Key Takeaways & Executive Findings
- •• • 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.
Abstract
This study investigates pollutant removal characteristics and kinetic behaviors in sewage networks, and analyzes their impact on the carbon-to-nitrogen ratio (C/N, as COD/TN) of influent to wastewater treatment plants. Source water quality sampling at drainage outlets revealed spatial and temporal variations in C/N, with domestic sewage exhibiting higher C/N than industrial sewage, and diurnal peaks reaching 6.92 versus 4.71 during off-peak hours. Using a pilot-scale adjustable sewage network system in Kunshan, experiments were conducted under high (0.491 m·s−1) and low (0.089 m·s−1) flow velocities, monitoring pollutant removal over 144 hours. Pseudo-first-order kinetics were applied to model COD and TN removal. Results showed that COD (including SCOD and PCOD), BOD5, and SS achieved approximately 80% removal within 144 h, with higher removal at low flow velocity. TN, NH3-N, and TP exhibited lower overall removal rates. Kinetic fitting revealed that COD removal rate constants (kCOD) were significantly higher than those for TN (kTN), and both decreased with increasing flow velocity: at low velocity, kCOD=0.0167 h−1 and kTN=0.0029 h−1; at high velocity, kCOD=0.0127 h−1 and kTN=0.0020 h−1. Simulations based on actual source pollutant concentrations indicated that the time for C/N to drop to the denitrification critical value of 4.50 was 12.24 h at high velocity, but shortened to 9.49 h at low velocity. These findings demonstrate that increasing flow velocity effectively retards the decline of C/N. Therefore, regulating network flow velocity to reduce hydraulic retention time is a key strategy for maintaining adequate C/N at the terminal and ensuring denitrification efficiency in wastewater treatment plants.
1. Introduction
Municipal wastewater treatment plants across China are increasingly challenged by low influent carbon-to-nitrogen ratios (C/N), which compromise biological nitrogen removal and necessitate costly external carbon supplementation. Previous studies have documented that pollutants undergo partial removal within sewer networks, with reported COD removal contributions up to 15.91%, and that COD changes correlate positively with hydraulic retention time. However, the dynamic impact of in-sewer transformation on terminal C/N has not been systematically quantified, leaving a critical gap in understanding the root causes of carbon deficiency at treatment plants.
This study addresses this bottleneck by employing a controlled pilot-scale sewer system to simulate real municipal networks under two distinct flow regimes, monitoring pollutant removal over 144 hours. By applying pseudo-first-order kinetics to COD and TN removal and integrating actual source water quality data, the research quantifies how flow velocity governs the rate of C/N decline. The findings provide a mechanistic basis for optimizing sewer operation—specifically, increasing flow velocity to shorten hydraulic retention time—as a viable strategy to preserve carbon availability and enhance downstream denitrification efficiency, offering a practical pathway for integrated sewer-plant management.
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FENG Ruodan, LI Cuimei, ZHU Chunwei, ZHOU Lifen, ZUO Shu, ZHANG Jian (2026). Kinetic Analysis and Simulation of Pollutant Removal in Sewage Networks. Chinese Journal of Environmental Engineering. https://doi.org/10.12030/j.cjee.202507064
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Frequently Asked Questions
What are the specific removal efficiencies for COD, BOD5, and SS under low and high flow velocities after 144 hours?
The study reports that COD (including SCOD and PCOD), BOD5, and SS all achieved approximately 80% removal within 144 hours. Notably, removal efficiencies were higher under low flow velocity (0.089 m·s−1) compared to high flow velocity (0.491 m·s−1), likely due to enhanced sedimentation and biofilm contact at lower shear stresses.
How do the pseudo-first-order rate constants for COD and TN removal differ, and what are their implications for C/N dynamics?
At low flow velocity, kCOD was 0.0167 h−1 and kTN was 0.0029 h−1; at high flow velocity, kCOD was 0.0127 h−1 and kTN was 0.0020 h−1. COD removal is approximately 5-6 times faster than TN removal, meaning carbon is consumed preferentially, leading to a progressive decline in C/N as sewage travels through the network. This explains why longer retention times exacerbate carbon deficiency at treatment plants.
What is the critical C/N threshold for denitrification, and how does flow velocity affect the time to reach it?
The critical C/N threshold for denitrification is 4.50 (COD/TN). Based on simulations using actual source water concentrations, the time for C/N to drop to this threshold was 12.24 hours under high flow velocity (0.491 m·s−1) and 9.49 hours under low flow velocity (0.089 m·s−1). Thus, higher flow velocity extends the time before carbon limitation occurs, providing a buffer for downstream treatment.
What are the practical implications of these findings for sewer network operation and wastewater treatment plant performance?
The results suggest that by increasing flow velocity in sewer networks, operators can reduce hydraulic retention time, thereby slowing the rate of C/N decline. This helps maintain adequate carbon availability at the treatment plant influent, reducing the need for external carbon addition and improving denitrification efficiency. This supports integrated management of sewer networks and treatment plants to optimize overall performance and reduce operational costs.
How were the experiments designed to simulate real sewer conditions, and what were the key controlled parameters?
The study used a pilot-scale sewer system with pipe diameters of 500 mm (inner loop) and 400 mm (outer loop), total lengths of 81.92 m and 195.3 m, and slopes of 0.37% and 0.23%, respectively. Two flow regimes were established by adjusting water levels: high flow with a fill ratio of ~25% and average velocity of 0.491 m·s−1, and low flow with full fill and average velocity of 0.089 m·s−1. The system operated in recirculation mode, drawing real municipal sewage from a pump station, and pollutant concentrations were monitored over 144 hours.
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