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Wastewater Treatment Processes

The wastewater process replicates the cleansing action of a flowing river within a series of tanks.

Preliminary Treatment/Influent Screening - The influent sewage is initially screened to remove large objects, fibrous materials and grit. Those objects are collected and stored in a waste disposal bin for disposal at a sanitary landfill.

Before basic wastewater treatment begins, large pipes carry the wastewater into the plant. The preliminary stage involves separating trash and large objects from the wastewater. Large screens remove paper (mostly toilet tissue), wood, rocks, can, rags and even dead animals that sometimes flow in with the wastewater.

Primary Treatment - The primary stage uses large tanks to slow the flow so that the heavier solids settle and the lighter materials float to the top. Clarifiers or settling tanks hold the wastewater while gravity takes over and heavy particles, "stuff that sinks," collect in the bottom of the tanks. Grease, oil, soap and grit (scum) float to the top. Rotating arms remove both for further treatment. Some wastewater requires additional processing before it is discharged.

Secondary Treatment - Secondary treatment employs a biological process whereby a large population of micro-organisms help convert the remaining organic material into other forms that can be easily separated into solids and a clear liquid. The primary affluent flows through a series of large rectangular aeration tanks that have been seeded with bacteria and other microbes (tiny organisms that exist naturally in plant and animal life). Filtered air is pumped through the liquid to enable the microbes to breathe and grow. In the constantly churning water, these microbes flourish and multiply, eating the remaining organic materials and nutrients in the wastewater. This mixture of microbes and water flows into a secondary settling tank. The microbes, now stabilized, clump together and settle to the bottom of the tank where they become part of the organic residuals and are removed. Approximately 85% of these microbes are recycled to the start of the aeration tanks to begin the biological treatment process for the primary effluent. The cleaned water flows out of the top of the secondary settling tank to be returned to the waterway or to the tertiary treatment process.

Tertiary Treatment - After the secondary treatment process, the wastewater may be designated to go through the tertiary treatment facility. Here, water is recycled through filtration and reverse osmosis. This process serves as a treatment to conserve water by recycling it for use in industries and irrigation purposes on demand. This water is not for human consumption.

Ultraviolet Disinfection system transfers electromagnetic energy from a mercury arc lamp to an organism's genetic material (DNA and RNA). When UV radiation penetrates the cell wall of an organism, it retards the cell's ability to reproduce. 

Disposal - The treated water (effluent) is discharged to the ocean, a reservoir or underground injection wells as mandated by the Clean Water Act of 1972, regulated by the U.S. Environmental Protection Agency. Effluent from Sand Island, Honouliuli, Kailua and Waianae treatment plants go to deep ocean outfalls. Effluent treated to tertiary level at Wahiawa WWTP goes to the Wahiawa Reservoir. Effluent from Waimanalo, Kahuku, Laie and Paalaa Kai treatment plants are injected into the ground.

Solids Handling - Solid waste matter is kept in enclosed heated tanks and pressure-cooked in order to break the bond with water. Bacteria then break it down even further, reducing volume, odors and microorganisms that can cause disease. The remains are sent to a landfill or recycled as fertilizer.

Last Reviewed: Tuesday, May 10, 2011