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Disposal & Reuse

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Treated Effluent

  Underground Diffusion

  Inland Ocean Diffusion
Most of Oahu's treated effluent is pumped into the ocean or underground. The Sand Island WWTP discharges 1.7 miles off shore, 230 feet deep. Here the remaining particles are rapidly disbursed by ocean currents. There is virtually no recreational use of the ocean water in that area. In addition, Honouliuli, Waianae and Kailua wastewater treatment plants dispose treated effluent through deep ocean outfalls, which together (with Sand Island) account for about 90 percent of Oahu's total disposal.

Treated effluent from Wahiawa WWTP is discharged into the Wahiawa Reservoir. Paalaa Kai, Kahuku, Laie and Waimanalo release treated efflluent through underground injection wells.

ENV Ocean Team samples and tests water from the outfalls about twice a week to make sure clean water standards are maintained. The cleaned effluent at all treatment facilities consistently meets federal requirements when it leaves the plant. If there is a break to a pipe or sanitary sewer overflow (SSO), the monitoring is increased, even to three times a day, until the problem is rectified.

Sludge Handling
Dissolved oxygen is forced into a treated effluent. Bubbles attach to the remaining solids and rise to the surface, making the organic matter easier to separate out and compress. The sludge is then heated slowly for hours in an oxygen-free environment—just the opposite of the earlier process. As the wastewater, the remaining bacteria thrive and convert the solids into biologically rich compounds. Liquid is squeezed out of the sludge, resulting in a more concentrated product called biosolids. The more water that is removed, the more concentrated it becomes and the less it costs to process or haul away.

Thickening - Dissolved oxygen is forced into treated effluent. Bubbles attach to the remaining solids and rise to the surface, making the organic matter easier to separate out and compress.  This is only one process that is not used at all plants, dissolved-air flotation thickening.


Anaerobic Digestion - Sludge is heated slowly for hours in an oxygen-free environment—just the opposite of the earlier process. As the wastewater, the remaining bacteria thrive and convert the solids into biologically rich compounds. This anaerobic process is not used at all plants.


Dewatering - Liquid is squeezed out of the sludge, resulting in a more concentrated product called biosolids. The more water that is removed, the more concentrated it becomes and the less it costs to process or haul away.

Last Reviewed: Tuesday, May 10, 2011