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Advanced Treatment

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Secondary Treatment


Wahiawa Treatment Plant: Sand Filters


Aeration Tanks


UV Radiation Unit


Cleaning UV Bulbs


Secondary Clarifiers

Oahu's treated wastewater may be discharged into a reservoir or ocean without adverse environmental effects. In fact, the cleansed water is often cleaner than the water where it's disposed.

Seven of Oahu's smaller wastewater facilities - Kailua, Waimanalo, Wahiawa, Kahuku, Laie, Waianae and Paalaa Kai Wastewater Treatment Plants - provide a second stage of treatment.

After primary treatment, wastewater still contains small amounts of solid matter. Secondary treatment speeds up what occurs naturally. The primary-treated water enters a series of aeration tanks, the heart of the secondary treatment system. Some secondary treatment plants (Honouliuli, Kailua, Waianae) use biotowers (also called trickling filters) to perform this function.

Microorganisms that break down waste are saturated with "food" (wastewater) and oxygen to break down the pollutants more quickly. Filtered air is pumped into the liquid enabling the microbes to flourish and grow, and they consume most of the pollutants. Chemicals may also be added to improve treatment.

Disinfection kills any remaining harmful bacteria. The processed water is released underground or to a nearby stream or oceanóreturning the water and a few left-over impurities back into our ecosystem.

Odor control
As the wastewater moves along, it produces hydrogen sulfide gas (H2S), which emits odor, rusts manhole covers and, in large doses, may cause illness at higher concentrations. Air and gases in the plant are monitored and cleansed following the wastewater's initial screening (and throughout the process) before the air is released through 100-foot towers into the atmosphere. An alarm sounds when H2S reaches high levels.

Trickling Filter/Solids Contact
At Honouliuli WWTP, giant pumps lift the wastewater to the top of multi-story biotowers. The water trickles down over plastic cubes where bacteria grow. The prolonged exposure to air and oxygen speeds the break up of solid particles.

Last Reviewed: Monday, June 25, 2012