Heavy Rains Trigger 286,000-Gallon Wastewater Spill at Waimea Plant

A powerful Kona low wrung 285,750 gallons of wastewater from the Waimea Wastewater Treatment Plant between 8 p.m. Friday and 6 a.m. Saturday, as heavy rains overwhelmed the facility's capacity and forced an overflow from its effluent tank.

The spill was not raw sewage. Officials said the wastewater had already completed the biological treatment process and passed through ultraviolet disinfection before backing up into the effluent tank and overflowing. The discharge was confined on-site, though officials acknowledged some of it may have entered adjacent drainage ditches that do not connect to state waters. Additional disinfection was performed in the affected areas.

The cause, according to Kaua'i County officials, was a combination of high flows from the storm's rainfall and heavy infiltration and inflow into the sewer system driven by the Kona low. That same storm system knocked down trees in Pahoa and Nanawale Estates on Hawai'i Island and triggered multiple wastewater overflows on O'ahu, where the Honolulu Department of Environmental Services reported that storm runoff and increased sewer inflow led to several discharges. Honolulu crews were still calculating the total volume released.

The Kaua'i County Wastewater Management Division said it is pursuing immediate repair projects to address the infiltration and inflow vulnerabilities the storm exposed. The division is also working with the Hawai'i Department of Health to identify options and expand the capacity of injection wells and reuse distribution systems to handle elevated storm flows in the future. Separately, Kaua'i County Public Works Roads Division crews spent Friday evening clearing drains, culverts, roadways, and other county infrastructure as the storm bore down on the island.

Officials cautioned that the storm had not fully passed and that additional spills remained possible depending on where the heaviest rain fell next. Despite some improvement in conditions, residents were urged to stay vigilant and monitor weather updates through local media and official government channels.

Anyone with questions about the Waimea spill can contact Donald Fujimoto with the Wastewater Management Division at 808-241-4083 or dfujimoto@kauai.gov.