Water Supply and Distribution
Water Supply and Distribution
Water supply systems must provide adequate quantity, pressure, and quality. Candidates must understand demand calculations (average day, maximum day, peak hour), storage requirements (fire flow plus equalization), distribution system hydraulics (pipe sizing, head loss, pressure zones), pump stations, and water quality (treatment, disinfection, cross-connection control). Key regulations include the Safe Drinking Water Act and local design standards.
Traps include: confusing maximum day demand with peak hour demand. Maximum day is the highest 24-hour demand; peak hour is the highest hour within a day. Storage must handle fire flow plus maximum day, while distribution mains must meet peak hour plus fire flow. Another trap is fire flow: required fire flow depends on building type, area, and construction; it is often 1,000-3,500 gpm for commercial areas. Also, pressure must be maintained: typical minimum residual pressure is 20 psi during fire flow, and normal operating pressure is 40-80 psi. Head loss calculations (Hazen-Williams) are common; the formula is h_f = 4.73 × L × (Q/C)^1.852 / d^4.87 (in SI) or similar in English units.
Worked example: A community has an average day demand of 0.5 mgd, a maximum day factor of 1.8, and a peak hour factor of 2.5. Fire flow requirement is 1,500 gpm for 2 hours. What is the required storage volume? Storage = fire flow volume + equalization volume. Fire flow volume = 1,500 gpm × 120 min = 180,000 gallons = 0.18 MG. Equalization volume is often taken as 25% of maximum day demand: 0.25 × (0.5 × 1.8) = 0.25 × 0.9 = 0.225 MG. Total storage ≈ 0.18 + 0.225 = 0.405 MG. Some methods use peak hour demand for equalization; always follow local criteria.
Rule of thumb: For small systems, storage ≈ maximum day demand plus fire flow. A common rule is to provide at least 1 day of average demand for emergency storage.