Wastewater Treatment & Disposal

The Honouliuli Wastewater Treatment Plant (WWTP), located in southwestern Oʻahu, is a cornerstone of the island’s wastewater system. Built in 1978, the plant has served the growing communities of West Oʻahu for decades. Its deep ocean outfall, completed in February 1979, extends 9,300 feet (1.76 miles) offshore into the Pacific Ocean, where treated effluent is safely discharged at a depth of 200 feet.

The outfall system consists of 19,719 lineal feet of combined land and ocean pipe. The ocean segment includes approximately 10,553 feet of reinforced concrete pipe running from the junction box at Oneʻula Beach Park to the flap gate structure at the ocean terminus. From the junction box to the start of the diffusers is 8,776 feet of 78-inch diameter pipe, with an additional 1,777 feet of diffuser piping ranging in diameter from 78 to 48 inches. The diffuser ports, positioned at the pipe’s springline, range from 3.4 to 6 inches in diameter and allow for effective dispersion of the treated water.

The Honouliuli WWTP services the West Mamala region, which stretches from Hālawa to Makakilo, and includes Barbers Point, ʻEwa, Kapolei, and Mililani. Wastewater reaches the plant through five major interceptor sewer lines from Makakilo (west), Waipio Peninsula (east), ʻEwa Gentry (north), ʻEwa Beach (south), and Barbers Point (southwest, U.S. Navy).

From Primary to Full Secondary Treatment

Originally designed for primary treatment with a capacity of 51 million gallons per day (mgd), Honouliuli took a major step forward in 1996 with the addition of a secondary treatment facility capable of treating up to 13 mgd. This treated wastewater is further polished through tertiary treatment at the Board of Water Supply’s ʻEwa Reclamation Facility, supporting non-potable reuse in the Kapolei and ʻEwa regions.

Major Upgrades Under the 2010 Consent Decree

In accordance with the 2010 Consent Decree between the City and County of Honolulu, the U.S. Environmental Protection Agency, and the state of Hawaiʻi, the city committed to fully upgrading Honouliuli WWTP to secondary treatment by June 2024. The Notice to Proceed for construction was issued in December 2018, and work was completed ahead of schedule.

Key Secondary Treatment Facilities Completed:

• Six Secondary Clarifiers
• Aeration Basins 
• Secondary Blower Building
  
• Secondary Process Pump Station
  
• High-Rate Biological Contactors (HRBC)
  
• Activated Sludge System
  
• Ultraviolet (UV) Disinfection
  
• Mixed Liquor Distribution Box
• Effluent Division Box, Connection Box, and Junction Box
  
• Dual Secondary Effluent Lines
  
• Primary Effluent Line and Channel Modifications
  
• Primary Switchgear Building
  
• Centralized Generator Building
  

Modernizing Energy and Solids Processing

The upgrades go beyond treatment capacity. Honouliuli WWTP now features robust systems to reduce waste, generate energy, and promote sustainability:

Energy Enhancements:

• Biogas Cleaning and Storage Facility
  
• Combined Heat and Power (CHP) System
  
• Vehicle Fueling with Recovered Biogas
  
• Photovoltaic (PV) Solar Arrays
  
• Electric Vehicle Charging Stations
  

Solids Handling Improvements:

• Centrifuges for Efficient Dewatering
  
• Sludge Dryer for Volume Reduction
  
• Thermal Hydrolysis Process (THP) for enhanced biosolids management
  

Supporting Infrastructure for a Modern Facility

To support long-term operations and workforce needs, the city also constructed:

• A new laboratory facility
  
• A maintenance and administration building
  
• A refuse convenience center to support nearby communities
  

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With these enhancements, the Honouliuli Wastewater Treatment Plant now operates as one of Hawaiʻi’s most advanced wastewater facilities—fulfilling regulatory mandates, supporting environmental sustainability, and preparing for the needs of future generations.

The Honouliuli WWTP operates under National Pollutant Discharge Elimination System (NPDES) Permit (No. HI0020877), effective October 1, 2020, and expires September 30, 2025. The plant discharges a mixture of secondary and R-1 treated wastewater from the facility to receiving waters in West Mamala Bay in accordance with the 2010 Consent Decree effluent limitations, monitoring requirements and other permit conditions.