Wastewater Treatment & Disposal

The Wahiawā Wastewater Treatment Plant (WWTP), originally constructed in 1928, is the oldest municipal treatment facility on Oʻahu and remains the only one that discharges into a freshwater body – Wahiawā Reservoir. Serving Wahiawā Town, Whitmore Village, and the U.S. Navy’s NCTAMS EASTPAC facility, the plant plays a critical role in Central Oʻahu’s wastewater management system.

Over the decades, the facility has undergone significant upgrades, transitioning from a primary treatment plant to a secondary activated sludge system in 1967, and eventually to a tertiary-level treatment plant in 2002 – one of the highest levels of wastewater treatment.  Although the Wahiawā WWTP is not officially certified by the Hawaiʻi Department of Health (DOH) as an R-1 facility.

The facility underwent two major upgrades – the “front end” and “back end” projects. The completed front end project upgraded the influent pump station and headworks with new bar screens, wet wells, pumps, grit removal systems, and hard-piped flow equalization facilities.  A biofilter system was also installed to minimize odors for nearby residents, and the plant’s SCADA system was modernized to support a membrane bioreactor (MBR) system, which replaced the old secondary treatment process.

After the effluent is treated by the MBR system, it flows to a new low pressure, high intensity ultraviolet (UV) disinfection facility that is up to three times more energy efficient than the old system, providing both secondary and tertiary treatment exceeding R-1 quality requirements.  The treated effluent is then discharged to the Wahiawā Reservoir or may be eventually pumped to future recycled water users via a new R-1 water pump station.  The project also includes conversion of the secondary clarifiers into flow equalization basins; a new bio-trickling filter for odor control; and associated electrical, instrumentation, paving, and grading.

The plant processes approximately 1.30 million gallons per day (mgd) and is designed for an average capacity of 2.49 mgd.  Solids are thickened on site and hauled to the Honouliuli WWTP for further processing.  Final effluent is discharged into Wahiawā Reservoir through a 24-inch outfall equipped with diffusers located 44 feet below the surface.

The Wahiawā Reservoir itself is a historic irrigation resource, formed by a dam built in 1906 by Waialua Sugar Company’s predecessors.  It captures the confluence of the north and south forks of Kaukonahua Stream and remains a key freshwater body for the region.

The WWTP operates and is authorized to discharge under a Consent Decree that applies the requirements of a National Pollutant Discharge Elimination System (NPDES) permit, which expired March 1, 1994.  The Consent Decree authorizes the City and County of Honolulu to discharge tertiary treated wastewater from the facility to the Wahiawā Reservoir in accordance with the effluent limitations, monitoring requirements, and permit conditions.