The ‘Ewa district on the island of O‘ahu has undergone a significant amount of development in recent years. What was once fertile agriculture land has now given way to numerous residential, commercial and industrial developments. These developments have seriously impacted the region’s available water resources in two ways: (1) by reducing the amount of recharge that the caprock aquifer receives because of reduced agricultural activity, and (2) increasing the potable water demands placed on O‘ahu’s potable water aquifers.
As more and more land is being developed, the amount of water being applied to the ground in the form of agricultural irrigation is being reduced. This reduction ofgroundwater recharge has markedly reduced the amount of water replenishing the existing aquifer. As a result of this activity, the salinity of the aquifer has been gradually rising as withdrawals from existing wells continue. This has prompted the Hawai‘i State Department of Land and Natural Resources (DLNR) to develop plans to limit new well permits and well permit renewals in the region.Development in the ‘Ewa area includes a number of golf courses that are currently pumping brackish water from the caprock aquifer for irrigation. New residential subdivisions are increasing the demand for potable water in the region, and accelerating the withdrawal of water from potable water aquifers. In addition, industrial activities at the Campbell Industrial Park also place demands on potable water aquifers. This area contains many industrial facilities that consume large amounts of potable water for their industrial processes. Recognizing these demands, the Honolulu Board of Water Supply (BWS) entered into the water recycling business in 2000 by purchasing the Honouliuli Water Recycling Facility. Water recycling is one element of a broader BWS strategy to protect O‘ahu’s aquifers and to conserve water resources through conservation and development of new water supplies. The facility is now irrigating golf courses that were once using brackish water, including West Loch, ‘Ewa Villages, Hawai‘i Prince, and Coral Creek. The facility is also providing recycled water to industries at Campbell Industrial Park.
WASTEWATER TREATMENT IN THE ‘EWA DISTRICT
|The City and County of Honolulu (City) owns and operates the Honouliuli Wastewater Treatment Plant (WWTP), which is the regional wastewater treatment facility for the ‘Ewa district. The Honouliuli WWTP is located along Geiger Road directly east of the former Naval Air Station Barbers Point. The service area of the plant encompasses a total area of approximately 76,000 acres and ranges from Red Hill along its eastern boundary up to Mililani on its northern boundary, and extends to Makakilo City, Honokai Hale, and Ko Olina on its western boundary.||
Service Area of Honoluliuli WWTP
All residential, commercial, industrial and agricultural areas within these boundaries are included in the service area except for Pearl Harbor, Campbell Industrial Park and several small pockets that are served by cesspools or septic tanks. Wastewater treated at the Honouliuli WWTP is discharged into West Mamala Bay through a deep ocean outfall.
The Honouliuli WWTP was originally put into service in December 1984. The plant was initially designed to treat up to 25 million gallons per day (mgd) of wastewater to the primary level only. Over the years, the plant capacity has been expanded to meet increasing and expected future flows. The plant presently has a design average dry weather flow capacity of 38 mgd, with future plans to further expand it’s capacity to 51 mgd.
Treatment processes at the Honouliuli WWTP include preliminary treatment, primary treatment, secondary treatment, effluent screening, solids treatment and handling, and odor control. The preliminary treatment processes include influent screens at the headworks, aerated grit removal, and preaeration facilities. Primary treatment processes include four circular clarifiers which utilize flotation and sedimentation to remove floating and settleable solids from the wastewater. The secondary treatment processes at Honouliuli includes two biotowers that are fed by the biotower pump station, a solids contactor and sludge reaeration tank, and two circular secondary clarifiers. The secondary treatment facilities are currently capable of treating up to 13 mgd. The effluent screens are located immediately upstream of the ocean outfall pipe, and ensure that no large objects are discharged through the outfall.
The solids treatment and handling processes include thickening, storage/blending, stabilization, dewatering and disposal. Incineration was originally used at the plant to further reduce the volume of the dewatered sludge before disposal, but was taken offline in 1995 due to public concerns about air quality. Odor control at the plant consists of four main systems, 1) headworks, 2) central system for the grit removal/preaeration tanks, primary clarifiers, gravity thickeners, blending tanks, 3) secondary treatment system, and 4) solids system.
HONOULIUILI AND 309 CONSENT ORDER
The secondary treatment facilities at the Honouliuli WWTP were constructed as a result of a State of Hawai‘i Department of Health (DOH) consent order which was signed in 1993 between the DOH and the City. The main objective of the consent order was to establish secondary treatment facilities at the plant to allow for those treated portions of the wastewater flow to be reused. The secondary treatment facilities at Honouliuli were completed in 1996 and initial water reuse was initiated in 1998 with approximately 2 mgd being used for in-plant demands.
Additionally, in 1995, the U.S. Environmental Protection Agency (EPA), DOH, and the City entered into an agreement known as the 309 Consent Decree, which required a significant commitment by the City to improve its wastewater system. As part of the 309 Consent Decree requirements, the City was faced with spending at least $20 million to develop an effluent reuse system that would need to recycle 10 mgd of water by July 2001. The Honouliuli WWTP was selected for implementation of the water reuse requirements because of the demands on the ‘Ewa caprock aquifer, the termination of sugar cultivation which led to the significant decrease in groundwater recharge, and the proximity of the facility to potential users of reclaimed water.
WATER RECYCLING FACILITIES
Construction of water recycling facilities within the Honouliuli WWTP commenced in January 1998 and was completed in the summer of 2000. The facility was officially dedicated in August 2000. Prior to its dedication, the water recycling facility was purchased by the Honolulu Board of Water Supply (BWS) in July 2000 for $48.1 million from USFilter Operating Services.
|The entire water recycling facility is located adjacent to the City & County of Honolulu’s Honouliuli WWTP. Water recycling components include a Reuse Pump Station, a Sand Filter Structure which includes rapid mixing tanks and chemical flocculators, Ultraviolet Light (UV) Disinfection, a Microfiltration/Reverse Osmosis Building, Storage Tanks, and a Product Delivery Pump Station.|
The facility currently has a capacity of 12 million gallons per day (mgd) and produces two grades of recycled water. R-1 water is used for irrigational uses, and Reverse Osmosis (RO) for industrial uses. The facility is currently capable of producing up to 10 mgd of R-1 water, which is the highest level of treatment as designated by the Hawai‘i DOH. R-1 water is currently used throughout the state of Hawai‘i for golf course irrigation, landscaping, and agriculture. On the other hand, RO water is intended strictly for industrial uses such as boiler feed water, cooling tower make-up water, and process water for refineries. The facility currently has an RO capacity of 2 mgd. Both types of recycled water begin with secondary treated effluent from the Honouliuli WWTP.
THE R-1 PROCESS
The R-1 process includes the following components:
- Rapid Mix Tanks
- Chemical Flocculators
- Sand Filters
- Ultraviolet (UV) Light Disinfection
- R-1 Transfer Pumps
- Two 2.5 Million Gallon R-1 Storage Tanks
- R-1 Product Delivery Pumps
Secondary effluent flows by gravity from the existing Parshall flume box into the Reuse Pump Station. Lift pumps in the Reuse Pump Station convey effluent to the Sand Filter Structure, where polyaluminum chloride is added and rapidly mixed in one of two mixing tanks. The effluent then flows into one of three flocculation tanks to facilitate the coagulation of suspended and dissolved particles to form larger and/or denser particles. The effluent then flows into one of seven sand filter cells. Filtered effluent is collected in an underdrain system to a clearwell at the rear of the Sand Filter Structure, then flows by gravity to the UV Disinfection system (consisting of 4 banks). Disinfected effluent is then conveyed by the R-1 Transfer Pumps to one of two 2.5 million gallon storage tanks. The R-1 effluent is then pumped to customers via the R-1 Product Delivery Pump Station.
THE REVERSE OSMOSIS PROCESS
The Reverse Osmosis (RO) process includes the following components:
- Reverse Osmosis
- One 0.5 Million Gallon Storage Tank
- RO Delivery Pumps
The production of RO water utilizes the membrane filtration process. Membrane filtration consists of a permeable membrane that allow particles smaller than the membrane pores to pass through, but captures particles that are larger than the membrane pores. In general, relatively high pressures are required to force the effluent through the permeable membrane.
The reverse osmosis process typically uses higher pressures and smaller pore sizes than microfiltration. Consequently, reverse osmosis can remove particles with lower molecular weights than microfiltration. Because of the high pressures involved in reverse osmosis, the process is capable of de-ionizing water and is typically used for desalination. Microfiltration is used as a pretreatment separation process downstream of the secondary treated effluent from the adjacent Honoluliuli WWTP secondary units. Self cleaning strainers at the Reuse Pump Station remove larger-sized particles that could clog the microfilters. The microfiltration process then removes additional residual suspended solids in the secondary effluent by utilizing a 0.2 micro hollow fiber membrane that is cleaned monthly using citric acid. This enables the reverse osmosis process to remove higher levels of ionic constituents from the effluent, thus reducing the degree of post-treatment required by RO water users to meet their water quality requirements. The microfiltration pretreatment process also prolongs the life of the reverse osmosis membranes by reducing the amount of membrane fouling and consequent cleaning required.
CURRENT USES OF RECYCLED WATER
The Honouliuli WRF currently supplies the City golf courses of West Loch and ‘Ewa Villages with 1 mgd each of R-1 water. Recycled water is pumped to the West Loch course at night, while the ‘Ewa Villages course is supplied during the day. The pumping rate to each course is 150 gallons per minute (gpm) at a pressure of 68 pounds per square inch (psi).
For more information about the Honouliuli Water Recycling Facility, please visit the official Honolulu Board of Water Supply Water. Organizations interested in touring the Facility may contact the Honolulu Board of Water Supply’s Water Recycling Program at phone (808) 527-6156.