Interceptor sewers
The paper addresses the problem of beach and shoreline pollution from combined storm overflow (CSO) structures located on long coastline-tracking interceptor sewers. In particular, it examines the scope of active control, so-called Real-Time-Control [RTC], against normal, passive, fixed-control methods in reducing the pollution impacts along the coastline.
A novel optimal pollution control (OPC) methodology, based on dynamic real time control of storm overflow spill gate releases, is developed to minimise total spill load to receiving waters. This is derived from a formal optimisation applied to a simplistic hydraulic representation of the long trunk sewer, under a so-called ‘slug flow’ approach. Combined sewer overflow (CSO) structures, at intermediate points of connection of contributory sewer catchments, are represented and the model tracks the pollution concentrations in the CSOs as well as incoming pollution loads in arriving at its optimal spill strategy at each such structure. In essence, in the current application, the strategy strives to defer spill to coast or river as far as is possible by utilising any available storage volumes and, when unavoidable, to first release sewage from CSO structures carrying the lowest pollution loads for minimum receiving water impact.
This paper gives a resume of an application of the approach to a case study formed by part of the Liverpool Interceptor Sewer system, running along the northern bank of the River Mersey estuary in the UK. Performance of the system is synthesised for a typical year of operation when subject to a relevant long-term rainfall regime rainfall. Four control procedures are considered: fixed (passive) local control (FLC), variable (dynamic) local control (VLC), restricted VLC, and the dynamic and global optimal pollution control (OPC).
The results show that by adopting dynamic real-time-control, and specifically the OPC methodology, substantial reductions in pollution over-spill load can be achieved when measured against normal (and existing), passive, fixed local control strategies. This reduction is as much as 70% in the case study presented. Furthermore, in application to moving storms, resulting in greater temporal and spatial variability in the arrival of run-off from contributing catchments, the superior capabilities of the global OPC strategy are amply demonstrated against the other candidate approaches. The OPC approach is computationally efficient and it remains only for practicability constrains relating to CSO control gate operations to be incorporated before its implementation could be actively promoted.
The authors would like to thank United Utilities (North West Water) Limited and Liverpool City Engineers Department for their assistance in the supply of data relating to the Liverpool Interceptor Sewer system. The work on application of the Unit Hydrograph flow synthesis procedure was partly funded by the UK EPSRC under contract GR/L88269.