Characteristics of water quality variation in sewer networks

Simulated sewer networks system was used to study variation of water quality in sewer system. Results showed that under the condition of 6 h of hydraulic flow, the total COD (TCOD) removal efficiency in urban sewage is 10.79% because of sedimentation and biodegradation process. Furthermore, 68.85% of the COD removal is attributed to sedimentation effect while the rest is related to 31.15%. Meanwhile, removal efficiency of TN in sewer is 11.15% and sedimentation and biodegradation process account for 50% respectively. Transformation of nitrogen pollutants caused by biodegradation were mainly conversion of organic nitrogen to NH₃-N (NH₃-N rises from 35.51 mg/L to 38.38 mg/L) and removal of NO₃-N caused by denitrification process(NO₃-N drops from 2.36 mg/L to 0.84 mg/L). The elimination of the total phosphorus is relatively small, which is because of stable anaerobic environment in the sewer networks.