Collection System Operations and Maintenance
What the exam expects
Operations and maintenance questions test whether you understand how a collection system behaves as a whole, not just individual components. Know the difference between preventive maintenance (scheduled inspection, cleaning, root control, lubrication) and corrective maintenance (repairing a failure after it occurs). Understand that flow velocity drives most O&M decisions: too slow and you get solids deposition and sulfide generation; too fast and you get scour and hydrogen sulfide corrosion. A common rule of thumb is to keep velocity between about 2 ft/s (self-cleansing) and 10 ft/s (scour threshold) in gravity sewers.
Concepts to master
- Gravity sewer cleaning methods: hydraulic flushing, mechanical rodding, bucket machines, and high-velocity jetting; know when each is appropriate.
- Root intrusion control, grease control, and the role of the O&M program in SSO prevention.
- Infiltration vs. inflow: infiltration enters through defects (groundwater), inflow is direct stormwater connection.
- Capacity, surcharging, and the difference between a maintenance hole and a manhole.
- Odor and corrosion control basics: sulfide generation in slow, warm, anaerobic flow.
Traps
Questions often confuse infiltration and inflow, or ask which cleaning method suits a specific blockage. Read for the pipe material, size, and nature of the obstruction. Another trap: assuming more cleaning frequency is always better; the exam wants risk-based scheduling.
Worked example
A 12-inch gravity sewer carries flow at 1.5 ft/s. Is it self-cleansing? No - below the ~2 ft/s target, so schedule more frequent cleaning and investigate the cause (low flow, upstream diversion, or infiltration).