About the Project
The Core Area Wastewater Treatment Plant (CAWTP) Conveyance System serves to connect two treatment facilities operating within the Capital Regional District’s (CRD) recently constructed WWTP. This Conveyance System is considered one of the longest sludge transfer systems of its type in the world and offers the advantage of locating the CAWTP’s biosolids recovery facility beyond the urban setting at the existing CRD landfill facility. Federally mandated requirements for wastewater system effluent discharges established a firm CAWPT program deadline of December 31, 2020 and both the WWTP and RTF would rely on the Conveyance System for commissioning and startup.
The 19km alignment between the WWTP and RTF is characterized as a transition between a sea level urban setting and an elevated rural environment where slurry pumping equipment must overcome 165m of static head. The RSF design criteria was established to transport a sludge product composed of 1.5% to 2.0% solids concentration at a target flowrate of 57 L/s. Conveyance of residual solids over such an extended distance provided design challenges in predicting sludge rheology and designing for dynamic solids concentration that could see fluctuation between Newtonian and non-Newtonian fluids. Chemical composition of the residual solids is also subject to change given the long (10-12 hour) conveyance time. Significant H2S production and nuisance odors were expected challenges in transporting residual solids and a dependable odor control system would be heavily relied upon.
Centrate recovered at the RTF was returned to the municipal sewage system in a parallel pipeline. Complexities associated with this pipeline topography could manifest in unstable flow regimes that would effectively limit pipeline capacity if left unaccounted.
The $60M Conveyance System was designed by Parsons’ Victoria-based team and supported by a variety of specialists working in Vancouver, Seattle, Toronto, and the United Kingdom.
An early assessment indicated 4 interconnected pump stations would be required to convey residual solids between the WWTP and RTF. A submersible duplex wet-well configuration was selected given the benefit of managing complex flow controls using a wireless SCADA system. A PID control loop was used to modulate pump speed based on wet well level to improve WWTP batch release cycles and reduce sludge settling time.
While the preferred operating range and fluid rheology created a challenging operating point, Vaughan 125HP submersible chopper pumps offered an efficient and robust solution. Leak detection and overflow protection was accomplished using a combination of flow and pressure monitoring between stations. Flow disparities could provide early warning of possible leaks or backflow, and sudden pressure drops could register line ruptures. These controls were monitored through SCADA and allowed for automatic line shut-in using actuated system plug valves.
The design team selected a multi-stage bio-trickling and activated carbon odor control unit (OCU). Significant H2S and nuisance odor removals were possible using this technology and a well-defined performance-based specification was developed to allow flexibility from potential OCU vendors.
The design team addressed hydraulic concerns associated with the centrate return line by establishing the system as a pressurized drain. Downstream controls were established with modulating valves to prevent atmospheric breaks.
The CAWTP Conveyance System successfully and safely delivered residual solids to the RTF and returned centrate well in advance of program deadlines. Design philosophies and equipment selection prepared by the design team continue to have a lasting effect on the predictable transfer of complex wastewater fluids over a long, challenging pipeline route. The CAWTP Conveyance System will continue to serve the region for many years and operate as a preeminent case study in sludge conveyance.