Law of Pollutant Erosion and Deposition in Urban Sewage Network

In order to investigate the scouring and deposition law of pollutants in urban sewer systems, the investigation of sewer systems was carried out in Xi'an. The results showed that the thickness of sediment in the branch pipe and the main pipe varied obviously. At the peak of the drainage, the velocity increased gradually, and the thickness of the pipeline sediment decreased. At the bottom of the drainage trough, the changes of the velocity and the thickness of the pipeline sediment were opposite, with variations of 0-24 mm and 0-12 mm, respectively. And the probability of granular contaminants sedimentation and scouring in sewer systems was high, while the thickness of sediment in sewage main pipe was less than the above mentioned pipes, the variation of which was 0-7 mm. In addition, the sedimentation and scouring in the main pipeline kept relatively balanced and the thickness of sediment remained stable. In order to clarify the relationship between the concentration of pollutants and the flow velocity, a pilot sewer system was established. The effects of different flow rates, which were 0.1, 0.3, 0.6, 0.9, 1.2 m·s^-1, respectively, on the contents of carbon (organic), nitrogen and phosphorus in the pipeline were studied. The results showed that with the increase of the flow velocity, the scour intensity increased and the concentration of pollutants in the pipeline also increased sharply. As shown by the monitoring results of the static light scattering particle size analyzer, the carbon organic pollutants in the pipeline were easily adsorbed on the larger particles, while nitrogen and phosphorus pollutants were easily adsorbed on the particles with smaller size. Analysis on the change of the urban sewage pipeline showed, when the flow rate was less than 0.6 m·s^-1, the sedimentation of granular pollutants in the sewage was greater than the scouring effect. When the flow velocity was higher than 0.6 m·s^-1, the water scouring intensity increased and the scouring action was greater than the sedimentation, meanwhile, the sediments were carried by the water and the thickness of the sediments was reduced. The increase of the organic pollutants in the sewage was bigger than the nitrogen and phosphorus pollutants, so that the existing sewage carbon source was improved.