Cost-effective advanced solutions for reducing combined sewer overflows (CSOs) in wastewater networks

SUEZ SEVESC
The issue
SEVESC (Société des Eaux de Versailles et de Saint Cloud), a SUEZ company, is in charge of operating and maintaining the wastewater collection network of Paris' western suburbs. The company's network manages around 650 km of sewer drains and 40 pump/lift stations. Within this network are two types of sewer system: combined sewer areas (30% of the system) and sanitary sewer areas (70% of the system).

In order to manage and control combined sewer overflows (CSOs), the combined sewer areas hold structures, such as regulators, low-level openings, and high-level overflow weirs, to help regulate the flow of water in the sewer system to prevent issues.

One cause for concern for SEVESC, however, is the frequency and severity of CSO events in the area that are causing significant problems. Apart from adverse environmental issues, these events also posed a threat to the reliability of the wastewater network. Moreover, the existing CSO control structures needed modifications to reduce spills without resorting to costly pipe replacements.
The solution
Recognising the need for a comprehensive solution, SEVESC collaborated with Optimatics to tackle the CSO problem head-on. Optimatics, a SUEZ company, is the world's leading provider of infrastructure planning software for water and wastewater utilities. It offers OptimizerTM,  a cloud-based software solution for optimisation that provides a thorough analysis of water distribution and collection systems.

The project employed OptimizerTM for ICM, an advanced decision support tool for collection systems. The optimisation process considered various types of summer storm events that occur at different intervals – every 6 months, 1 year, 2 years, 5 years, and 10 years. This approach was taken as the majority of the network's CSO discharge volume occurs during these storm events.

The team assessed two separate catchments – System 1 and System 2. The key to success for System 1 lay in maximising the use of existing infrastructure and optimising the existing CSO control structures. Rather than opting for high-cost replacements, the focus was on strategic modifications that included adjustments to regulator structures, lift stations, weirds, pumps, and sluice gates. These decisions were configured as Real Time Control (RTC) decisions as well as manual settings.

For System 2, additional pumps, in-line storage, and extension pipes (both with diameter range options) were considered with associated costs.
The result
The optimisation process resulted in a range of solutions, significantly enhancing the hydraulic performance of the system. The selected strategy showed a predicted 42% decrease in CSO discharge volume for System 1 and a 14% decrease for System 2.

Notably, the selected strategy for System 2, involving an additional pump and existing conditions, is expected to achieve a remarkable 23% reduction in CSO discharge volume.

These results underscore the success of the collaborative effort between Optimatics and SEVESC in addressing CSO challenges in the western suburbs of Paris. The reduction in CSO spills was accomplished without compromising the hydraulic integrity of the region, proving the effectiveness of the implemented modifications.