Influence of discharge modes of the tail water from a municipal sewage plant on river water quality

The establishment of the municipal sewage plants has significantly improved the urban water environment, while their tail water has a certain impact on urban river´s water quality, especially on the near-field. Currently, there are two main discharge modes of the tail water, namely centralized and decentralized discharges. As the increase of river restoration projects, for example riverbank treatment systems, the discharge modes of the tail water are diversified. In this study, the COD concentration field is simulated in near-field of sewage outlet under three discharge modes, including single-hole (S1), five-holes (S2) and uniform overflow (S3) along a 150m riverbank. Results show that relative concentration (C/C₀, C is the average concentration of observation section; C₀ is the concentration of sewage outfall) of COD at the downstream sections accords with S3<;S2<;S1; comparing with S1, the high concentration area in near-field under the condition of S3 and S2 are obviously decreased by 1.53% and 1.393% respectively during open-gate period and by 0.683% and 0.683% during close-gate period. Decentralized discharge mode speeded up the diffusion of pollutants and decreased significantly the high concentration area in the near-field of sewage outlet. Under a permit condition, the uniform overflow (S3) would be a proposed discharge mode.