WAK003A Wake Island Sewer Redesign

The Coffman team developed three working options for this project that provided the redesign of the previous sanitary sewer system design for Wake Island to minimize the impact of unexploded ordinances (UXO) requirements by limiting the depth of site excavation. The team developed three working options for evaluation, with estimated construction costs for each, to determine Life Cycle Cost Analysis for each option. The three options were evaluated by the end user to find a solution that provided long-term ease of maintenance while minimizing the upfront costs and project risks. Due to the remote location of Wake Island, the goal was to provide a simple and robust design.

Wake Island is a land mass of 2.7 square miles located approximately 2,300 miles west of Honolulu. Based on the remote location, new equipment and structures on the air base are uniquely designed to resist the severe wind loads and other load criteria as adopted by the Unified Facilities Criteria (UFC). The original design scope allowed the AE team to evaluate the existing distribution and treatment systems to provide a viable, cost-effective, and long-term solution to the island’s ailing infrastructure.

Challenges of this project included documenting and understanding the systems constructed over a period of 60 years, with many additions and undocumented changes made along the way. Based on the field assessment by the AE team, several of the existing buildings slated for connection to the new system had severely deteriorated and were not code-compliant waste piping systems. The system design was further complicated by the need to import sand and gravel for fill and to make concrete as the local coral aggregate was inadequate for construction use. The new design of the systems and equipment enclosures withstand typhoon-force wind loads; building materials and equipment were selected to withstand the highly corrosive environment on the island. The AE team provided design options for advanced effluent treatment and pipe routing based on cost, functionality, and island maintenance ability. A modular, expandable treatment system was designed to allow for future expansion.

The civil scope of the project included over six miles of sewer piping. The sewer system redesign was challenging because the island has very little elevation change. Based on coordination between the AE and project stakeholders, the system was redesigned utilizing vacuum sewer technology to allow for routing wastewater in a flat area while minimizing excavation depth. This approach reduced the project risk of exposure to UXO, minimizing the mechanical equipment required and consolidating the vacuum pumps and lift station in one area. The final solution was also designed to accommodate future growth by providing seamless connection points for future infrastructure.

Due to its corrosion-resistant properties, a pre-manufactured fiberglass building was utilized for the vacuum building. All reinforcing elements were epoxy coated, and all other metals were either nonferrous or properly coated to perform adequately in Wake Island’s extremely corrosive environment. In addition to the vacuum building, a concrete drying bed and concrete ballast for buried tankage were designed to support the advanced sewage treatment system. Seismic restraint was also provided for mechanical and electrical equipment. All systems were designed to reduce the effects of the corrosive saltwater environment of the island.

The mechanical scope included effluent flow design calculations for each building connection, lift station design, coordination with the waste treatment modules, and coordination with onsite utilities. The electrical scope included providing power and communications connections to the lift vacuum building, equipment and waste treatment modules, and close coordination with onsite electrical utilities.