Division of Wastewater Treatment & Disposal
Around-the-Clock Operation, Island-Wide Impact
The City and County of Honolulu operates nine wastewater treatment plants across Oʻahu—kept running 24/7 by a dedicated team of highly trained operators, mechanics, electricians, and technicians.
These frontline professionals are responsible for the safe and efficient treatment of millions of gallons of wastewater each day, protecting public health and preserving our island’s natural resources.
At the heart of operations is the SCADA Operations Control Center at the Sand Island Wastewater Treatment Plant. From this advanced facility, crews monitor and manage systems across the entire island in real time—responding quickly to changes and ensuring consistent, reliable service.
But it’s not just about maintaining the system—it’s about improving it. The team is always exploring new technologies and innovative practices to:
Boost operational efficiency
Enhance water quality
Strengthen environmental protection
With a commitment to excellence and a future-focused mindset, our plant teams are working every day to keep Oʻahu clean, safe, and sustainable.
First Stop: Removing the Big Stuff
When wastewater enters the treatment plant, the first job is to protect the equipment by removing large and harmful objects.
Big screens catch all kinds of surprising items—like cans, sticks, rags, diapers, batteries, even fish heads and the occasional lost jewelry. After that, gravity helps settle out paper products like toilet paper, wipes, cotton balls, dental floss, and other items that shouldn’t be flushed.
A mechanical rake or screen collects the waste and moves it to a hopper, where it’s compacted to remove excess water. From there, it’s safely trucked to a landfill for disposal.
Odor Control
Controlling Odors and Protecting Health
As wastewater flows through the system, it can release hydrogen sulfide gas (H₂S)—a gas that smells bad, can damage equipment like manhole covers, and may be harmful in high amounts.
To keep the air safe and clean, special systems monitor and treat the air at every stage of the treatment process. After the wastewater is screened, the air is cleaned before being released through 100-foot tall exhaust towers.
If hydrogen sulfide levels get too high, an alarm will sound, alerting staff to take immediate action.
These systems help protect workers, equipment, and the surrounding community from harmful gases.
Aeration
How Honouliuli Cleans Wastewater with Biotowers
At the Honouliuli Wastewater Treatment Plant, powerful pumps lift the wastewater to the top of tall biotowers—some as high as a multi-story building.
From there, the water trickles down through layers of plastic cubes that are covered in helpful bacteria. As the water slowly moves through, the bacteria and constant flow of air help break down solid waste naturally and efficiently.
This step is an important part of cleaning the water before it moves on to the next stage.
Primary Clarifiers
Settling Out the Solids
In the next step of treatment, wastewater is sent to large open-air settling tanks where the water flow slows down.
As the water becomes still, gravity takes over:
Heavier solids sink to the bottom—this is called primary sludge
Oil and grease float to the top—this is called scum
Revolving arms in the tanks slowly scrape the solids from the bottom and skim the grease off the top.
At the same time, microorganisms naturally present in the water begin to break down the organic waste, helping clean the water even more.
Each step in this process removes more and more solid material—getting the water ready for the next phase of treatment.
Disinfection
Final Step: Disinfecting the Water
After most of the solids are removed, the remaining liquid goes through disinfection to kill any harmful bacteria that could cause illness.
Some treatment plants use chlorine, while others—like Sand Island—use a more advanced method with ultraviolet (UV) light. The water flows through a closed channel, passing under UV lamps that kill or weaken bacteria, stopping them from reproducing. This process also helps remove phosphorus, which can harm ocean life.
Since Sand Island switched to UV disinfection in 2005, nearly 90% of the City’s treated wastewater now uses this cleaner, more modern technology.
After disinfection, the clean water is safely released into the deep ocean through outfalls from Sand Island and Honouliuli. There, natural ocean currents break down and disperse the remaining particles, returning them to the environment in a safe, natural way.
Secondary Treatment
What Happens to Treated Wastewater?
Once Oʻahu’s wastewater has been cleaned, it can safely be released into the ocean, a stream, or underground—often cleaner than the water it’s flowing into. This treated water meets strict environmental standards and does not harm the environment.
Seven of Oʻahu’s smaller treatment plants—Kailua, Waimānalo, Wahiawā, Kahuku, Lāʻie, Waiʻanae, and Paʻalaʻa Kai—provide secondary treatment, which takes the cleaning process a step further.
How Secondary Treatment Works
After the first stage (primary treatment), the water still contains tiny bits of waste. Secondary treatment speeds up nature’s process of breaking it down:
The water flows into aeration tanks or biotowers, depending on the facility.
Helpful microorganisms—tiny living organisms—are added and given oxygen to help them grow.
These microbes “eat” the remaining pollutants, cleaning the water even more.
In some cases, chemicals are added to improve the results.
Finally, the cleaned water goes through disinfection to kill any leftover harmful bacteria.
Once it’s fully treated, the water is safely released—underground, into nearby streams, or into the ocean—where it reenters the natural environment with minimal impact.
Odor Control
Controlling Odors and Protecting Health
As wastewater flows through the system, it can release hydrogen sulfide gas (H₂S)—a gas that smells bad, can damage equipment like manhole covers, and may be harmful in high amounts.
To keep the air safe and clean, special systems monitor and treat the air at every stage of the treatment process. After the wastewater is screened, the air is cleaned before being released through 100-foot tall exhaust towers.
If hydrogen sulfide levels get too high, an alarm will sound, alerting staff to take immediate action.
These systems help protect workers, equipment, and the surrounding community from harmful gases.
Trickling Filter/Solids Contact
How Biotowers Help Clean Wastewater
At the Honouliuli Wastewater Treatment Plant, powerful pumps lift the wastewater to the top of tall structures called biotowers.
Inside the towers, the water trickles down over stacks of plastic cubes that are covered in helpful bacteria. As the water flows through, it’s exposed to plenty of air and oxygen, which helps the bacteria break down any remaining solid waste.
This natural, air-powered process helps clean the water before it moves on to the next stage of treatment.
What Is Tertiary Treatment?
Tertiary treatment is the final step in the wastewater cleaning process. It takes water that has already gone through secondary treatment and gives it a final “polish” to make it even cleaner.
During this process:
The water is filtered through fine sand to remove tiny particles
UV light is used to disinfect the water
Nutrients like nitrogen and phosphorus are also removed to protect the environment
This advanced level of treatment removes up to 95% of remaining suspended matter and is required when the cleaned water is released into freshwater sources, like lakes or reservoirs.
Two facilities on Oʻahu use this high-level treatment:
Wahiawā Wastewater Treatment Plant, which releases water into the Wahiawā Reservoir
Lāʻie Water Reclamation Facility, recently acquired by the city
These plants are among the most advanced on the island, helping ensure our water remains clean and our ecosystems stay healthy.
Treated Effluent
Where Does Treated Wastewater Go?
Most of Oʻahu’s cleaned wastewater—called treated effluent—is safely released into the ocean or underground, following strict environmental standards.
At the Sand Island Wastewater Treatment Plant, treated water is pumped 1.7 miles offshore and 240 feet deep into the ocean, where strong currents quickly disperse any remaining particles. This area is not used for recreation.
Other plants—Honouliuli, Waiʻanae, and Kailua—also use deep ocean outfalls, and together with Sand Island, they handle about 90% of the island’s wastewater disposal.
Other methods include:
Wahiawā WWTP: discharges into the Wahiawā Reservoir
Paʻalaʻa Kai, Kahuku, Lāʻie, and Waimānalo: use underground injection wells to safely release treated water below ground
Monitoring to Protect the Environment
The city’s Ocean Team regularly collects and tests water samples near the outfalls—about twice a week—to make sure water quality stays within safe, clean standards.
If there’s ever a sewer overflow or pipe issue, testing is increased (even up to three times a day) until everything is safe again.
All of Oʻahu’s wastewater treatment plants consistently meet federal environmental standards, helping protect our ocean, land, and public health.
Sludge Handling
Turning Waste Into Biosolids
In this final stage, oxygen is added to the treated water, creating bubbles that stick to any leftover solids. These bubbles help the solids rise to the surface, where they can be easily separated and collected.
Next, the collected material—called sludge—is slowly heated in a sealed, oxygen-free environment. This allows natural bacteria to continue breaking it down, turning the waste into nutrient-rich biosolids.
To finish the process, liquid is squeezed out of the sludge to make it more solid and easier to handle. The more water removed, the more concentrated the biosolids—which means lower costs for processing and transportation.
Thickening
A Special Process: Dissolved-Air Flotation
At some treatment plants, a method called dissolved-air flotation is used to help remove leftover solids from the treated water.
In this process, oxygen is added to the water, forming tiny bubbles. These bubbles stick to the remaining solids, causing them to float to the surface. Once at the top, the solids are easier to remove and compress.
Not all plants use this method, but where it is used, it helps make the treatment process even more efficient.
Anaerobic Digestion
Breaking Down Sludge Without Oxygen
At some treatment plants, sludge is slowly heated for several hours in a closed, oxygen-free space—the opposite of earlier steps that use air and oxygen.
In this oxygen-free (or anaerobic) environment, naturally occurring bacteria continue to break down the solids, turning them into biologically rich material.
This special process isn’t used at every plant, but where it is, it helps reduce waste and create useful byproducts.
Dewatering
From Sludge to Biosolids
After treatment, liquid is squeezed out of the sludge to make it thicker and easier to handle. This drier material is called biosolids.
The less water it contains, the more concentrated it is—which means it’s cheaper to process and transport.
Advanced Monitoring and Control: Upgrading Honolulu’s SCADA Infrastructure
Since initiating its wastewater operations in 1901, the City and County of Honolulu’s Department of Environmental Services (ENV) has continually prioritized investment in resilient infrastructure to safeguard public health and environmental quality. As the complexity of Oʻahu’s wastewater systems has increased alongside urban growth, so too has the city’s commitment to adopting cutting-edge technology to ensure operational continuity, regulatory compliance, and rapid emergency response.
A major technological leap occurred in the mid-1980s, when Honolulu deployed a Supervisory Control and Data Acquisition (SCADA) system to centralize the monitoring and control of its wastewater collection and treatment network. This legacy system—Barrington SCADA—enabled the remote management of critical assets, reducing manual oversight and enabling more timely responses to equipment malfunctions and system anomalies.
SCADA System Overview
The SCADA architecture currently supports real-time monitoring and control across:
- 71 wastewater pump stations
- 2 storm drain stations
- 9 wastewater treatment plants
- 4 preliminary treatment facilities
Field equipment, including Remote Terminal Units (RTUs) and Programmable Logic Controllers (PLCs), transmit telemetry data via analog leased lines and digital protocols to a centralized operations control center. Operators at the SCADA Operations Control Center manage the system 24/7/365, responding in real-time to alerts related to flow rates, pump cycle anomalies, power faults, and mechanical failures.
The previous Barrington SCADA system, while robust and reliable for decades, has become increasingly obsolete in terms of hardware compatibility, data visualization capabilities, and integration potential with next-generation technologies.
Transition to Rockwell Automation FactoryTalk SCADA
To meet modern performance demands and enable scalable future enhancements, ENV has initiated a full migration from Barrington SCADA to the Rockwell Automation FactoryTalk® SCADA system—a highly configurable, industrial-grade platform offering advanced analytics, intuitive human-machine interfaces (HMI), and seamless PLC integration.
Key features of the upgraded FactoryTalk system include:
- Programmable Logic Controllers (PLCs) replacing legacy RTUs, allowing modular configuration and enhanced input/output expansion
- Digital high-speed Ethernet-based communication networks, improving data transfer rates and minimizing latency
- Redundant HMI and Historian servers, ensuring data integrity and uninterrupted visibility during primary server outages
- Real-time data visualization and diagnostics, offering proactive insights into asset performance, load balancing, and alarm trending
- Integrated alarm management, enabling automated escalations and response workflows that reduce mean time to resolution (MTTR)
This system significantly improves ENV’s ability to detect early-warning signals of pump station irregularities—such as unexpected cycle durations or flow pattern deviations—and triggers automated alarms that are displayed through high-visibility pop-up alerts at the command center.
Implementation and Progress
To date, FactoryTalk SCADA has been successfully installed at the Sand Island Wastewater Treatment Plant’s Disinfection Facility and Headworks Facility. Systemwide deployment is ongoing, with a phased transition designed to ensure zero operational downtime across the city’s wastewater infrastructure.
The new platform is fully compliant with cybersecurity best practices and supports future integration with AI-enabled predictive maintenance tools, geographic information systems (GIS), and digital twin modeling for smarter utility planning.
Enhancing Environmental Protection and Operational Resilience
The transition to the Rockwell FactoryTalk SCADA system ensures that ENV continues to exceed federal and state regulatory mandates while strengthening its emergency preparedness and environmental stewardship capabilities.
By leveraging state-of-the-art automation, Honolulu is better positioned to:
- Minimize the risk of sanitary sewer overflows (SSOs)
- Enhance resource efficiency and workforce productivity
- Support data-driven infrastructure planning
- Extend the operational life of its wastewater assets
With this upgrade, the city reaffirms its commitment to a smarter, safer, and more sustainable wastewater management system for current and future generations.
What Happens to Oʻahu’s Wastewater?
Every day, the City & County of Honolulu’s Department of Environmental Services (ENV) collects around 96 million gallons of wastewater from homes, schools, and businesses across Oʻahu. That’s enough water to cover Ala Moana Park and Magic Island with four feet of water!
This wastewater travels through 2,100 miles of pipelines, supported by 71 pump stations, and flows into nine treatment plants around the island.
Cleaning the Water
At the treatment plants, the wastewater goes through several steps:
Primary Treatment – removes large solids, fats, oils, and grease
Secondary Treatment – uses air and helpful microorganisms to break down smaller waste
Tertiary Treatment – further filters and disinfects the water for the cleanest result
Once treated, the clean water—called effluent—is released into the ocean, a lake, or underground wells. Some of it is also recycled and reused.
Working with Nature
Wastewater treatment mimics nature’s own process of cleaning water, just much faster. Microorganisms help break down waste, and oxygen is added to speed things up. This is especially true in secondary treatment, used at eight facilities across the island: Kailua, Honouliuli, Wahiawā, Kahuku, Lāʻie, Waiʻanae, Waimānalo, and Paʻalaʻa Kai.
At Sand Island, which handle over 55% of the island’s wastewater, primary treatment along with UV treatment is used. Their treated water is discharged into the deep ocean, where strong currents and natural oxygen continue the cleanup process.
What Happens to the Leftover Solids?
The solid waste left behind is called sludge. It contains organic material and nutrients like nitrogen and phosphorus—but also trace metals and bacteria. In enclosed tanks, the sludge is heated and broken down by bacteria, reducing volume and odor. Water is removed, and the remaining biosolids are either sent to a landfill or recycled into fertilizer.
A public-private partnership with Synagro turns some of these biosolids into nutrient-rich fertilizer pellets at a facility on the Sand Island WWTP grounds. These safe, Class A pellets are used by golf courses, landscapers, and commercial growers across the island.
Recycled Water for Reuse
Recycled water—also called R-1 water—is disinfected using ultraviolet (UV) light and reused for:
Irrigating golf courses and roadways
Industrial uses at Campbell Industrial Park
Dust control at construction sites
Plant operations at wastewater treatment facilities
Lāʻie, Wahiawā, and Honouliuli all help produce this water, supporting the city’s goal of smarter, sustainable reuse.
Keeping the System Safe
ENV’s Regulatory Control Branch works with businesses to make sure what they send into the sewer system won’t cause harm. Restaurants and food businesses, for example, must install systems to manage fats, oils, and grease (FOG) before the wastewater reaches the city’s lines.
Turning Grease into Fuel
Through a partnership with Pacific Biodiesel, used cooking oil from restaurants is collected for free, then turned into clean-burning biodiesel. This fuel powers city vehicles and private fleets—reducing emissions, improving mileage, and supporting local energy production.
Greening the Government
As Oʻahu grows, the city is focused on sustainability and moving closer toward zero waste. From clean water to renewable energy and recycling, ENV is helping Honolulu build a greener, healthier future—for our residents, our environment, and generations to come.
Wastewater Plants
A Growing System for a Growing Island
The City and County of Honolulu operates nine wastewater treatment plants (WWTPs) across Oʻahu, working 24/7 with the support of more than 300 skilled workers who keep the system running safely and efficiently.
The first plant, located in Wahiawā, was built back in 1928, when Oʻahu was still mostly rural and sparsely populated. As the island grew and the economy developed, new plants were built in Kailua and Waiʻanae during the 1960s to meet increasing demand. Back then, many coastal communities had individual systems, and septic tanks and cesspools were common.
By the 1970s, growing environmental concerns led to stronger protections. The federal Clean Water Act, passed in 1972 and updated in 1977, set the stage for major improvements in how wastewater was handled nationwide.
Oʻahu responded by building larger, more advanced facilities:
Sand Island WWTP, our largest, opened in 1976
Honouliuli WWTP followed in 1980
In the 1990s, major upgrades modernized the system even further—including the installation of a high-tech SCADA monitoring system, which now provides centralized control of all wastewater pumping and treatment operations from the Sand Island Operations Control Center.
From humble beginnings to a modern island-wide network, Honolulu’s wastewater system continues to evolve to protect public health and our environment.
