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Primary Treatment

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Honouliuli Treatment Plant:  Large objects removed

Odor Control Towers

Primary clarifiers

Revolving arm

Both Sand Island and Honouliuli are Advanced Primary treatment plants.

As wastewater enters the plant, the first step is to remove large objects to protect the plant's equipment from damage.

Large screens eliminate an amazing array of materials: cans, rocks, sticks, rags, diapers, batteries, lumber, fish heads, money and even jewelry. Gravity settles out the paper waste (toilet paper and towels, cotton balls, dental floss, condoms and feminine products) onto a conveyer belt. A mechanical arm or screen rakes the trash to a hopper. It's compacted to reduce the water content and trucked to a landfill.

Odor Control
As the wastewater moves along, it produces hydrogen sulfide gas, which emits odor, rusts manhole covers, and in large doses, can cause illness. 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 levels are too high.

At Honouliuli, 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 breakup of solid particles.

Primary Clarifiers
Wastewater is pumped into large open-air settling tanks, slowing the movement of the water. As the water stills, gravity pulls organic solids to the bottom (primary sludge). Oil and grease (scum) float to the top.

Revolving arms scrape the solids from the bottom and skim the grease from the top.

Each step of the treatment removes more solid matter from the wastewater. Along the way, microorganisms attack the organic matter from our toilets and garbage disposals and break it down further.

Finally, liquid from the clarifiers is disinfected to destroy bacteria that can cause disease. Some plants use chlorine. At others the liquid runs in a closed channel under ultraviolet cylinders. The procedure removes phosphorus and kills or cripples harmful bacteria to the point where they cannot reproduce.

With the conversion of the Sand Island plant to ultraviolet disinfection in 2005, nearly 90 percent of all the wastewater that's treated by the City & County will use this more advanced disinfection.

From Sand Island and Honouliuli WWTPs, the spent water goes to the deep ocean, where nature does the rest. That is, the remaining particles are rapidly disbursed by ocean currents and reduced to their basic elements.

Last Reviewed: Tuesday, May 10, 2011