100+ Free Wastewater Treatment Operator Class I Practice Questions
Pass your ABC/WPI Wastewater Treatment Operator Class I Certification Exam exam on the first try — instant access, no signup required.
Operator math: A 1 MGD plant has an aeration basin of 250,000 gallons. What is the hydraulic detention time?
Explore More ABC Water Operator Certification
Continue into nearby exams from the same family. Each card keeps practice questions, study guides, flashcards, videos, and articles in one place.
More From This Family
Videos and articles for deeper review.
Key Facts: Wastewater Treatment Operator Class I Exam
100
Scored Questions
WPI standardized Class I wastewater exam outline
180 minutes
Time Limit
WPI ABC standardized exam policy
70%
Passing Score
Typical across WPI/ABC state programs
40+
States Using WPI Exams
Water Professionals International
25 mg/L
Typical CBOD5 Monthly NPDES Limit
EPA Secondary Treatment Standards (40 CFR 133)
1500–4000 mg/L
Typical MLSS Operating Range
WPI Need-to-Know Criteria — Activated Sludge
Water Professionals International (WPI, formerly the Association of Boards of Certification, or ABC) develops the standardized Wastewater Treatment Operator exam series used by more than 40 state certifying authorities. Class I is the entry-level grade and covers the smallest wastewater treatment plants — commonly those with the lowest design flow or the simplest process trains. The Class I exam covers preliminary treatment (bar screens, comminutors, grit chambers), primary clarifiers, secondary biological treatment with a heavy emphasis on activated sludge (MLSS, MLVSS, F/M ratio, SRT/MCRT, SVI, RAS/WAS, DO control, bulking and foaming), trickling filters and SBRs, secondary clarification, disinfection (chlorine, dechlorination, UV), solids handling (thickening, digestion, dewatering, Class A vs Class B biosolids under 40 CFR Part 503), NPDES effluent limits for CBOD5/TSS/ammonia, sampling and lab basics (BOD5, TSS, DMR reporting), operator math (flow, detention time, surface loading, organic loading), and plant safety including confined space, H2S, chlorine gas, and methane in digesters. The exam consists of 100 scored multiple-choice questions plus up to 10 unscored pretest items administered in a 3-hour (180-minute) time window, and most state programs require a 70% passing score.
Sample Wastewater Treatment Operator Class I Practice Questions
Try these sample questions to test your Wastewater Treatment Operator Class I exam readiness. Each question includes a detailed explanation. Start the interactive quiz above for the full 100+ question experience with AI tutoring.
1What is the primary purpose of a bar screen at the headworks of a wastewater treatment plant?
2Which treatment step typically follows preliminary treatment in a conventional wastewater plant?
3Why are grit chambers placed early in the treatment train?
4What does BOD5 measure in a wastewater sample?
5What does TSS stand for and what does it measure?
6What is the typical BOD5 of domestic raw wastewater entering a treatment plant?
7What does MLSS stand for in an activated sludge process?
8Which value represents the active biological fraction of mixed liquor?
9What is RAS in an activated sludge plant?
10What is WAS in an activated sludge plant?
About the Wastewater Treatment Operator Class I Exam
The ABC/WPI Wastewater Treatment Operator Class I exam is the entry-level standardized certification exam for operators of the smallest wastewater treatment plants. It covers preliminary and primary treatment, activated sludge fundamentals, disinfection, biosolids handling, sampling, operator math, NPDES permits, and plant safety.
Assessment
100 scored multiple-choice questions plus up to 10 unscored pretest items
Time Limit
180 minutes
Passing Score
70%
Exam Fee
Varies by jurisdiction; typically $80–$175 (Water Professionals International (WPI / formerly ABC))
Wastewater Treatment Operator Class I Exam Content Outline
Preliminary and Primary Treatment
Bar screens (coarse 50–150 mm, fine 6–25 mm), comminutors, grit chambers (aerated, vortex, horizontal), primary clarifiers (1.5–2.5 hr detention, 30–40% BOD and 50–70% TSS removal), scum collection.
Secondary (Biological) Treatment
Activated sludge fundamentals — MLSS 1500–4000 mg/L, MLVSS, F/M 0.2–0.5, SRT 4–15 days, SVI 80–150, DO 1.5–3 mg/L, RAS/WAS, aeration systems, extended aeration, SBRs, trickling filters, RBCs, microbial indicators, bulking and foaming control.
Disinfection and Effluent
Chlorine (gas, NaOCl, Ca(OCl)2), chlorine contact tanks (30–60 min), dechlorination with SO2 or NaHSO3, UV dose 30–100 mJ/cm², NPDES limits (CBOD5 25 mg/L monthly, TSS 30 mg/L typical secondary).
Solids Handling and Biosolids
Thickening (gravity, DAF, GBT — 1–2% to 4–6% solids), aerobic digestion 15–30 days, anaerobic digestion, belt filter press/centrifuge/drying beds (15–25% cake solids), Class A vs Class B biosolids under 40 CFR Part 503, land application at agronomic rate.
Sampling, Lab, and Operator Math
24-hour flow-proportional composite vs grab samples, BOD5/CBOD5 5-day bottle test, TSS gravimetric, ammonia, DO and pH meters, monthly DMR reporting, flow conversion, detention time, surface loading, organic loading, F/M and SRT calculations.
Safety, Collection, and Regulations
Confined space entry (O2 19.5–23.5%, LEL <10%, H2S <10 ppm), chlorine gas handling, methane explosion hazard in digesters, gravity sewers vs force mains, lift stations, I/I, SSO vs CSO, NPDES permit framework, Clean Water Act.
How to Pass the Wastewater Treatment Operator Class I Exam
What You Need to Know
- Passing score: 70%
- Assessment: 100 scored multiple-choice questions plus up to 10 unscored pretest items
- Time limit: 180 minutes
- Exam fee: Varies by jurisdiction; typically $80–$175
Keys to Passing
- Complete 500+ practice questions
- Score 80%+ consistently before scheduling
- Focus on highest-weighted sections
- Use our AI tutor for tough concepts
Wastewater Treatment Operator Class I Study Tips from Top Performers
Frequently Asked Questions
What is the ABC/WPI Wastewater Treatment Operator Class I exam?
It is a standardized multiple-choice exam developed by Water Professionals International (formerly the Association of Boards of Certification, ABC) for entry-level wastewater treatment operators. Class I is the lowest grade and covers the smallest wastewater treatment plants — commonly those with the lowest design flow and simplest process trains. More than 40 state certifying authorities use the WPI/ABC standardized exam.
How many questions are on the Class I wastewater treatment exam?
The current standardized format uses 100 scored multiple-choice questions and may include up to 10 unscored pretest items. The 3-hour (180-minute) time limit and 70% passing score apply in most state programs that use the WPI standardized exam.
What plants does Class I certification cover?
Class I wastewater treatment certification covers the smallest wastewater treatment plants — typically small package plants and lagoon systems. State classification rules vary, but Class I focuses on fundamental operator knowledge for plants using basic preliminary/primary treatment, conventional or extended aeration activated sludge (or equivalent), simple disinfection, and basic solids handling.
What is MLSS and what range should I memorize?
MLSS is Mixed Liquor Suspended Solids — the concentration of solids (mostly microorganisms) in the aeration tank. Conventional activated sludge plants run 1500–3000 mg/L; extended aeration plants run higher at 2000–4000 mg/L. MLVSS (Mixed Liquor Volatile Suspended Solids) is the organic fraction, typically about 70–80% of MLSS.
What is the difference between Class A and Class B biosolids?
Under EPA 40 CFR Part 503, Class A biosolids are treated to reduce pathogens below detection limits and have no site restrictions — they can be bagged and sold to the public. Class B biosolids still contain measurable pathogens; they require site restrictions on public access, crop type, animal grazing, and harvest timing after land application.
What causes activated sludge bulking and how do operators control it?
Bulking is excess filamentous bacteria growth that causes poor settling (SVI >150–200 mL/g). Common causes are low F/M ratio, low DO, septic conditions, or nutrient deficiency. Control methods include RAS chlorination, polymer addition, increasing DO, adjusting F/M, and using a selector zone to favor floc-forming bacteria over filaments.