To reduce combined sewer overflows, the City of Edmonton implemented real time control (RTC) in its wastewater collection system. The RTC#4 and RTC#6 sites are designed to control flows and take advantage of the available inline retention in the upstream collectors. However, both sites demonstrated operational issues, such as instability in the process and excessive gate movements. Attempts to stabilize control failed. Tetra Tech CSO was then commissioned to review the existing control logics and implement a new control strategy to resolve the persistent problems.
Tetra Tech CSO carried out a detailed analysis of the actual control logics, existing control strategies and performances, including a review of raw data from past rainfalls, equipment specifications, field installations, and the ladder logic code programmed in the programmable logic controllers (PLC). The findings confirmed that some of the control strategies were not optimum regarding the site hydraulic behaviors and would need to be revised. To overcome the risks and limitations of onsite testing, a new methodology and tool were developed to perform off site simulations to test different control strategies and fine-tune the PID controllers for difficult real-time application under various rainfall events. The simulations were programmed to mimic the actual hydraulic conditions at the sites, which took into account all equipment specifications (i.e. gate size, actuator speed and instrumentation location).
The optimal control strategy was programmed by Tetra Tech CSO, developed through the offline simulation onsite at RTC#4 in October 2011. Performance monitoring in wet weather conditions at RTC#4 through the spring of 2012 showed that optimum performance was reached using the new controllers. Consequently, the new control strategy for RTC#6 was programmed and implemented in June 2012, and also demonstrated optimum performance. All operational and control issues were resolved at these two sites.
“It is great to work with people who understand these RTC systems as well as your team does. Your team has a great understanding of the RTC facility’s purpose, and the mechanical equipment, hardware and software used to run them. The results have been great, and have come faster and with fewer problems than past experiences with other consultants. Your quick response to our facility issues was much appreciated.”
– Sid Lodewyk, General Supervisor of Pumpwells, Drainage Operations, City of Edmonton