Contact-time (CT) calculation and disinfection byproduct formation and control first appear at D3.
For the operators who keep treated water moving through the pipes, pumps, valves and storage between the plant and the customer.
Start studying D3| Content area | % of the exam | |
|---|---|---|
| Disinfection | 20 | |
| Distribution System Design / Hydraulics | 15 | |
| Equipment Operation / Maintenance / Inspections | 25 | |
| Regulations / Management / Safety | 15 | |
| Water Mains and Piping | 15 | |
| Water Quality / Water Source | 10 |
58% of examinees passed (415 of 719) from July 1, 2025 to June 30, 2026. Source: SWRCB Drinking Water Operator Certification Program, Water Distribution Exam Results (through 6/30/2026). The agency updates these tables every six months.
Disinfection: contact time (CT) · an original practice item written only from publicly available sources, not a real exam question
Water flows at 3 million gallons per day (MGD) through a 24-inch transmission main 2 miles long before the first customer, carrying a free chlorine residual of 0.8 mg/L. Treating the full pipe as plug flow, what CT does the main provide?
A. 95 mg·min/L
Pipe volume = 0.785 × 2 ft × 2 ft × 10,560 ft = 33,158 ft³ × 7.48 = 248,020 gal. Flow = 3,000,000 ÷ 1,440 = 2,083 gallons per minute (gpm), so contact time T = 248,020 ÷ 2,083 = 119 min. CT = 0.8 mg/L × 119 min = 95 mg·min/L. 119 reports T alone, 149 divides T by the residual instead of multiplying, and 381 treats the 24-inch diameter as the radius, which quadruples the volume.
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Topic coverage follows Expected Range of Knowledge for Drinking Water Distribution Exam (SWRCB Division of Drinking Water, rev. 4/2020). Practice questions are written only from publicly available sources (that outline and the regulations and references it cites); they are not real exam questions, which are confidential. Check current application requirements, fees and exam dates on the certifying body's own page: official certification page.
Questions, flashcards, and explanations are AI-written from publicly available sources only: the exam content outlines and candidate materials each certifying body publishes without restriction, and the regulations and technical references they cite. No confidential material is used, including actual exam questions, recollections of any exam sat by the author, or any handbook or document released under a confidentiality agreement. Content is then put through repeated review — regulatory values checked against primary sources, every calculation recomputed, and errors corrected — plus spot checks by the author, who holds California water operator certifications and a California civil engineering licence. Those spot checks are sampling, not a professional review: nothing here is stamped, sealed, or offered as engineering advice, and not every item has been individually verified, so mistakes can still slip through.