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Wastewater Treatment Plant:  Sand Island WWTP

 Plant Detail

Sand Island Service Area

Sand Island Wastewater Treatment Plant (1976)

  • Advanced Primary level of treatment
  • Processed 57.62 million gallons per day (FY13)
  • Deep ocean outfall (12,518 ft. offshore/240 ft. deep)
  • Estimated 20-year CIP funding: $866 million

The Sand Island Wastewater Treatment Plant serves metropolitan Honolulu to (but not including) Hawaii Kai. This region generates 60 percent of Oahu's wastewater. The basin runs from the Koolau Mountains to the sea.

SIWWTP, a primary treatment facility, is the largest wastewater treatment plant on Oahu.  The facility was put into service, starting in 1976 and competed in 1980. 


The plant has gone through numerous upgrades since 1980, including the addition of ultraviolet (UV) disinfection before disposing of effluent through its 78-inch diameter pipe via a deep ocean outfall 1.7 miles offshore at an average depth of 240 feet.


SIWWTP processes an average of approximately 57.62 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 70 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 Oahu Regional Park, and two storm drain pumping stations.


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 as of Nov. 2, 1998, and expired Nov. 3, 2003.  The NPDES permit and applicable Zone of Mixing (ZOM) authorized the City and County of Honolulu to discharge primary treated wastewater from the SIWWTP to Mamala Bay through its deep ocean outfall in accordance with the effluent limitations, monitoring requirements and other permit conditions.  The permit was issued under 40 CFR Section 301(h) that allows the discharge of less than secondary treated effluent if it meets specified parameters as set forth in the regulations.  The permit was administratively extended while the EPA processed the application for renewal.  On December 17, 2010, the City entered into a Consent Decree to upgrade the treatment plant to comply with secondary treatment standards by December 31, 2035. The SIWWTP continues to operate under the existing NPDES permit with more stringent Consent Decree "interim limits" until the State Department of Health issues the next NPDES permit.

Last Reviewed: Monday, September 23, 2013