Distribution System Operation and Maintenance
Core concepts
Master routine O&M: main flushing (unidirectional and conventional), valve exercising and recordkeeping, hydrant inspection and flow testing, leak detection and surveys, main repair and disinfection of new/repared mains, storage tank inspection and turnover, and corrosion/scale control. Understand how operations affect water quality-stagnation, dead ends, low velocity, and biofilm growth-and how to manage them (flushing programs, looping, tank mixing). Know the elements of a cross-connection-free, pressure-maintained system and the importance of maintaining positive pressure during repairs.
Traps to watch
A classic trap: flushing is not just for dirty water-it manages stagnancy and disinfectant residual. Distinguish unidirectional flushing (higher velocity, directed flow, better scour) from conventional flushing. Questions may test that new and repaired mains must be disinfected, flushed, and bacteriological-sampled before service. Do not confuse valve exercising (operability) with valve inspection. Remember that water quality complaints (taste, odor, discoloration) often trace to hydraulic or maintenance causes, not treatment failure.
Worked example / rule of thumb
Flushing velocity rule of thumb: target at least 2.5–3 ft/s in the main to scour sediment; compute flow needed from pipe area. Example: an 8-inch main (area ≈ 0.349 ft²) at 3 ft/s needs about 1.05 cfs ≈ 470 gpm. If your hydrant cannot deliver that, the segment may not scour effectively.
Exam strategy
Link each O&M activity to its purpose (scour, residual, operability, public health) and pick the answer that protects pressure and quality.