Collection System Model Refines Scope of Alternatives to Control SSOs on Oahu

The City and County of Honolulu (CCH) Department of Environmental Services is in the process of developing the Honouliuli/Waipahu/Pearl City Wastewater Facilities Plan, which covers the sewershed tributary to the Honouliuli Wastewater Treatment Plant (WWTP) in the developed areas around Pearl Harbor on the island of Oahu. Previous planning activities had identified the need for equalization storage at three major pump stations in the Honouliuli WWTP tributary area to control surcharging and SSOs in the 2-year, 6-hour storm. Subsequent to those planning activities, a new alternative was identified that would involve a storage/conveyance tunnel that would eliminate the need for near-surface storage tanks, and would also eliminate the need for a number of existing pump stations. As part of the work to further evaluate the tunnel alternative, a calibrated InfoWorks model was developed for use in sizing and evaluating the alternatives. Use of the model has allowed for a more sophisticated dynamic evaluation of combinations of near-surface equalization with interceptor relief, as well as a more rigorous evaluation of the performance of the conveyance/storage tunnel alternative. As a result, the investment in developing a calibrated InfoWorks model has reduced the scope and cost of potential alternatives to control SSOs in the design storm event.