|Wastewater Treatment Plant: Wahiawa WWTP|
Wahiawa Wastewater Treatment Plant (1928)
- Tertiary level of treatment
- Processes 1.62 million gallons per day (FY13)
- Freshwater (Wahiawa Reservoir) outfall
- Estimated 20-year CIP funding: $28 million
The Wahiawa Wastewater Treatment Plant (WWTP) is the oldest treatment facility on Oahu built in 1928. It is the only City treatment facility that discharges into a fresh water body – the Wahiawa Reservoir.
Originally a primary treatment plant, it was expanded and upgraded to provide activated sludge secondary treatment in 1967 and raised it from secondary treatment to tertiary - one of the highest levels of wastewater treatment - in December 2002.
The design was intended to follow the state of Hawaii Department of Health's (DOH) Guidelines for the Treatment and Use of Reclaimed Water, dated Nov. 22, 1993. The Wahiawa WWTP does not meet DOH Class 1 recycled water (R-1) quality effluent requirements and cannot be certified by DOH as a R-1 WWTP, as defined in the May 15, 2002, DOH water reuse guidelines.
Currently, the Wahiawa WWTP produces DOH Class 2 recycled water (R-2) quality effluent, although DOH has not certified it as such; Final Central Oahu Wastewater Facilities Plan (pp. 1-4, June 2008); however, the tertiary treatment improvements recently completed at the plant allow the facility to generate R-1 type quality effluent, although DOH has not certified it as R-1 quality.
The plant is going through two major renovations called the "front end" and "back end" projects.
The front end project was recently completed upgrading the existing influent pump station and headworks facility in advance of a planned upgrade to the secondary treatment process, which will install membrane bioreactors (MBR) in lieu of the current activated sludge process.
Equipment installed consist of new bar screens, new wetwell and influent pumps, grit removal and development of hard piped flow equalization facilities. A biofilter is also being provided to take care of any nuisance odor issues given the proximity of the project to the neighboring fence line. The project will also set up the plant's SCADA system to accommodate the future MBR project.
The back end project, expected to be completed in late summer of 2013, involves the construction of a new MBR system, modification of the existing Reclamation Building, providing six new blower for aeration and scouring the air, construction of a new untraviolet (UV) system, new off-spec water storage tank and R-1 (final effluent) pump station, relocation of the existing chlorine contact basin sump pump, modification of the existing Dewatering Building, providing two new gravity belt thickeners (GBTs), providing two new stored waste activated sludge (SWAS) pumps and two new stored thickened waste activated sludge pumps (STWAS), modification of existing dissolved air flotation tanks (DAFTS) in to sludge holding tanks, providing a new biofilter, modification of existing secondary clarifiers into flow equalization basins, modification of existing TBS pump station, construction of one new distribution box and asisting with startup of the new SCADA system.
Wahiawa WWTP serves Wahiawa Town, Whitmore Village and the U.S. Naval Computer and Telecommunications Area Master Station (NCTAMS EASTPAC) military facility near Whitmore Village.
The WWTP processes approximately 1.62 million gallons per day (mgd) and is designed to handle an average daily load of 2.49 mgd. Solids are flotation thickened and trucked to Honouliuli WWTP for further processing and final effluent disposal is through a 24-inch outfall with diffusers at a depth of 44 feet at the bottom of the reservoir.
The dam that forms the Wahiawa Reservoir was constructed in 1906 by the predecessors of the Waialua Sugar Company to be used for irrigating sugarcane. The reservoir impounds the waters of the North and South Forks of Kaukonahua Stream at a point just downstream of the confluence of the two stream branches.
The WWTP operates and is authorized to discharge under a Consent Decree that applies the requirements of a National Pollutant Discharge Elimination System (NPDES) permit, which expired March 1, 1994. The Consent Decree authorizes the City and County of Honolulu to discharge tertiary treated wastewater from the facility to the Wahiawa Reservoir in accordance with the effluent limitations, monitoring requirements and permit conditions.