Water Treatment Operator Practice Test

100 questions of our own, and under every answer the section of the public source it was written from.

What does direct filtration leave out that conventional filtration includes?

Pick an answer, then press Check answer. Then you see Correct or Not quite, with the reason. Keys: A to D pick, Enter checks.

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Exam mode: 100 questions, 180 minutes, 70 right to pass (70%).

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Unofficial practice questions, not affiliated with Water Professionals International (formerly ABC). Bank checked October 7, 2026.

The real exam, as its owner describes it

Questions
100
To pass
70 of 100 right (70%)
Time limit
180 minutes
Content areas
Treatment Process 31 questions, Laboratory Analysis 16, Equipment Operation and Maintenance 26, Source Water 15, Security, Safety and Administration 12.
Calculations
About 10 percent of the Class I exam, in US and metric units. A formula and conversion table is handed out.
Unscored questions
Up to 10 extra pre-test questions may be mixed in. They are not marked out and do not count toward the score.
Format
Multiple choice with four options, closed book. A non-programmable calculator is allowed.
Time and pass mark
Three hours and 70 percent are what the exam owner states for its own certification. A state or province that uses the exam sets its own.
The ABC name
The owner was the Association of Boards of Certification (ABC) and is now Water Professionals International. Its testing service is still ABC Testing.

Checked October 7, 2026 against Water Professionals International's Need-to-Know Criteria for the Class I exam and its certification pages. Rules change, so confirm with the agency itself before your test.

The exam owner's Need-to-Know Criteria

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All 100 questions

Treatment Process 31

  • A filter has just been backwashed and put back in service. For the first several minutes its effluent turbidity is usually: Pennsylvania DEP operator training, Module 14: Conventional Filtration (2014), Unit 4, Filter Ripening
  • A filter is 20 ft by 25 ft and filters 1,500 gpm. What is its filtration rate? Pennsylvania DEP operator training, Module 14: Conventional Filtration (2014), Unit 4, Filtration Rate; our arithmetic
  • A filter with 400 sq ft of surface is backwashed at a rate of 15 gpm/sq ft for 8 minutes. How much water does the backwash use? Pennsylvania DEP operator training, Module 14: Conventional Filtration (2014), Unit 4, Backwash Rate; our arithmetic
  • A plant treats 2.5 MGD and feeds alum at 12 mg/L. How much alum does it use? Pennsylvania DEP operator training, Module 14: Conventional Filtration (2014), Unit 2, Example 2.1 (the pounds formula); our arithmetic
  • A sedimentation basin is 60 ft long, 20 ft wide and 10 ft deep. The flow is 500 gpm. What is the theoretical detention time? Pennsylvania DEP operator training, Module 14: Conventional Filtration (2014), Unit 3, Theoretical Detention Time; our arithmetic
Show all 100 questions (95 more)

Treatment Process, continued

  • A storm changes the raw water overnight. Which test shows the operator what coagulant dose to feed now? Pennsylvania DEP operator training, Module 14: Conventional Filtration (2014), Unit 2, Process Control
  • At a surface water plant that filters, the disinfectant residual in water entering the distribution system may not be below 0.2 mg/L for more than how long? 40 CFR 141.72(b)(2), disinfection, systems that filter
  • Calcium hypochlorite granules are 65 percent chlorine. How many pounds of the granules are needed to supply 13 lb of chlorine? Pennsylvania DEP operator training, Module 25: Hypochlorite (2022), Unit 3, Math Principles and Process Calculations; our arithmetic
  • Chlorine has been added up to the breakpoint. What happens to any chlorine added after that? Pennsylvania DEP operator training, Module 24: Gas Chlorination (2022), Unit 3, Breakpoint chlorination
  • Coagulation is poor because the raw water is low in alkalinity. Which chemical is fed to add alkalinity? Pennsylvania DEP operator training, Module 14: Conventional Filtration (2014), Unit 2, Basic Coagulant Chemistry
  • Floc that formed well in the first flocculation compartment is tearing into fine pieces in the last one. Which change addresses the cause? Pennsylvania DEP operator training, Module 14: Conventional Filtration (2014), Unit 2, Flocculation
  • Flow is 1.6 MGD. The chlorine demand is 1.8 mg/L and the target residual is 1.2 mg/L. About how many pounds of chlorine are needed in a day? Pennsylvania DEP operator training, Module 24: Gas Chlorination (2022), Unit 3, chlorine dose = demand + residual; the pounds formula is in Module 14, Unit 2; our arithmetic
  • For conventional or direct filtration, combined filter effluent turbidity must be at or below what level in at least 95 percent of a month's measurements? 40 CFR 141.173(a), filtration; the same limits for smaller systems are in 141.551
  • Free chlorine is the stronger disinfectant under which condition? Pennsylvania DEP operator training, Module 24: Gas Chlorination (2022), Unit 1, forms of chlorine in solution
  • The free chlorine residual at the end of the clearwell is 0.8 mg/L and the contact time is 45 minutes. What is the CT? 40 CFR 141.2, definition of CT; our arithmetic
  • How does a corrosion inhibitor such as orthophosphate protect pipes? Pennsylvania DEP operator training, Module 21: Chemical Addition (2022), Unit 1, Chemicals Used in Water Treatment
  • How does potassium permanganate help remove dissolved iron and manganese? Pennsylvania DEP operator training, Module 22: Inorganics Removal Basics (2018), Unit 3, Inorganic Removal Chemistry
  • How is a slow sand filter cleaned? Pennsylvania DEP operator training, Module 17: Slow Sand Filtration, Unit 1, Scraping
  • Powdered activated carbon is fed ahead of the treatment processes for what purpose? Pennsylvania DEP operator training, Module 15: Direct Filtration, Unit 1, Direct Filtration Overview
  • Raw water turbidity, pH and alkalinity shift suddenly. Which processes should the operator review at once? Pennsylvania DEP operator training, Module 14: Conventional Filtration (2014), Unit 5, Abnormal Operation
  • What causes air binding in a filter? Pennsylvania DEP operator training, Module 14: Conventional Filtration (2014), Unit 4, Filter Problems
  • What does direct filtration leave out that conventional filtration includes? 40 CFR 141.2, definition of direct filtration
  • What is a SCADA system? Pennsylvania DEP operator training, Module 11: Administration of Water Treatment Plants, Unit 9, SCADA and Instrumentation
  • What is the job of the rapid mix (flash mix) step? Pennsylvania DEP operator training, Module 14: Conventional Filtration (2014), Unit 2, Mixing
  • What is the purpose of filtering to waste after a backwash? Pennsylvania DEP operator training, Module 14: Conventional Filtration (2014), Unit 4, Filter to Waste
  • Where must a plant send the spent filter backwash water that it recycles? 40 CFR 141.76(c), recycle provisions, treatment technique
  • Which list gives the steps of conventional filtration treatment in the order water passes through them? 40 CFR 141.2, definition of conventional filtration treatment
  • Which membrane process has the tightest membrane and removes dissolved salts? Pennsylvania DEP operator training, Module 19: Membrane Filtration, Unit 1, levels of membrane filtration
  • Which reading on a running filter signals that it is clogging and due for a backwash? Pennsylvania DEP operator training, Module 14: Conventional Filtration (2014), Unit 4, Backwashing
  • Why are tube or plate settlers installed in a sedimentation basin? Pennsylvania DEP operator training, Module 14: Conventional Filtration (2014), Unit 3, Tube or Plate Settlers
  • Why is a chemical disinfectant still used at a plant that disinfects with UV light? Pennsylvania DEP operator training, Module 26: UV Disinfection, Unit 1, Use of UV with Other Disinfectants

Laboratory Analysis 16

  • A routine sample comes back total coliform-positive. What must the system do next? 40 CFR 141.858(a)(1), repeat monitoring
  • At what temperature are water samples kept from collection until analysis? Pennsylvania DEP operator training, Module 31: Laboratory Supervisor Highlights (2014), Unit 1, Sample Collection Tips
  • For a total coliform sample, how much time may pass between collection and the start of incubation at the laboratory? 40 CFR 141.852(a)(3), analytical methods, total coliform samples
  • How often should a laboratory pH meter be calibrated, and with what? Pennsylvania DEP operator training, Module 31: Laboratory Supervisor Highlights (2014), Unit 1, Laboratory Basics Review
  • The secondary standard for odor is 3. In what unit is it stated? 40 CFR 143.3, secondary maximum contaminant levels
  • Total chlorine residual minus free chlorine residual equals which value? Pennsylvania DEP operator training, Module 25: Hypochlorite (2022), Unit 4, Chemical Feed
  • Total hardness is caused mainly by salts of which two metals? Pennsylvania DEP operator training, Module 4: Water Quality and Characteristics, Unit 3, Water Quality Classifications (key terms)
  • What are the secondary maximum contaminant levels for iron and for manganese? 40 CFR 143.3, secondary maximum contaminant levels
  • What does a chain-of-custody record document? Pennsylvania DEP operator training, Module 31: Laboratory Supervisor Highlights (2014), Unit 1, Sample Collection Tips
  • What does alkalinity measure? Pennsylvania DEP operator training, Module 29: General Chemistry (2013), Unit 4, Acids and Bases
  • What does the DPD colorimetric test measure? Pennsylvania DEP operator training, Module 5: Disinfection, Unit 2, Measurement
  • What step does a true color test need that an apparent color test does not? Pennsylvania DEP operator training, Module 4: Water Quality and Characteristics, Unit 4, Water Quality Parameter Sampling and Monitoring
  • When a graduated cylinder is read, where on the curved liquid surface is the volume taken? Pennsylvania DEP operator training, Module 31: Laboratory Supervisor Highlights (2014), Unit 1, Laboratory Basics Review
  • Which instrument measures the cloudiness of water by the light its particles reflect? Pennsylvania DEP operator training, Module 14: Conventional Filtration (2014), Unit 1, Conventional Water Treatment Overview
  • Which pair gives the maximum contaminant levels for total trihalomethanes and for five haloacetic acids? 40 CFR 141.64(b), maximum contaminant levels for disinfection byproducts
  • Why does a coliform sample bottle for chlorinated water hold sodium thiosulfate? 40 CFR 141.852(a)(4), analytical methods, total coliform samples

Equipment Operation and Maintenance 26

  • A bearing has a grease relief plug. What is done with the plug when the bearing is greased? Pennsylvania DEP operator training, Module 10: General Maintenance, Unit 4, Lubrication
  • A chemical feed pump rated at 60 gallons per day is set to 50 percent stroke and 80 percent speed. What is its theoretical output? Pennsylvania DEP operator training, Module 21: Chemical Addition (2022), Unit 3, pump output formula; our arithmetic
  • A mechanical seal on a pump starts to leak. What does good maintenance call for? Pennsylvania DEP operator training, Module 10: General Maintenance, Unit 4, Mechanical Seals
  • A pump is running but no water is coming out. Which of these is a likely cause? Pennsylvania DEP operator training, Module 1: General Overview, Volume III (2023), Unit 3, Typical Pump Maintenance Procedures
  • A round tank is 20 ft across and holds water 10 ft deep. About how many gallons is that? Pennsylvania DEP operator training, Module 14: Conventional Filtration (2014), Unit 4, Example 4.4 (area of a circle) and Unit 3 (7.48 gallons per cubic foot); our arithmetic
  • A sludge collector's motor is running, but the collector is not moving. What should be checked? Pennsylvania DEP operator training, Module 14: Conventional Filtration (2014), Unit 3, Basin Types
  • How is a small chlorine gas leak located? Pennsylvania DEP operator training, Module 24: Gas Chlorination (2022), Unit 2, Chlorine Handling and Safety
  • How large is a spill containment area for liquid chemicals typically made? Pennsylvania DEP operator training, Module 21: Chemical Addition (2022), Unit 4, Chemical Feed System Schematics
  • How should valves be operated to avoid water hammer? Pennsylvania DEP operator training, Module 1: General Overview, Volume III (2023), Unit 4, Distribution Networks
  • Maintenance done on a regular basis and scheduled in advance, to avoid costly repairs, is called: Pennsylvania DEP operator training, Module 10: General Maintenance, Unit 2, Types of Maintenance
  • The output of a diaphragm chemical feed pump is set by which two adjustments? Pennsylvania DEP operator training, Module 21: Chemical Addition (2022), Unit 3, Adjusting Chemical Feed Pump Dosage
  • The packing gland nuts are fully taken up and the gland still leaks too much. What is the next step? Pennsylvania DEP operator training, Module 10: General Maintenance, Unit 4, Shaft Seal Adjustment
  • Vibration analysis on a pump can reveal which kind of problem? Pennsylvania DEP operator training, Module 10: General Maintenance, Unit 4, Condition Assessment
  • What are the two jobs of a filter underdrain? Pennsylvania DEP operator training, Module 14: Conventional Filtration (2014), Unit 4, Filter Underdrains
  • What do surface wash and air scour systems add to a backwash? Pennsylvania DEP operator training, Module 14: Conventional Filtration (2014), Unit 4, Supplemental Filter Wash Systems
  • What does a foot valve on a pump's suction line do? Pennsylvania DEP operator training, Module 21: Chemical Addition (2022), Unit 3, Chemical Feed System Components
  • What does an anti-siphon valve on a chemical feed line prevent? Pennsylvania DEP operator training, Module 21: Chemical Addition (2022), Unit 3, Chemical Feed System Components
  • What does over-tightening the packing on a pump shaft lead to? Pennsylvania DEP operator training, Module 10: General Maintenance, Unit 4, Shaft Seal Adjustment
  • What is a megger used to test? Pennsylvania DEP operator training, Module 10: General Maintenance, Unit 4, electrical testing
  • What is the calibration cylinder on a chemical feed pump used for? Pennsylvania DEP operator training, Module 21: Chemical Addition (2022), Unit 3, Chemical Feed System Components
  • What is the fusible plug in a chlorine cylinder valve for? Pennsylvania DEP operator training, Module 24: Gas Chlorination (2022), Unit 2, Chlorine Handling and Safety
  • What is the most chlorine gas that should be drawn from one 150-pound cylinder in 24 hours? Pennsylvania DEP operator training, Module 24: Gas Chlorination (2022), Unit 3, Maximum Feed Rates
  • Where is the exhaust fan placed in a chlorine room, and why? Pennsylvania DEP operator training, Module 24: Gas Chlorination (2022), Unit 2, Personnel Safety Protection
  • Which chemical feed accessory offsets the surges from a diaphragm metering pump? Pennsylvania DEP operator training, Module 21: Chemical Addition (2022), Unit 3, Chemical Feed System Components
  • Which chlorine emergency repair kit fits 100 and 150 pound cylinders? Pennsylvania DEP operator training, Module 24: Gas Chlorination (2022), Unit 2, Chlorine Handling and Safety
  • Why should an electric motor not be over-lubricated? Pennsylvania DEP operator training, Module 10: General Maintenance, Unit 4, electric motors

Source Water Characteristics 15

  • A well's static water level is 38 ft below ground. While the pump runs, the level is 61 ft below ground. What is the drawdown? U.S. Geological Survey, Water Science School, Water Science Glossary, entry "drawdown"; our arithmetic
  • A well's turbidity and temperature shift quickly after rain, and its water contains algae. How may the well be classified? 40 CFR 141.2, definition of ground water under the direct influence of surface water
  • After a lake's fall or spring turnover, which raw water problem becomes more likely? Pennsylvania DEP operator training, Module 3: Surface Water Sources, Unit 2, Considerations for Use of Surface Water as a Source of Supply
  • How do the federal drinking water rules define surface water? 40 CFR 141.2, definition of surface water
  • In a well drilled into a confined aquifer, where does the water level stand? Pennsylvania DEP operator training, Module 2: Groundwater Sources of Supply and Protection, Unit 2, Types of Wells
  • Nitrate found in a sand and gravel aquifer generally comes from where? Pennsylvania DEP operator training, Module 2: Groundwater Sources of Supply and Protection, Unit 2, Sand and Gravel Aquifers
  • Pumping a well draws down the water table around it. What is that lowered zone called? Pennsylvania DEP operator training, Module 2: Groundwater Sources of Supply and Protection, Unit 3, General Well Hydraulics
  • Reservoir stratification is a natural source of which two metals in raw water? Pennsylvania DEP operator training, Module 4: Water Quality and Characteristics, Unit 1, General Overview, and Unit 4
  • Well water smells like rotten eggs. Which gas is the likely cause, and which process treats it? Pennsylvania DEP operator training, Module 22: Inorganics Removal Basics (2018), Unit 3, Inorganic Removal Chemistry
  • What is the maximum contaminant level for nitrate, measured as nitrogen? 40 CFR 141.62(b), maximum contaminant levels for inorganic contaminants
  • What removal or inactivation of Giardia lamblia cysts must surface water treatment reliably achieve? 40 CFR 141.70(a), general requirements, treatment technique
  • What usually happens to raw water turbidity when heavy rain flushes into a reservoir? Pennsylvania DEP operator training, Module 14: Conventional Filtration (2014), Unit 1, Turbidity
  • Which of these particles found in surface water is the smallest? Pennsylvania DEP operator training, Module 14: Conventional Filtration (2014), Unit 3, Settling Characteristics
  • Why is a perched water table a poor place for a well? Pennsylvania DEP operator training, Module 2: Groundwater Sources of Supply and Protection, Unit 2, Types of Aquifers and Types of Wells
  • Why is water tested for coliform bacteria and not for every pathogen? Pennsylvania DEP operator training, Module 5: Disinfection, Unit 1, Overview of Disinfection

Security, Safety, and Administrative Procedures 12

  • After an emergency is over, what is done with the emergency response plan? Pennsylvania DEP operator training, Module 11: Administration of Water Treatment Plants, Unit 2, Developing Standard Operating Procedures
  • Before anyone enters a confined space, its air is tested for three things. In which order? 29 CFR 1910.146(c)(5)(ii)(C), pre-entry atmospheric testing
  • Below what oxygen level is the air in a confined space oxygen deficient? 29 CFR 1910.146(b), definitions
  • By what date each year must a community water system distribute its consumer confidence report? 40 CFR 141.155(j)(1), report delivery
  • For how long must records of microbiological and turbidity analyses be kept? 40 CFR 141.33(a), record maintenance
  • How does a maximum contaminant level goal (MCLG) differ from a maximum contaminant level (MCL)? 40 CFR 141.2, definitions of maximum contaminant level and maximum contaminant level goal
  • How soon must a Tier 1 public notice go out after the system learns of the violation? 40 CFR 141.202(b), Tier 1 public notice; Tier 2 is 141.203(b) and Tier 3 is 141.204(b)
  • Which respiratory protection is preferred when responding to a chlorine leak? Pennsylvania DEP operator training, Module 24: Gas Chlorination (2022), Unit 2, Personnel Safety Protection
  • Which section of a safety data sheet gives first-aid measures? 29 CFR 1910.1200(g)(2), safety data sheets
  • Who removes a lockout device when the repair is finished? 29 CFR 1910.147(e)(3), lockout or tagout devices removal
  • Why is a leaking chlorine cylinder never put in water? Pennsylvania DEP operator training, Module 24: Gas Chlorination (2022), Unit 2, Chlorine Handling and Safety
  • Why must powdered activated carbon never be mixed or stored with potassium permanganate? Pennsylvania DEP operator training, Module 21: Chemical Addition (2022), Appendix, chemical storage (incompatible chemicals)

How this list was compiled: Our own questions, in the five content areas and the counts of the Need-to-Know Criteria for the Water Treatment Operator Class I exam (Water Professionals International, 2025, https://gowpi.org/services/2025-need-to-know-criteria). Facts were read in 40 CFR Parts 141 and 143 and 29 CFR Part 1910 on ecfr.gov, in the Drinking Water Operator Certification Training workbooks of the Pennsylvania Department of Environmental Protection (https://files.dep.state.pa.us/Water/BSDW/OperatorCertification/TrainingModules/) and in the U.S. Geological Survey's Water Science Glossary, all on 2026-10-07. Each question names the section it was written from. Licence: Own work. Every question, answer choice and explanation was written for this site from the public sources named on it. Nothing is taken from the exam, from its owner's sample questions or study guides, from the training workbooks' own exercises, or from another practice test.

How it works

This water treatment operator certification practice test follows the content areas of the Need-to-Know Criteria for the Class I exam. It is unofficial and not affiliated with, endorsed by or sponsored by Water Professionals International (formerly ABC).

Read the full guide

Classes II, III and IV

The share of each content area shifts with the class: treatment process grows from 31 questions in Class I to 35 in Class IV, and calculations from about 10 percent to 15.

The calculations

Our calculation questions use US units, and each explanation writes out its formula: the pounds formula, detention time, filtration rate, CT. Work them with the calculator you will bring.

Where the answers come from

Rule questions cite 40 CFR Part 141 or 143 by section, or an OSHA standard. Process and equipment questions cite Pennsylvania's public operator training modules, because the exam's listed references are commercial textbooks. Your state may add rules of its own.

  • Learn the pounds formula first: flow in MGD x dose in mg/L x 8.34. Run backward, it gives a dose from the pounds fed.
  • Keep the turbidity numbers together: 0.3 NTU in 95 percent of the month, never over 1 NTU.

Questions and answers

Is this an ABC water treatment practice test?

It follows the outline of the exam many operators still call the ABC exam, after its owner's former name. The questions here are ours, not theirs.

How many questions are on the water treatment operator exam?

The 2025 Need-to-Know Criteria list 100 scored questions for each class, I to IV, and up to 10 unscored pre-test questions. An exam your state writes for itself may differ.

What score do you need to pass the water operator exam?

Water Professionals International asks 70 percent or higher for its own certification. A state or province that uses the exam sets its own pass mark, so ask your certifying authority.

Does my state use this exam?

Each certifying authority chooses. Some give the standardized exam, some build a custom one from the same question banks, and some do not use the testing service. Your state's drinking water program can say which.

Is there a practice test for water treatment operator Class 2, 3 or 4?

Not as separate pages here. The higher classes share these five content areas, so this bank works as a first pass. Add your class's outline and harder calculations.

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