• • COD, BOD5, and SS achieved approximately 80% removal within 144 h in sewage networks, with higher removal at low flow velocity (0.089 m·s−1) compared to high flow (0.491 m·s−1), indicating that longer retention enhances physical sedimentation and biodegradation of particulate organics.
• • Pseudo-first-order kinetic rate constants for COD removal (kCOD) were 0.0167 h−1 at low velocity and 0.0127 h−1 at high velocity, while TN removal constants (kTN) were 0.0029 h−1 and 0.0020 h−1, respectively, demonstrating that COD removal is 5-6 times faster than TN, leading to preferential carbon consumption and C/N decline.
• • Source water C/N exhibited significant temporal variation: peak hours reached 6.92, while off-peak dropped to 4.71, and domestic sewage had higher C/N than industrial sewage, highlighting the need for real-time monitoring and management of discharge patterns.
• • Simulation based on actual source concentrations showed that C/N dropped to the denitrification threshold of 4.50 after 12.24 h at high flow velocity versus 9.49 h at low flow velocity, proving that increasing flow velocity can extend the time before carbon limitation occurs, thereby preserving denitrification capacity in downstream treatment plants.