Overview
PE depth exam study guide for drinking water: storage sizing, distribution networks, coagulation, sedimentation, filtration, and disinfection CT.
This topic accounts for 6 out of 40 questions on the PE Civil Water Resources Depth (PM) exam.
Key Concepts
Storage Tank Sizing
Total storage = equalizing + fire + emergency. Equalizing storage handles peak hour demand exceeding supply rate. When supply = max day demand, equalizing ≈ 15-25% of max day. Fire storage = required fire flow x duration (from ISO/IFC tables). Emergency storage = 25% of average daily demand (typical). Average daily demand = population x per capita demand (gpcd).
Distribution System Analysis
Hazen-Williams for pipe sizing. Minimum pressure: 20 psi at fire flow, 35 psi normal. Minimum pipe: 6-inch for mains. Hydraulic grade line analysis: HGL = elevation + pressure head. Pressure at any node = (HGL - elevation) / 2.31. For parallel pipes: same head loss, flows add. For series pipes: same flow, head losses add.
Coagulation and Flocculation
Coagulation destabilizes particles (rapid mix: 1-3 min, G = 300-1000 s^-1). Common coagulants: alum (Al2(SO4)3), ferric chloride (FeCl3), polyaluminum chloride (PACl). Chemical dose: lb/day = MGD x mg/L x 8.34. Flocculation promotes particle growth (slow mix: 20-40 min, G = 20-80 s^-1). Gt product: 10,000-100,000.
Sedimentation
Type I (discrete): Stokes' law settling. Type II (flocculent): requires settling column tests. Overflow rate (SOR) = Q/A controls removal of discrete particles. Design SOR: 500-1000 gpd/ft^2. Detention time: 2-8 hours. Tube settlers increase effective surface area (SOR = Q/A_projected). Lamella plates work similarly.
Filtration
Rapid sand filtration: rate = 2-6 gpm/ft^2. Effective size (D10) = 0.45-0.55 mm. Uniformity coefficient (Cu = D60/D10) ≤ 1.65. Filter run time: 24-72 hours. Backwash rate: 15-25 gpm/ft^2 for 5-15 min. UFRV (unit filter run volume) = filtration rate x run time. Net production = gross - backwash volume. Slow sand: 0.05-0.10 gpm/ft^2.
Disinfection (CT Values)
Required CT from EPA tables depends on: organism (Giardia, viruses, Crypto), temperature, pH, disinfectant type. For free chlorine at 10°C pH 7: 3-log Giardia ≈ 104 mg·min/L. CT_actual = C x T10 = C x BF x V/Q. Multiple barriers: log credits add (e.g., 2-log filtration + 1-log chlorine = 3-log total). Surface water treatment rule: 3-log Giardia, 4-log virus removal/inactivation.
Common Exam Question Types
- Size a water storage tank (equalizing + fire + emergency)
- Calculate pressure at a node in a distribution system
- Determine chemical feed rate for coagulant dosing
- Design a sedimentation basin (SOR, detention time, depth)
- Calculate net filter production and UFRV
- Verify CT compliance for disinfection
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