Collection System Operation and Maintenance
How gravity collection systems work
A sanitary sewer carries flow by gravity, so performance depends on slope, roughness, diameter and the actual flow reaching the pipe. Designers aim for a self-cleansing velocity-commonly about 0.6 m/s (2 ft/s) at design flow-to keep solids moving, while avoiding excessive scour at peak flow. Actual velocity varies through the day and drops during dry weather, which is when sediment, grease and roots accumulate.
Condition assessment and maintenance
CCTV inspection records defects such as cracks, offset joints, root intrusions and sags. Smoke testing finds storm connections and illegal drains; dye testing confirms specific connections; flow monitoring quantifies capacity and rainfall response. Cleaning methods include hydraulic jetting, mechanical rodding and bucket machines, with root cutting and grease control as required.
Infiltration and inflow
Infiltration is groundwater entering through defects; inflow is direct stormwater entry through manhole covers, roof leaders and cross-connections. Both consume capacity and raise treatment costs. Hydrogen sulfide is a double concern: odour and toxicity, plus conversion to sulfuric acid that corrodes concrete at the crown.
Traps and a worked example
Questions often confuse infiltration with inflow, assume a design slope equals as-built slope, or imply that jetting alone restores capacity without inspection. Jetting without downstream protection can flood a basement or send debris to a pump station.
Worked example: a 300 mm inside-diameter sewer flowing full at 0.9 m/s. Area = π(0.15 m)² = 0.0707 m²; Q = A × V = 0.0707 × 0.9 = 0.064 m³/s, about 64 L/s. If a new development adds 20 L/s, check the resulting depth and velocity, not just whether the pipe is “big enough”.