Drinking water filtration and disinfection practice questions
20 free practice questions from 40 CFR Part 141 Subpart H. Each answer is explained and linked to the rule it comes from.
Drinking water filtration and disinfection
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Scope and general requirements
1. In addition to complying with Subpart H, which subpart does 141.70(e) say systems serving fewer than 10,000 people must also comply with?
- A. Subpart P
- B. Subpart T
- C. None; the same requirements apply whatever the population served
- D. Subpart P, but only after the State approves
Show answer
Answer: B. Subpart T
141.70(e) states: "In addition to complying with requirements in this subpart, systems serving fewer than 10,000 people must also comply with the requirements in subpart T of this part." Subpart P is named in 141.70(d) for "systems serving at least 10,000 people".
Source: eCFR: 40 CFR 141.70, General requirements (as of Nov 3, 2026)
2. Under 141.70(b), a subpart H system that filters is considered in compliance with the treatment technique requirements of 141.70(a) if it meets which pair of requirements?
- A. The criteria for avoiding filtration in 141.71 and the disinfection requirements in 141.72(b)
- B. The filtration requirements in 141.73, with no disinfection requirement
- C. The disinfection requirements in 141.72(a) alone
- D. The filtration requirements in 141.73 and the disinfection requirements in 141.72(b)
Show answer
Answer: D. The filtration requirements in 141.73 and the disinfection requirements in 141.72(b)
A system is considered in compliance with 141.70(a) if "It meets the filtration requirements in § 141.73 and the disinfection requirements in § 141.72(b)." The other route, for unfiltered systems, is meeting "the requirements for avoiding filtration in § 141.71 and the disinfection requirements in § 141.72(a)".
Source: eCFR: 40 CFR 141.70, General requirements (as of Nov 3, 2026)
3. Under 141.70(c), whose requirements must the personnel operating a surface water or GWUDI system meet?
- A. Requirements set by the system's own governing board
- B. Requirements set by the laboratory that analyzes the system's samples
- C. Requirements specified by the State
- D. Requirements set by the system's largest wholesale customer
Show answer
Answer: C. Requirements specified by the State
Each such system "must be operated by qualified personnel who meet the requirements specified by the State."
Source: eCFR: 40 CFR 141.70, General requirements (as of Nov 3, 2026)
Criteria for avoiding filtration
4. An unfiltered system measures both fecal coliforms and total coliforms in its source water. Under 141.71(a)(1), which criterion must it meet?
- A. The fecal coliform criterion, but not the total coliform criterion
- B. The total coliform criterion, but not the fecal coliform criterion
- C. Both the fecal and the total coliform criteria
- D. Whichever criterion the system chooses each month
Show answer
Answer: A. The fecal coliform criterion, but not the total coliform criterion
"If a system measures both fecal and total coliforms, the fecal coliform criterion, but not the total coliform criterion, in this paragraph must be met."
Source: eCFR: 40 CFR 141.71, Criteria for avoiding filtration (as of Nov 3, 2026)
5. How does 141.71(a)(2) define a turbidity "event" for source water?
- A. Any single turbidity measurement above 1 NTU
- B. Any calendar month whose average turbidity exceeds 5 NTU
- C. A series of consecutive days during which at least one turbidity measurement each day exceeds 5 NTU
- D. Any four-hour period in which every measurement exceeds 5 NTU
Show answer
Answer: C. A series of consecutive days during which at least one turbidity measurement each day exceeds 5 NTU
Under 141.71(a)(2), an event "is a series of consecutive days during which at least one turbidity measurement each day exceeds 5 NTU."
Source: eCFR: 40 CFR 141.71, Criteria for avoiding filtration (as of Nov 3, 2026)
6. Under 141.71(b)(3), who must conduct the annual on-site inspection of an unfiltered system's watershed control program and disinfection treatment process?
- A. The system's chief operator, without any State involvement
- B. Either the State or a party approved by the State
- C. The laboratory certified to analyze the system's coliform samples
- D. The system's largest customer
Show answer
Answer: B. Either the State or a party approved by the State
"Either the State or a party approved by the State must conduct the on-site inspection."
Source: eCFR: 40 CFR 141.71, Criteria for avoiding filtration (as of Nov 3, 2026)
Disinfection requirements
7. 141.72(a)(1) requires an unfiltered system's disinfection to ensure 3-log Giardia lamblia and 4-log virus inactivation every day it serves water, with what exception stated in the rule?
- A. Except any one day each month
- B. Except any two consecutive days
- C. Except one week in each calendar quarter
- D. Except on days when the source water turbidity is below 1 NTU
Show answer
Answer: A. Except any one day each month
Disinfection must ensure the inactivation "every day the system serves water to the public, except any one day each month."
Source: eCFR: 40 CFR 141.72, Disinfection (as of Nov 3, 2026)
8. Under 141.72(a)(2), an unfiltered system's disinfection system must have either redundant components (including an auxiliary power supply with automatic start-up and alarm) or what other safeguard?
- A. A second certified operator on site at all times
- B. A doubled disinfectant dose during peak hourly flow
- C. A daily report of residuals to the State
- D. Automatic shut-off of delivery of water to the distribution system whenever the residual disinfectant concentration is less than 0.2 mg/l
Show answer
Answer: D. Automatic shut-off of delivery of water to the distribution system whenever the residual disinfectant concentration is less than 0.2 mg/l
The alternative is "automatic shut-off of delivery of water to the distribution system whenever there is less than 0.2 mg/l of residual disinfectant concentration in the water."
Source: eCFR: 40 CFR 141.72, Disinfection (as of Nov 3, 2026)
9. For the distribution system residual requirement in 141.72(b)(3)(i), what water is deemed to have a detectable disinfectant residual?
- A. Water in which no total coliforms are found
- B. Water with a turbidity of 1 NTU or less
- C. Water with a heterotrophic bacteria concentration less than or equal to 500/ml, measured as HPC
- D. Water with a pH between 6.5 and 8.5
Show answer
Answer: C. Water with a heterotrophic bacteria concentration less than or equal to 500/ml, measured as HPC
"Water in the distribution system with a heterotrophic bacteria concentration less than or equal to 500/ml, measured as heterotrophic plate count (HPC) as specified in § 141.74(a)(1), is deemed to have a detectable disinfectant residual".
Source: eCFR: 40 CFR 141.72, Disinfection (as of Nov 3, 2026)
Filtration and turbidity limits
10. Under 141.73(a)(1), what must the State determine before it may substitute a turbidity limit higher than 0.5 NTU for a system using conventional or direct filtration?
- A. That the system has had no source water turbidity event in the past 12 months
- B. That the system is capable of achieving at least 99.9 percent removal and/or inactivation of Giardia lamblia cysts at that higher turbidity level
- C. That the system returns its spent filter backwash water through the filters
- D. That the system's source water turbidity never exceeds 1 NTU
Show answer
Answer: B. That the system is capable of achieving at least 99.9 percent removal and/or inactivation of Giardia lamblia cysts at that higher turbidity level
The State may substitute a higher limit "if the State determines that the system is capable of achieving at least 99.9 percent removal and/or inactivation of Giardia lamblia cysts at some turbidity level higher than 0.5 NTU in at least 95 percent of the measurements taken each month". Even then, "in no case may the State approve a turbidity limit that allows more than 1 NTU in more than 5 percent of the samples taken each month".
11. Under 141.73(b)(1), on what basis may the State substitute a turbidity limit higher than 1 NTU for a slow sand filtration system?
- A. If the State determines there is no significant interference with disinfection at the higher turbidity level
- B. If the system serves 500 or fewer persons
- C. If the source water turbidity has never exceeded 1 NTU
- D. If the system installs continuous turbidity monitoring
Show answer
Answer: A. If the State determines there is no significant interference with disinfection at the higher turbidity level
The 1 NTU limit applies "except that if the State determines there is no significant interference with disinfection at a higher turbidity level, the State may substitute this higher turbidity limit for that system."
12. A system demonstrates to the State that an alternative filtration technology meets 141.73(d). The requirements of which paragraph of 141.73 then apply to it?
- A. Paragraph (a), conventional filtration treatment or direct filtration
- B. Paragraph (b), slow sand filtration
- C. Paragraph (c), diatomaceous earth filtration
- D. None; no turbidity limit applies to an approved alternative technology
Show answer
Answer: B. Paragraph (b), slow sand filtration
"For a system that makes this demonstration, the requirements of paragraph (b) of this section apply." Paragraph (b) covers slow sand filtration.
Monitoring
13. For a filtered system serving 500 or fewer persons, when does 141.74(c)(1) allow the State to reduce filtered water turbidity sampling to once per day?
- A. Only when the system uses slow sand filtration
- B. Automatically, without any State determination
- C. Only after the system has reported 12 months of data with no measurement above 1 NTU
- D. Regardless of the type of filtration used, if the State determines that less frequent monitoring is sufficient to indicate effective filtration performance
Show answer
Answer: D. Regardless of the type of filtration used, if the State determines that less frequent monitoring is sufficient to indicate effective filtration performance
"For systems serving 500 or fewer persons, the State may reduce the turbidity sampling frequency to once per day, regardless of the type of filtration treatment used, if the State determines that less frequent monitoring is sufficient to indicate effective filtration performance."
Source: eCFR: 40 CFR 141.74, Analytical and monitoring requirements (as of Nov 3, 2026)
14. A filtered system serving 3,300 or fewer persons takes grab samples of the residual entering the distribution system in lieu of continuous monitoring, at the daily frequencies in the 141.74(c)(2) table. What does the table's footnote say about each day's samples?
- A. All of the day's samples must be taken within the same hour
- B. The day's samples may be composited into one bottle before analysis
- C. The day's samples cannot be taken at the same time, and the sampling intervals are subject to State review and approval
- D. The day's samples need to be taken only on business days
Show answer
Answer: C. The day's samples cannot be taken at the same time, and the sampling intervals are subject to State review and approval
The footnote states: "The day's samples cannot be taken at the same time. The sampling intervals are subject to State review and approval."
Source: eCFR: 40 CFR 141.74, Analytical and monitoring requirements (as of Nov 3, 2026)
15. Under 141.74(a), who must conduct measurements of pH, turbidity, temperature and residual disinfectant concentrations used to show compliance with Subpart H?
- A. A person approved by the State
- B. Only EPA staff
- C. Any person the system hires, with no approval needed
- D. The system's wholesale water supplier
Show answer
Answer: A. A person approved by the State
"Measurements for pH, turbidity, temperature and residual disinfectant concentrations must be conducted by a person approved by the State."
Source: eCFR: 40 CFR 141.74, Analytical and monitoring requirements (as of Nov 3, 2026)
Reporting to the State
16. Under 141.75(b)(3)(ii), what must a filtered system do, and by when, if its turbidity exceeds 5 NTU at any time?
- A. Include the result in its next monthly report, with no earlier contact
- B. Consult with the primacy agency only if the turbidity stays above 5 NTU for more than 4 hours
- C. Consult with the primacy agency as soon as practical, but no later than 24 hours after the exceedance is known
- D. Wait for the next annual on-site inspection to discuss it
Show answer
Answer: C. Consult with the primacy agency as soon as practical, but no later than 24 hours after the exceedance is known
"If at any time the turbidity exceeds 5 NTU, the system must consult with the primacy agency as soon as practical, but no later than 24 hours after the exceedance is known, in accordance with the public notification requirements under § 141.203(b)(3)."
Source: eCFR: 40 CFR 141.75, Reporting and recordkeeping requirements (as of Nov 3, 2026)
17. After the residual entering the distribution system falls below 0.2 mg/l, what else must the system tell the State by the end of the next business day under 141.75(b)(3)(iii)?
- A. The name of the operator on duty at the time
- B. The planned date of its next sanitary survey
- C. The cost of any repairs to the disinfection equipment
- D. Whether or not the residual was restored to at least 0.2 mg/l within 4 hours
Show answer
Answer: D. Whether or not the residual was restored to at least 0.2 mg/l within 4 hours
"The system also must notify the State by the end of the next business day whether or not the residual was restored to at least 0.2 mg/l within 4 hours."
Source: eCFR: 40 CFR 141.75, Reporting and recordkeeping requirements (as of Nov 3, 2026)
18. What deadline does 141.75(a)(3) set for an unfiltered system's report summarizing its compliance with the watershed control program requirements?
- A. Within 10 days after the end of each month
- B. No later than ten days after the end of each Federal fiscal year (September 30)
- C. By December 31 of each calendar year
- D. Within 30 days after each on-site inspection
Show answer
Answer: B. No later than ten days after the end of each Federal fiscal year (September 30)
"No later than ten days after the end of each Federal fiscal year (September 30), each system must provide to the State a report which summarizes its compliance with all watershed control program requirements".
Source: eCFR: 40 CFR 141.75, Reporting and recordkeeping requirements (as of Nov 3, 2026)
Recycle provisions
19. Which systems do the recycle provisions of 141.76 apply to?
- A. All subpart H systems that employ conventional or direct filtration and recycle spent filter backwash water, thickener supernatant, or liquids from dewatering processes
- B. Every public water system that backwashes filters, whatever its source
- C. Only subpart H systems using slow sand filtration
- D. Only ground water systems that recycle backwash water
Show answer
Answer: A. All subpart H systems that employ conventional or direct filtration and recycle spent filter backwash water, thickener supernatant, or liquids from dewatering processes
141.76 applies to "All subpart H systems that employ conventional filtration or direct filtration treatment and that recycle spent filter backwash water, thickener supernatant, or liquids from dewatering processes".
Source: eCFR: 40 CFR 141.76, Recycle provisions (as of Nov 3, 2026)
20. Under 141.76(c), where must a system return recycled spent filter backwash water, thickener supernatant, or liquids from dewatering processes?
- A. Directly into the clearwell after filtration, without State approval
- B. Into the distribution system after disinfection, without State approval
- C. Into any point the system chooses, without State review
- D. Through the processes of the system's existing conventional or direct filtration system, or at an alternate location approved by the State
Show answer
Answer: D. Through the processes of the system's existing conventional or direct filtration system, or at an alternate location approved by the State
The system "must return these flows through the processes of a system's existing conventional or direct filtration system as defined in § 141.2 or at an alternate location approved by the State".
Source: eCFR: 40 CFR 141.76, Recycle provisions (as of Nov 3, 2026)