Sand Island Wastewater Treatment Plant

The Sand Island Wastewater Treatment Plant serves metropolitan Honolulu to (but not including) Hawai‘i Kai. This region generates more than 55 percent of O‘ahu’s wastewater. The basin runs from the Ko‘olau Mountains to the sea.

SIWWTP, a primary-plus treatment facility, is the largest wastewater treatment plant in the state. The facility went into service starting in 1976 and completed in 1980.

The plant has gone through numerous upgrades since 1980, including the addition of ultraviolet (UV) disinfection before disposing of effluent through its 84-inch diameter pipe via a deep ocean outfall 2.3 miles offshore at an average depth of 240 feet.

Currently, SIWWTP is going through several upgrades, including to its In-Vessel Bioconversion facilities, Synagro, and to upgrade the plant to full secondary treatment by the Dec. 31, 2035, deadline in the 2010 Consent Decree.

The Synagro project is valued at $174 million and is underway with a completion date of October 2028. The scope of work includes relocating and reconstructing the Effluent Water Reuse Pump Station, removing the old pump station and various underground utilities, and supplying power to Maintenance and Lab Buildings. Additionally, the project will construct new underground utilities, electrical improvements, and site civil work. Key upgrades include two new anaerobic digesters, two sludge storage tanks, a Digester Control Building, and a new tunnel and basement. Other enhancements involve installing a new boiler system, hot water pumps, modifying existing digesters and piping, relocating and installing flares, a propane gas tank, and new chemical dosing systems.

The upgrade to full secondary treatment is in two phases totaling $2.5 billion. Phase 1 consists of five major segments: an intermediate pump station and support building; sludge processing, thickening building and sludge storage tanks; odor control systems; secondary treatment effluent flow routing; and, a membrane bioreactor facility (MBR).

The MBR facility will showcase state-of-the-art technology creating R-1 type recycled water, arguably the highest quality in the industry, and is anticipated to be completed by Dec. 30, 2026. The Phase 1 portion of construction costs about $700 million.

Phase 2, an estimated $1.8 billion, is in the planning stage and will take the plant out to 2035.

SIWWTP processes an average of approximately 56.01 million gallons per day (mgd) of wastewater. It serves the communities from Kuliouou in the east to Salt Lake in the west, including Kahala, Kaimuki, Waikiki, Manoa, Makiki, downtown Honolulu, and Kalihi. The design average flow for this facility is 90 mgd with a design peak wet weather flow capacity of 200 mgd. Capacity of the outfall is 270 mgd.

The plant houses the Supervisory Control and Data Acquisition (SCADA) system, which continuously collects data from 72 wastewater pump stations, nine wastewater treatment plants and four preliminary treatment facilities. SCADA also monitors the Hanauma Bay Lift Station, Kuhio Beach Comfort Station, Central O‘ahu Regional Park, and two storm drain pumping stations. SCADA will be moving its base to the Honouliuli WWTP in late 2025.

The sludge generated at SIWWTP is processed on site by Synagro, an In-Vessel Bioconversion Facility that transforms the solids into a beneficial soil amendment pellet. The product, called Granulite, is used by golf courses, agriculture, turf, and landscaping industries.

The SIWWTP operates under National Pollutant Discharge Elimination System (NPDES) permit No. HI0020117, effective Jan. 1, 2015, modified Sept. 1, 2018, and expired Nov. 11, 2019. The City and County of Honolulu entered into a Consent Decree effective Dec. 17, 2010, to upgrade the treatment plant to comply with secondary treatment standards by Dec. 31, 2035. The NPDES permit and the Consent Decree authorized the city to discharge disinfected primary treated effluent from the SIWWTP to Mamala Bay through its deep ocean outfall in accordance with the effluent limitations, monitoring requirements and other permit conditions.