine.

    Source: 29 CFR 1910.147 (Control of Hazardous Energy, lockout/tagout) — energy isolation and verification of zero energy; WPI 2025 NTK, Safety area, lockout/tagout of conveyorsReport a problem with this question

18. 18. Why is a standby generator exercised under load rather than simply started and idled?

    • A.An idling run keeps the batteries and block heater charged
    • B.A loaded run proves it can carry the critical plant loads✓ Answer
    • C.An unloaded run is enough to prove the transfer switch works
    • D.A loaded run is only needed after the fuel supply is changed

    Only a loaded exercise proves the engine, the automatic transfer switch and the designated critical loads — headworks and influent pumping, aeration or blowers, return sludge, disinfection, and alarms and SCADA — will actually come up together during an outage. Light or no-load running also lets a diesel wet-stack with unburned fuel and carbon, which fouls the engine over time.

    Source: CSUS/Office of Water Programs, Operation of Wastewater Treatment Plants, Vol. 1, 8th ed. — standby power: exercising generators under load and critical load transfer; WPI 2025 NTK, Equipment area, 'Auxiliary power (generator)'Report a problem with this question

Practice questions written against the 2025 standardized Wastewater Treatment Operator Need-to-Know Criteria published by Water Professionals International (formerly the Association of Boards of Certification) and standard references from the CSUS Office of Water Programs and the Water Environment Federation. This site is not affiliated with or endorsed by WPI/ABC, WEF, or the US EPA. Operator certification is issued by your state's certifying authority, which sets plant classification tiers, operator grades, eligibility, and the passing standard — confirm those with your state before testing, and confirm which edition of the exam your program has adopted. Effluent limits, monitoring frequencies and reporting requirements come from an individual discharge permit and are revised over time, so no answer here should be relied on as a current regulatory or permit value; consult the permit in force for your facility. This bank covers the wastewater treatment exam only — drinking-water treatment, water distribution, and wastewater collection are separate certifications. About the Need-to-Know Criteria →