18 Solids: Digestion, Dewatering & Disposal Practice Questions & Answers

1. 1. An operator's log for a dewatering unit records, for each of eight loads per shift, a fill step, a press step, and an open-and-drop-cake step. Which unit is being described?

   • A.Solid-bowl decanter centrifuge
   • B.Belt filter press
   • C.Plate and frame filter press✓ Answer
   • D.Screw press

   The plate and frame press is a batch device: each cycle fills the chambers with conditioned sludge through the center feed line, presses, and then the plate stack is opened and the cake released before the next load can begin. The belt press, screw press and solid-bowl decanter all run continuously, with uninterrupted feed and uninterrupted cake and filtrate discharge, so none of them has a discrete open-and-empty step.

   Source: WPI 2025 Need-to-Know Criteria, Wastewater Treatment Class II — Equipment: Solids treatment equipment (plate and frame press); WEF Wastewater Treatment Fundamentals 2, Solids HandlingReport a problem with this question

2. 2. Good floc is visible leaving the polymer mixer, but the sludge arriving at the wedge zone of a belt filter press is much wetter than normal. What should the operator check FIRST?

   • A.Plugged washwater spray nozzles that have blinded the drainage belt✓ Answer
   • B.A belt travel speed set much slower than the current sludge feed rate
   • C.A polymer metering pump running above its calibrated stroke setting
   • D.A sludge feed rate that has fallen well below the press design range

   Floc quality upstream is good, so conditioning is not the problem — the water simply is not leaving through the belt. A blinded belt, most often caused by plugged high-pressure washwater nozzles, closes the drainage openings so free water rides through the gravity zone into the pressure section. Slower belt speed and reduced feed both increase drainage time and would produce a drier, not wetter, product.

   Source: CSUS Operation of Wastewater Treatment Plants Vol. 1, 8th ed., belt filter press operation and troubleshooting; WPI 2025 NTK Class II — Equipment: belt pressReport a problem with this question

3. 3. An aerobic digester is being re-equipped with a diffused aeration system. Why are coarse-bubble diffusers normally selected over fine-bubble diffusers for this service?

   • A.Coarse bubbles hold digester pH up by stripping less alkalinity
   • B.Thick digester sludge plugs fine pores, and coarse units also mix✓ Answer
   • C.Coarse bubbles are needed because sludge foams under fine bubbles
   • D.Oxygen transfer per volume of air is better with coarse bubbles

   Digester sludge is far thicker than mixed liquor and will foul and plug fine-pore media, and the aeration system in an aerobic digester must supply mixing as well as oxygen. Coarse-bubble diffusers resist plugging and release the larger bubbles that deliver the mixing energy needed to keep the tank homogeneous. Fine-bubble units actually transfer oxygen more efficiently, which is why they belong in aeration basins rather than digesters.

   Source: WPI 2025 NTK Class II — Equipment: Aerobic digesters; WEF Wastewater Treatment Fundamentals 2, Solids Handling & Support Systems (digester aeration and mixing)Report a problem with this question

4. 4. Each day an operator shuts the air off to an aerobic digester for several hours, then draws liquid off the surface and sends it to the headworks. What is that step accomplishing?

   • A.Letting solids settle so supernatant is drawn, thickening the batch✓ Answer
   • B.Resting the blowers so their motors can cool between running periods
   • C.Working grit down to the floor where it can be pumped to the drain
   • D.Stripping nitrogen gas so the digester contents will stop floating

   An aerobic digester thickens its own contents by decanting: with mixing and aeration off, the solids settle and the clarified supernatant above them is withdrawn, leaving a more concentrated sludge in the tank and freeing volume for the next feed. The decant is a sidestream, so the load it carries returns to the head of the plant rather than leaving the site.

   Source: WPI 2025 NTK Class II — Equipment: Aerobic digesters (decant/supernatant); CSUS Operation of Wastewater Treatment Plants Vol. 1, 8th ed.Report a problem with this question

5. 5. On the plates of a plate and frame filter press, what is the function of the raised areas known as bosses?

   • A.They hold the filter cloth flat so cake releases cleanly from the plate
   • B.They seal each plate face so conditioned sludge cannot escape sideways
   • C.They carry filtrate out of each chamber to the filtrate collection line
   • D.They stiffen the plates so the stack resists deformation under pressure✓ Answer

   Bosses are reinforcing projections cast into the plate face; their job is structural, adding rigidity so the plates do not deform when the stack is squeezed at high pressure. Filtrate is carried by the plate's drainage channels and ports, the cloth is retained by its own attachment method, and sealing is done by the machined plate faces and gaskets.

   Source: WPI 2025 NTK Class II — Equipment: Solids treatment equipment (plate and frame press); WEF Wastewater Treatment Fundamentals 2, Solids HandlingReport a problem with this question

6. 6. An operator describes the plant's dewatering centrifuge as a horizontal bowl with an internal screw that moves settled solids toward one end while liquid overflows the other. Which machine is this?

   • A.Perforated-bowl centrifuge
   • B.Solid-bowl decanter centrifuge✓ Answer
   • C.Basket centrifuge
   • D.Disc-nozzle centrifuge

   The solid-bowl decanter turns on a horizontal axis: solids thrown to the bowl wall are scraped up and conveyed by the internal scroll to the tapered discharge end, while clarified centrate overflows weirs at the opposite end, so the machine dewaters continuously. Basket and disc-nozzle centrifuges turn on a vertical axis, and the basket unit accumulates solids inside the bowl and must be stopped to unload them.

   Source: WPI 2025 NTK Class II — Treatment Process: Dewatering (centrifuges); WEF Wastewater Treatment Fundamentals 2, Solids Handling (centrifuge types)Report a problem with this question

7. 7. Digested sludge is now being decanted to a much thicker consistency, and the existing centrifugal sludge pump has lost most of its capacity. Which pump type BEST suits the thicker sludge?

   • A.A larger centrifugal pump, run at a higher impeller speed
   • B.A self-priming centrifugal pump, fitted with a larger suction line
   • C.A centrifugal pump, fitted with a mechanical seal instead of packing
   • D.A positive displacement pump, whose output resists changes in head✓ Answer

   A centrifugal pump depends on the fluid flowing freely through its impeller, so capacity falls off sharply as sludge thickens and viscosity and friction losses rise. Positive displacement pumps — progressive cavity, plunger, diaphragm, peristaltic — move a fixed volume per revolution or stroke, so their output stays nearly constant against changing head and viscosity, which is why they are the standard choice for thick and variable sludge.

   Source: WPI 2025 NTK Class II — Equipment: Pumps (centrifugal, positive displacement); CSUS Operation of Wastewater Treatment Plants Vol. 1, 8th ed.Report a problem with this question

8. 8. Volatile solids in an aerobic digester continue to fall for weeks after the last feed sludge was added. Which mechanism accounts for that continued reduction?

   • A.Residual conditioning polymer breaks down and releases its bound water
   • B.Nitrifiers oxidize ammonia to nitrate, which removes volatile solids
   • C.Organisms enter endogenous respiration and consume their own cell mass✓ Answer
   • D.Inert grit keeps settling out and lowers the volatile fraction present

   Once the external biodegradable food is exhausted, the organisms respire their own protoplasm; cells lyse and release degradable organics that the surviving organisms then consume, so the mass of volatile organic solids keeps declining and ends up as carbon dioxide, water and non-degradable residue. Nitrification only changes the form of nitrogen, and settling grit shifts the volatile ratio without destroying organic mass.

   Source: WPI 2025 NTK Class II — Treatment Process: Solids treatment processes (aerobic digestion); CSUS Vol. 1, 8th ed., sludge stabilization by endogenous respirationReport a problem with this question

9. 9. Over several weeks an aerobic digester's pH has drifted downward. Dissolved oxygen is high, decant ammonia is falling and decant nitrate is rising. What is the MOST likely cause?

   • A.Conditioning polymer carried in with the feed sludge is acidifying it
   • B.Decanting is removing alkalinity faster than the feed sludge adds it
   • C.Nitrification of ammonia is consuming alkalinity and lowering the pH✓ Answer
   • D.Volatile acids are accumulating because the contents have gone septic

   Nitrifying organisms destroy alkalinity as they oxidize ammonia to nitrate, and once the digester's buffering capacity is drawn down the pH begins to fall. The rising nitrate together with falling ammonia in the decant is the signature of active nitrification. Accumulating volatile acids would depress pH only in an oxygen-starved, septic sludge, which the high dissolved oxygen reading rules out.

   Source: WPI 2025 NTK Class II — Treatment Process: Aerobic digestion (process evaluation and adjustment); CSUS Vol. 1, 8th ed., alkalinity destruction by nitrificationReport a problem with this question

10. 10. Cake off a belt filter press is wet, and the filtrate returning to the headworks is visibly cloudy with solids. Which conditioning problem BEST fits both observations?

    • A.Polymer is anionic, and the sludge particles carry a positive charge
    • B.Polymer is under-dosed, so floc is weak and solids pass the belt✓ Answer
    • C.Polymer is over-dosed, so the floc has become too large and stiff
    • D.Polymer solution is over-aged, so its viscosity has become too high

    Conditioning polymer bridges fine particles into flocs large enough to be retained by the belt and to release their free water. When the dose falls short of what the solids demand, the floc is weak, water is not released in the gravity zone and unbound fines wash straight through into the filtrate, so cake dryness and solids capture fall together. Biological sludge particles carry a negative charge, so a cationic polymer is the normal choice.

    Source: WPI 2025 NTK Class II — Treatment Process: Dewatering and chemical dosing (coagulation/flocculation); WEF Wastewater Treatment Fundamentals 2, sludge conditioningReport a problem with this question

11. 11. A plant increases dewatering from two days a week to five. On the new dewatering days, ammonia in the aeration basin effluent rises. What is the BEST explanation?

    • A.Filtrate returned to the headworks adds an ammonia and solids load✓ Answer
    • B.Hauling more cake has lowered the aeration basin's mixed liquor level
    • C.Polymer in the filtrate is toxic to the plant's nitrifying organisms
    • D.Running the press longer has lowered the digester's detention time

    Filtrate, centrate, drying-bed underdrain and digester supernatant all return to the head of the plant, and they carry a concentrated load of suspended solids, BOD and especially the ammonia released as cells break down during digestion. Dewatering more often applies that recycle load to the liquid train more often, and the extra ammonia shows up in the effluent until the nitrifying population grows enough to carry it.

    Source: WPI 2025 NTK Class II — Treatment Process: Dewatering (sidestream return to plant); CSUS Operation of Wastewater Treatment Plants Vol. 1, 8th ed.Report a problem with this question

12. 12. Neighbours complain of odour whenever sludge is placed on the plant's open sand drying beds. Which operating practice MOST directly reduces the nuisance?

    • A.Placing thicker layers of sludge so less surface area is exposed
    • B.Adding conditioning polymer to the sludge just before it is placed
    • C.Flooding the beds with plant water to keep a seal over the sludge
    • D.Placing only well-digested sludge, and covering or enclosing the beds✓ Answer

    Open drying beds expose sludge to the air for days or weeks, so any readily biodegradable organic matter left in it will go septic and release odour. Stabilizing the sludge in the digester before it is placed removes most of that odour potential, and covering or enclosing the bed contains what remains. Thicker placements dry more slowly and turn septic more readily, and flooding or polymer addresses neither the odour source nor its escape.

    Source: WPI 2025 NTK Class II — Treatment Process: Dewatering (drying beds); CSUS Operation of Wastewater Treatment Plants Vol. 1, 8th ed., sludge drying bedsReport a problem with this question

13. 13. A landfill rejects a load of sludge cake because free liquid drains out of it during unloading. What does the plant need to change in its solids train?

    • A.Improve dewatering so that the cake holds no drainable free water✓ Answer
    • B.Increase digester detention so that more volatile solids are destroyed
    • C.Add lime to the cake so that its pH rises above the landfill's limit
    • D.Increase the polymer dose so that the cake is chemically stabilized

    A landfill accepts sludge as a solid waste and screens it for drainable free liquid, so it is the cake's water content — not its degree of stabilization or its pH — that decides acceptance. Dewatering is the unit process that removes that water; digestion stabilizes the solids and reduces odour but does not by itself produce a cake dry enough to pass a free-liquids check, and polymer only conditions the sludge for the dewatering device.

    Source: WPI 2025 NTK Class II — Treatment Process: Solids disposal (landfill); 40 CFR 258.28 free-liquids restriction for municipal solid waste landfillsReport a problem with this question

14. 14. A belt filter press discharges 12,000 lb (5,440 kg) of cake per day at 22% total solids. How many pounds (kilograms) of dry solids leave the plant each day?

    • A.264 lb/day (120 kg/day)
    • B.54,500 lb/day (24,730 kg/day)
    • C.2,640 lb/day (1,197 kg/day)✓ Answer
    • D.9,360 lb/day (4,243 kg/day)

    Dry solids equal the wet cake weight multiplied by the total solids expressed as a decimal: 12,000 lb x 0.22 = 2,640 lb/day, and independently 5,440 kg x 0.22 = 1,197 kg/day. The 9,360 lb figure is the water in the cake rather than the solids, and dividing by 0.22 instead of multiplying inflates the result more than twentyfold — the classic wet-tonnage versus dry-tonnage error.

    Source: WPI Wastewater Formula/Conversion Table — Total Solids, % (dry solids = wet weight x solids fraction); WPI 2025 NTK Class II calculation itemsReport a problem with this question

15. 15. A technician dries a sludge sample to constant weight at 103-105 C, weighs it, then ignites that residue in a muffle furnace and weighs it again. The weight lost on ignition represents which fraction?

    • A.The moisture remaining after the drying step
    • B.The total solids present in the original sample
    • C.The fixed (inert) solids in the sample
    • D.The volatile (organic) solids in the sample✓ Answer

    Drying at 103-105 C drives off water and leaves the total solids behind; igniting that dried residue burns off the organic matter, so the weight lost on ignition is the volatile solids and the ash remaining is the fixed or inert solids. This is exactly why the supplied formula reports volatile solids as the difference between dry solids and fixed solids, and it is the measurement that feeds volatile solids reduction on the digester.

    Source: WPI Wastewater Formula/Conversion Table — Total Solids, % and Volatile Solids, %; Standard Methods 2540 E/G (fixed and volatile solids ignited at 550 C)Report a problem with this question

16. 16. Feed sludge to an aerobic digester tests 72% volatile solids and the digested sludge tests 58% volatile solids, both as a percentage of total solids. Using the supplied formula, what volatile solids reduction has been achieved?

    • A.19%
    • B.46%✓ Answer
    • C.24%
    • D.14%

    The supplied formula is (VS in - VS out) divided by (VS in - (VS in x VS out)), times 100, and its footnote requires the values in decimal form: (0.72 - 0.58) / (0.72 - 0.4176) = 0.14 / 0.3024 = 0.46, or 46%. Treating the problem as a simple percent removal, (72 - 58) / 72, gives 19% and badly understates the destruction because it ignores the concentrating effect of the inert solids that remain behind.

    Source: WPI Wastewater Formula/Conversion Table — Reduction of Volatile Solids, % (Van Kleeck), with the footnote requiring all In and Out values in decimal formReport a problem with this question

17. 17. A digester has been drained and must be entered for cleaning. In what order MUST the atmosphere be tested before anyone enters?

    • A.Toxic gases, then oxygen, then flammability
    • B.Oxygen, then flammability, then toxic gases✓ Answer
    • C.Oxygen, then toxic gases, then flammability
    • D.Flammability, then oxygen, then toxic gases

    Oxygen is tested first because combustible-gas sensors need a known oxygen level to give a valid reading, and both oxygen deficiency and enrichment are immediately dangerous on their own. Flammability comes next because an explosive atmosphere can kill everyone present in an instant, and toxic gases such as hydrogen sulfide are tested last. Testing is done from outside the space, with continuous ventilation, an attendant and retrieval equipment in place.

    Source: WPI 2025 NTK Class II — Security, Safety and Administrative Procedures: confined space entry; 29 CFR 1910.146(d)(5)(iii) order of atmospheric testingReport a problem with this question

18. 18. Digester gas is a useful plant fuel, yet operators are trained to keep air out of the gas piping system. Why is that so important?

    • A.Methane mixed with air can reach an explosive range and ignite✓ Answer
    • B.Air cools the gas and lets condensate freeze inside the gas line
    • C.Air oxidizes the methane and lowers the gas's heating value
    • D.Air raises the moisture load and corrodes the gas piping through

    Digester gas is valuable because of its methane content, but methane burns or explodes only when it is mixed with air within its flammable range; held as nearly pure gas under slight positive pressure, there is no oxygen present for it to burn with. That is why gas systems are kept above atmospheric pressure and why the greatest explosion risk arises when air is drawn in through a leak, during draining, or while a unit is being opened for maintenance.

    Source: WPI 2025 NTK Class II — Security, Safety and Administrative Procedures: safety procedures, chemical handling and emergency response; CSUS Operation of Wastewater Treatment Plants Vol. 2, 8th ed., digester gas safetyReport 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 →