18 Tertiary Filtration & Disinfection Practice Questions & Answers

1. 1. A dual-media tertiary filter polishing secondary effluent has reached its terminal head loss, but filtrate turbidity is still low. What should the operator do?

   • A.Backwash the filter, because terminal head loss ends the run✓ Answer
   • B.Increase the filtration rate to force flow through the bed
   • C.Keep filtering, because turbidity has not yet broken through
   • D.Reduce the coagulant feed to lower head loss and keep filtering

   A filter run ends on whichever criterion arrives first: terminal head loss, turbidity breakthrough, or elapsed time. Once terminal head loss is reached, continued filtration pushes captured solids deeper and can produce breakthrough or negative head within the bed, so the filter is removed from service and backwashed even though filtrate quality still looks acceptable.

   Source: WPI 2025 Need-to-Know Criteria, Wastewater Treatment Class II — Treatment Process Evaluation and Adjustment: tertiary treatment, media filtration; CSUS/OWP Operation of Wastewater Treatment Plants, Vol. 1, 8th ed.Report a problem with this question

2. 2. Inspection of a tertiary sand filter finds hard agglomerated balls of media and solids in the upper bed; filter runs have shortened and filtrate turbidity is rising. The most likely cause is:

   • A.Filtration rate is too low, letting solids settle on the surface
   • B.Backwashing has been inadequate, allowing mudballs to form✓ Answer
   • C.Backwash rate is too high, over-expanding and mixing the bed
   • D.Media is too coarse, letting solids pass deep into the bed

   Mudballs are agglomerations of media grains and captured solids that form when backwash energy fails to break up and carry away the accumulated floc. They occupy bed volume, force flow into channels, and cause cracking, short runs and breakthrough. Adequate backwash rate combined with air scour or surface wash prevents them; badly mudballed media must be cleaned or replaced.

   Source: WPI 2025 NTK, Wastewater Treatment Class II — Equipment Evaluation, Maintenance and/or Operation: tertiary filters (sand, anthracite, disc); CSUS/OWP Operation of Wastewater Treatment Plants, Vol. 1, 8th ed.Report a problem with this question

3. 3. A single-media sand filter is rebuilt as a dual-media bed with anthracite over sand. At the same loading rate, compared with sand alone, this arrangement:

   • A.Stratifies with sand on top, so fine media traps solids first
   • B.Adsorbs dissolved organics, so effluent BOD falls sharply
   • C.Filters in depth, so runs are longer at lower head loss✓ Answer
   • D.Strains at the surface, so filtrate turbidity drops faster

   Anthracite is lighter and coarser than sand, so after backwash it restratifies above the finer, denser sand, giving a coarse-to-fine gradation in the direction of flow. Solids penetrate the anthracite and are stored throughout the depth of the bed instead of blinding the surface, which lengthens the run and slows head loss build-up at the same filtration rate.

   Source: WPI 2025 NTK, Wastewater Treatment Class II — Treatment Process: tertiary media filtration (sand, anthracite); WEF Wastewater Treatment Fundamentals 1 — Liquid TreatmentReport a problem with this question

4. 4. Three tertiary filters are backwashed one after another during peak plant flow. Within the hour, the primary clarifiers begin losing solids over the weirs. The most likely reason is:

   • A.Backwash air scour stripped dissolved oxygen from the return
   • B.Backwash water carried chlorine that killed the primary biomass
   • C.Backwash removed the biological film the filters need to work
   • D.Returned backwash water hydraulically overloaded the headworks✓ Answer

   Spent backwash water is returned to the head of the plant. Sending several backwashes back at once during peak flow spikes the hydraulic loading, raising surface overflow and weir overflow rates in the clarifiers so that settled solids are swept out. Backwashes should be staggered, equalized or decanted and scheduled during low-flow periods.

   Source: WPI 2025 NTK, Wastewater Treatment Class II — Treatment Process: flow equalization and tertiary filtration; CSUS/OWP Operation of Wastewater Treatment Plants, Vol. 1, 8th ed.Report a problem with this question

5. 5. Nitrification is lost and effluent ammonia climbs. With the chlorine feed rate unchanged, total residual leaving the contact tank falls sharply. The best explanation is:

   • A.Ammonia lowered chlorine demand, so the analyzer must be faulty
   • B.Ammonia raised chlorine demand, leaving less dose as residual✓ Answer
   • C.Ammonia raised effluent pH, which destroyed the chlorine residual
   • D.Ammonia converted combined residual into stronger free residual

   Ammonia reacts with hypochlorous acid to form chloramines and, as the ratio increases, is oxidized outright, so it is a major exerter of chlorine demand in secondary effluent. Because dose equals demand plus residual, a rise in demand at a fixed dose leaves less measurable residual; the operator must raise the dose or restore nitrification to hold the target residual.

   Source: WPI 2025 NTK, Wastewater Treatment Class II — Treatment Process: disinfection (chlorine, hypochlorite, chloramine); CSUS/OWP Operation of Wastewater Treatment Plants, Vol. 1, 8th ed.Report a problem with this question

6. 6. An operator must achieve a specified total chlorine residual at the end of the contact tank, and the supplied exam formula table does not list the needed relationship. Which relationship applies?

   • A.Dose equals demand plus residual✓ Answer
   • B.Demand equals dose plus residual
   • C.Dose equals demand minus residual
   • D.Residual equals dose plus demand

   Chlorine added to wastewater is first consumed by the demand exerted by ammonia, organics, sulfides, nitrite, ferrous iron and manganese; only what is left over appears as a measurable residual. Hence dose = demand + residual, and rearranged, residual = dose − demand. This relationship is not printed on the supplied formula table, so it must be memorized.

   Source: WPI 2025 NTK, Wastewater Treatment Class II — Treatment Process: disinfection process control (chlorine dose, demand and residual); CSUS/OWP Operation of Wastewater Treatment Plants, Vol. 1, 8th ed.Report a problem with this question

7. 7. Chlorine dose is raised in steps on an ammonia-bearing secondary effluent. Residual rises, then falls to a minimum, then rises again as free residual. That dip and turn-up mark:

   • A.The breakpoint, where chlorine oxidizes the ammonia present✓ Answer
   • B.The point where the demand of sulfides is finally satisfied
   • C.The onset of dechlorination by sulfite carried in the effluent
   • D.The point where all chloramines convert to monochloramine

   As dose increases, chlorine first combines with ammonia to form chloramines, so combined residual rises. Past a critical chlorine-to-ammonia ratio the added chlorine destroys those chloramines and oxidizes the ammonia, so measured residual drops to a minimum — the breakpoint. Beyond it, additional chlorine persists as free residual, which is why breakpoint chlorination also removes nitrogen.

   Source: WPI 2025 NTK, Wastewater Treatment Class II — Treatment Process: disinfection and chemical nitrogen removal (breakpoint chlorination); CSUS/OWP Operation of Wastewater Treatment Plants, Vol. 1, 8th ed.Report a problem with this question

8. 8. Two contact tanks receive the same effluent, the same chlorine dose and the same contact time, but one runs near pH 6.5 and the other near pH 8.5. Which disinfects better, and why?

   • A.Neither tank, because pH does not affect the chlorine kill
   • B.The pH 6.5 tank, because more residual exists as HOCl✓ Answer
   • C.The pH 8.5 tank, because alkalinity speeds up the reaction
   • D.The pH 8.5 tank, because more residual exists as HOCl

   Chlorine in water distributes between hypochlorous acid (HOCl) and hypochlorite ion (OCl−), and the equilibrium shifts toward HOCl as pH falls. HOCl is by far the stronger disinfectant, so a lower pH gives better kill at the same dose and contact time. Note that feeding chlorine gas lowers pH and destroys alkalinity, while hypochlorite feed raises pH.

   Source: WPI 2025 NTK, Wastewater Treatment Class II — Treatment Process: disinfection chemistry (HOCl/OCl− equilibrium, pH effect); CSUS/OWP Operation of Wastewater Treatment Plants, Vol. 1, 8th ed.Report a problem with this question

9. 9. A dye study on a chlorine contact tank shows dye at the outlet well before the theoretical detention time, and coliform results are erratic even when the residual is adequate. The most likely cause is:

   • A.Chlorine gas feed pushing tank pH above the effective range
   • B.Short-circuiting past missing or damaged baffles in the tank✓ Answer
   • C.Accumulated sludge in the tank increasing the detention time
   • D.The analyzer reading combined residual instead of free residual

   Contact tanks are baffled into a serpentine path to approach plug flow so every parcel of water receives the full contact time. Missing or damaged baffles let part of the flow travel straight to the outlet, and those organisms are under-exposed even though the residual measured at the outlet looks fine. Accumulated solids also short-circuit flow — they reduce effective volume and detention time, not increase it.

   Source: WPI 2025 NTK, Wastewater Treatment Class II — Equipment: chlorine contact tanks, baffling and short-circuiting; CSUS/OWP Operation of Wastewater Treatment Plants, Vol. 1, 8th ed.Report a problem with this question

10. 10. A plant chlorinates its filtered effluent and discharges to a small trout stream. Dechlorination before the outfall is required mainly because residual chlorine:

    • A.Adds measurable oxygen demand load to the receiving water
    • B.Raises the pH of the receiving stream above its normal range
    • C.Causes persistent foaming and turbidity at the outfall
    • D.Is toxic to fish and other aquatic life in the stream✓ Answer

    Both free and combined chlorine residuals are acutely toxic to fish and aquatic invertebrates at very low concentrations, so discharge permits for surface water effectively require the residual to be destroyed before it reaches the receiving stream. Dechlorination with sulfur dioxide, sodium bisulfite or sodium thiosulfate is therefore a mandatory step, and the permit — not the operator — sets the applicable limit.

    Source: WPI 2025 NTK, Wastewater Treatment Class II — Treatment Process: dechlorination and effluent disposal, surface water discharge; NPDES permit limits under 40 CFR 122.44(d)Report a problem with this question

11. 11. An operator doubles the sodium bisulfite feed 'for safety' after dechlorination. Effluent dissolved oxygen then falls and effluent pH drifts down. The reason is that excess sulfite:

    • A.Reacts with dissolved oxygen in the effluent and consumes it✓ Answer
    • B.Precipitates solids at the outfall that exert an oxygen demand
    • C.Forms chloramines that continue to exert an oxygen demand
    • D.Raises effluent temperature, driving dissolved oxygen out

    Sulfite reducing agents are dosed only slightly above the stoichiometric requirement because, once all chlorine residual is destroyed, the leftover sulfite goes on to react with dissolved oxygen in the effluent and the reaction releases acidity. Both effluent DO and pH therefore fall, which can put the plant out of compliance with its discharge permit even though disinfection worked.

    Source: WPI 2025 NTK, Wastewater Treatment Class II — Treatment Process: dechlorination (sodium bisulfite, sodium thiosulfate, sulfur dioxide); CSUS/OWP Operation of Wastewater Treatment Plants, Vol. 1, 8th ed.Report a problem with this question

12. 12. A plant is adding sulfur dioxide dechlorination downstream of an existing chlorine contact tank. How much contact volume does the dechlorination step itself require?

    • A.About the same as chlorination, since both need long contact
    • B.About twice chlorination, because sulfite reacts more slowly
    • C.None at all, because the reaction occurs only in the outfall
    • D.Very little, because the reaction is essentially instantaneous✓ Answer

    Sulfur dioxide, sodium bisulfite and sodium thiosulfate reduce chlorine residual almost instantly, so the design and operating problem is rapid, complete mixing at the injection point rather than detention volume. This is the opposite of chlorination, which needs a baffled contact tank sized for substantial detention at design flow; confusing the two is a common error.

    Source: WPI 2025 NTK, Wastewater Treatment Class II — Treatment Process and Equipment: dechlorination feed and mixing; CSUS/OWP Operation of Wastewater Treatment Plants, Vol. 1, 8th ed.Report a problem with this question

13. 13. Tertiary filters break through and effluent TSS rises from 5 to 25 mg/L. The UV system was serviced last week and lamp hours are low, yet coliform results begin to fail. The best explanation is:

    • A.High TSS raises UV transmittance, so lamps cycle off early
    • B.High TSS cools the lamps and shifts their output wavelength
    • C.High TSS lowers UV transmittance and shields organisms✓ Answer
    • D.High TSS increases the UV residual demanded downstream

    UV dose equals intensity multiplied by exposure time, and the intensity actually reaching organisms depends on how much UV light the effluent passes, measured as percent transmittance. Suspended solids absorb and scatter UV and physically shield organisms embedded in particles, so upstream filtration performance is the master variable controlling UV disinfection; there is no UV residual to demand.

    Source: WPI 2025 NTK, Wastewater Treatment Class II — Treatment Process and Equipment: UV disinfection (dose, intensity, transmittance); CSUS/OWP Operation of Wastewater Treatment Plants, Vol. 1, 8th ed.Report a problem with this question

14. 14. UV disinfection performance falls off. Flow, lamp hours and effluent turbidity are all normal, but the quartz sleeves carry a dull mineral film. This indicates that:

    • A.Lamps have reached end of life and the whole bank needs replacing
    • B.Sleeve fouling is blocking light and cutting delivered intensity✓ Answer
    • C.Exposure time is too long, so the lamps are overheating the water
    • D.Channel water level is too low, so the upper lamps run uncovered

    Delivered UV dose is intensity times exposure time. Calcium and magnesium carbonate scale, iron deposits, grease and biological slime on the quartz sleeves absorb and scatter the light before it enters the water, so intensity at the organisms drops even though lamp age, flow and water quality are unchanged. Routine mechanical or chemical sleeve cleaning restores the dose.

    Source: WPI 2025 NTK, Wastewater Treatment Class II — Equipment: UV lamps, quartz sleeves, modules and banks (cleaning and maintenance); CSUS/OWP Operation of Wastewater Treatment Plants, Vol. 1, 8th ed.Report a problem with this question

15. 15. Fecal coliform samples taken from an open, sunlit effluent channel downstream of a UV reactor read higher than samples taken right at the reactor outlet. The likely explanation is:

    • A.Photoreactivation lets damaged organisms repair in sunlight✓ Answer
    • B.A UV residual persists and feeds the surviving organisms
    • C.UV-treated effluent cannot be sampled in an open channel
    • D.Lamp intensity was too low to inactivate any organisms

    UV damages the nucleic acids of organisms without destroying the cell, and exposure to light in roughly the 310 to 500 nm range activates repair enzymes so some organisms regain viability — photoreactivation. Because UV leaves no residual, nothing in the effluent prevents this recovery, which is why counts can climb in open sunlit channels between the reactor and the outfall.

    Source: WPI 2025 NTK, Wastewater Treatment Class II — Treatment Process: UV disinfection limitations (photoreactivation, no residual); CSUS/OWP Operation of Wastewater Treatment Plants, Vol. 1, 8th ed.Report a problem with this question

16. 16. Alum is fed for coagulation ahead of the tertiary filters. Over the following week, effluent alkalinity and pH both fall. The operator should recognize that:

    • A.Alum is pH-neutral, so only the polymer feed can lower the pH
    • B.Alum raises pH, so the fall must be an analyzer calibration error
    • C.Alum consumes alkalinity, so pH falls and lime may be needed✓ Answer
    • D.Alum adds alkalinity, so the pH drop must have another cause

    Metal-salt coagulants such as alum and ferric chloride hydrolyze in water and release acidity, destroying alkalinity. Where alkalinity is limited, the buffer is used up and pH falls quickly, which can push the effluent toward the low end of the permitted range and also inhibit nitrification. Lime, caustic soda or soda ash is fed to restore alkalinity and raise pH.

    Source: WPI 2025 NTK, Wastewater Treatment Class II — Treatment Process: chemical dosing for coagulation/flocculation and pH adjustment; WEF Wastewater Treatment Fundamentals 1 — Liquid TreatmentReport a problem with this question

17. 17. A plant must lower total phosphorus in its discharge and has no true anaerobic zone in the process. The most direct chemical approach is to:

    • A.Raise the aeration DO so the bacteria take up more phosphorus
    • B.Feed methanol to the aeration basin to strip out the phosphorus
    • C.Feed sodium bisulfite to reduce the phosphorus, then settle it
    • D.Feed a metal salt to precipitate the phosphorus, then flocculate✓ Answer

    Alum, ferric chloride or lime convert soluble phosphate into an insoluble precipitate, which is then built into settleable, filterable floc with a polymer and removed with the sludge or on the tertiary filters. Enhanced biological phosphorus removal requires a true anaerobic zone the plant does not have; bisulfite is a dechlorination chemical and methanol is a carbon source for denitrification.

    Source: WPI 2025 NTK, Wastewater Treatment Class II — Treatment Process: chemical dosing for nutrient removal; WEF Wastewater Treatment Fundamentals 1 — Liquid TreatmentReport a problem with this question

18. 18. A plant that chlorinated and then dechlorinated its effluent converts to UV disinfection. At the surface-water outfall the operator now:

    • A.Must add contact volume to hold the UV dose at the outfall
    • B.May stop all effluent monitoring, since UV adds no chemical
    • C.Has no chlorine residual to destroy or to report at the outfall✓ Answer
    • D.Must still dechlorinate, because UV leaves its own residual

    UV inactivates organisms with light and adds nothing to the water, so no chemical residual travels to the receiving stream — which is exactly why dechlorination, chlorine storage and residual-chlorine reporting drop out when a plant converts. UV also needs only seconds of exposure rather than a long baffled contact tank, and all other permit monitoring, including bacteriological testing, continues unchanged.

    Source: WPI 2025 NTK, Wastewater Treatment Class II — Treatment Process: disinfection alternatives and effluent disposal, surface water discharge; CSUS/OWP Operation of Wastewater Treatment Plants, Vol. 1, 8th ed.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 →