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 of groundwater 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.
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.