The largest wastewater treatment plant serving the Vancouver, BC metropolitan area is located on Annacis Island within the Fraser River. The plant provides secondary wastewater treatment to over 1 million residents across 14 municipalities.

The original outfall at Annacis Island, constructed in the early 1970s, was installed by pipe-laying into a dredged trench on the riverbed. With a limited capacity of 14.5 m3/s (1,253 MLD), the plant was unable to meet the region’s growing population demands and the ever-increasing seismic resilience standards. To address these challenges, the Annacis Island WWTP New Outfall System Project (the Project) was designed and is being executed to provide both increased capacity and enhanced durability, while improving overall dilution of effluent in the Fraser River. Given the region’s seismic activity, soil liquefaction and the associated ground deformations are critical considerations when assessing the seismic resilience of existing and new infrastructure.

The Project consists of two 4.2-metre internal diameter conveyance tunnels, two on-land shafts, an in-river riser shaft, and two diffuser arm manifolds. In contrast to the original outfall, the new conveyance tunnels are positioned below the zone most vulnerable to liquefaction, minimizing concerns of susceptibility to ground deformations following a seismic event. Upon construction, the new outfall system will have an ultimate capacity of 25.3 m3/s (2,186 MLD), providing long-term reliability and improved wastewater management for the region.

The Project is being delivered in collaboration with Metro Vancouver, with Hatch overseeing project execution as the Construction Manager, and CDM Smith Canada leading the engineering design. Together, the team is working to successfully implement a new outfall system while prioritizing the region’s stakeholders in all decisions.

Taking an alternative approach to conventional outfall construction, which often relies on dredged trenches in riverbeds, the Project leverages innovative tunnelling technology to position the new outfall tunnels beneath the zone of highest potential for soil liquefaction.

A 5 m diameter slurry Tunnel Boring Machine (TBM) was selected for construction, allowing for precise ground control, along with handling and installation of the precast concrete segments that form the tunnel lining. Hatch and CDM Smith Canada collaborated closely to implement rigorous QA/QC measures, ensuring the tunnel lining’s longevity and reliability.

Connecting the Project’s tunnel drives is the Outfall Shaft, which has been designed with the flexibility to accommodate a future pump station. This proactive design and forward-thinking approach allows the new outfall system to address future sea level rise, mitigating potential challenges to gravitational discharge into the Fraser River.

The in-river diffuser manifolds are engineered and constructed for effective effluent dispersion, minimizing environmental impacts. The diffuser arms include innovative seismic steel pipe sections that can accommodate significant deflections before failure.

By integrating modern tunnelling techniques, a comprehensive seismic engineering design, and close collaboration with the Client, Contractor, and stakeholders, we successfully delivered a robust new outfall system.

The Project has significantly improved the existing wastewater treatment plant’s resilience, ensuring reliable operation after a seismic event and adaptability to future environmental changes. The in-river diffuser manifolds are designed to promote effective effluent dispersion into the Fraser River, minimizing environmental impacts. The Project also supports increasing overall capacity from 14.5 m3/s (1,253 MLD) to 25.3 m3/s (2,186 MLD).