Wastewater Treatment & Disposal

Serving as a cornerstone of environmental protection for Oʻahu’s Leeward Coast, the Waiʻanae Wastewater Treatment Plant (WWTP) plays a critical role in safeguarding public health and preserving the coastline’s natural beauty. Strategically located in Waiʻanae, the facility supports the wastewater treatment needs of surrounding communities, including Nānākuli, Lualualei, Māʻili, Waiʻanae, and Mākaha.

The plant began operations in 1968 as a primary treatment facility with a capacity of 1.72 million gallons per day (mgd). In response to growing community needs and evolving environmental regulations, major upgrades were implemented in 1988 and again in 1996. These improvements increased treatment capacity to 5.2 mgd with a peak flow of 13.8 mgd and elevated the facility to full secondary treatment capabilities, significantly enhancing the quality of treated effluent.

Wastewater is conveyed to the plant via a coastal pipeline that parallels Farrington Highway, supported by two pump stations in Nānākuli. After undergoing a rigorous treatment process, the treated effluent is safely discharged through a deep ocean outfall located 6,184 feet offshore at a depth of about 107.5 feet—minimizing impacts to nearshore waters and promoting marine ecosystem health.

The plant is also committed to responsible solids management. Biosolids produced during treatment are stabilized and dewatered before being either transported to H-POWER for renewable energy generation or disposed of at the Waimānalo Gulch Sanitary Landfill.

As part of ongoing efforts to modernize infrastructure and ensure regulatory compliance, the city implemented a series of Capital Improvement Projects (CIP). Under Upgrade 1, the following enhancements were completed:

• Replacement of headwork screens and associated equipment
• Rehabilitation of the grit removal system
• Refurbishment of pre-aeration tanks and channels
• Structural upgrades to the primary clarifiers and associated effluent boxes and piping

Other key improvements, include:

• Upgrades to the pumping systems of Digesters 1 and 2, along with structural reinforcement of Digester 2
• Installation of climate control systems for electrical components within the Influent Pump Station
• Integration of a new screw press dewatering system and associated piping to improve biosolids processing

Currently, the plant is in the midst of Upgrade 2, continuing its long-standing commitment to reliable service and environmental stewardship for future generations.

The WWWTP operates under a National Pollutant Discharge Elimination System (NPDES) Permit effective as of Dec. 1, 2016, and expires Nov. 30, 2021. The plant is under an administrative extension with the most recent issued June 28, 2024, and expires June 30, 2025.  The NPDES permit and applicable Zone of Mixing (ZOM) authorize the City and County of Honolulu to discharge secondary treated wastewater from the facility to receiving waters (Pacific Ocean) through its deep ocean outfall in accordance with the effluent limitations, monitoring requirements and other NPDES permit conditions.