Wastewater Treatment & Disposal

The Waimānalo Wastewater Treatment Plant (WWTP) is a secondary treatment facility that serves communities along the Windward coast of Oʻahu—from Makapuʻu Point to Bellows Air Force Station. While the WWTP supports a significant portion of the area, some neighborhoods continue to rely on individual systems such as cesspools and septic tanks.

Originally constructed in 1969 and placed into service in 1972, the facility was built by the state of Hawaiʻi’s Department of Land and Natural Resources (DLNR) and operated by the City and County of Honolulu under a long-standing agreement. In 2012, ownership of the plant was officially transferred to the city following a $28 million upgrade by the state to modernize operations.

Today, the plant processes wastewater through secondary treatment and advanced filtration. Treated effluent is safely discharged via 10 underground injection wells, each reaching depths of approximately 200 feet. Sludge generated during treatment is transported to the Kailua Regional WWTP for further processing before final disposal at H-POWER.

The Waimanalo WWTP operates under an Underground Injection Control (UIC) Permit (Permit No. UO-1259) issued August 30, 2023, and expiring August 29, 2028.  The UIC permit authorizes the City and County of Honolulu to operate an injection well system consisting of 10 injection wells to dispose of secondary treated domestic wastewater effluent in accordance with effluent limitations, monitoring requirements and other permit conditions.