Long Range Sewer Rehabilitation

Unlike potable water systems, wastewater systems are typically not completely sealed. These systems are typically built to manage both normal wastewater flows and a limited amount of additional water known as infiltration and inflow (I/I). Infiltration occurs when groundwater enters the system through deteriorated pipes, joints, or manholes, while inflow comes from surface sources such as manhole covers, cleanouts, or unauthorized connections like roof drains or area drains. 

During storm events, this added volume — referred to as “wet weather I/I”  —  has the potential to overwhelm the system’s hydraulic capacity, leading to sanitary sewer overflows (SSOs). These overflows pose a serious public health risk due to potential exposure to harmful pathogens present in untreated sewage. 

Addressing this issue posed a significant challenge for the City, involving a vast and aging infrastructure. At the time, the City’s system processed more than 120 million gallons of wastewater daily across approximately 1,800 miles of collection lines.  Any proposed rehabilitation strategy needed to correct existing structural deficiencies, account for anticipated future growth, and outline a phased, 20-year improvement plan that could be realistically implemented within the City’s financial and operational constraints. 

Fukunaga & Associates, Inc. (FAINC) led the most comprehensive sewer system evaluation ever undertaken in the State of Hawai'i, conducting a systematic analysis of the hydraulic, structural, and operational conditions of the City's wastewater infrastructure. 

Hydraulic Condition Assessment

The assessment began with a complete update of the sewer system inventory to identify key components and establish a critical sewer network. The system was divided into numerous collection basins, which served as units for an extensive sewer flow monitoring program. This program provided essential data for understanding how the collection system performed under both dry and wet weather conditions. Flow and rainfall data were carefully analyzed to distinguish between areas with significant issues and those operating effectively, helping prioritize locations for further evaluation.

The City deployed a monitoring network that included 65 permanent flow monitors, 50 temporary monitors, and 25 recording rain gauges. Over six years of continuous data collection provided insight into baseline dry weather flows and the impacts of wet weather events. This analysis revealed the extent and behavior of infiltration and inflow (I/I) in each basin - how much water entered the system, how rapidly, and for how long following storm events.

A major goal was to establish realistic design flow standards that the system should be able to accommodate. The most influential factor affecting system performance was the volume of wet weather I/I. It became clear that the City’s existing Sewer Design Standards were insufficient for peak wet weather conditions. A cost-performance analysis was conducted to determine the most effective design level. Results showed that designing the system to accommodate a two-year storm event would significantly reduce SSOs without incurring excessive additional costs.

With this benchmark, design flows were calculated for the entire system and used to evaluate system adequacy. Hydraulic modeling was performed to identify current and future deficiencies. Non-problematic segments were excluded from detailed analysis, while problem areas were assessed to develop and compare potential improvements. Recommended solutions were prioritized for inclusion in the City’s Long Range Sewer Rehabilitation Capital Improvements Program (CIP).

Structural Condition Assessment

The structural assessment focused on identifying sewers most at risk of failure. An inventory evaluation allowed FAINC to identify structurally critical segments for further inspection. Field assessments included approximately 3,000 manhole entries and 130,000 linear feet of closed-circuit television (CCTV) inspections - covering roughly 15% of the system.

Defects were categorized by severity. Urgent issues were addressed either through immediate emergency repairs or by inclusion in short-term CIP projects. Moderately deteriorated segments were scheduled for rehabilitation under the Long Range Sewer Rehabilitation CIP. Minor issues and stable lines were placed under observation for ongoing monitoring, while non-critical segments were incorporated into the City’s preventive maintenance schedule.

The City’s Long Range Sewer Rehabilitation Plan outlines a series of conceptual capital improvement projects designed to correct the hydraulic and structural deficiencies identified during the comprehensive assessment phase. The plan outlines a structured, 20-year implementation timeline (2000–2019) to systematically resolve the issues affecting the wastewater collection system. 

The total estimated cost of the rehabilitation program is $890 million. While substantial, this investment aligns with the City’s long-term financial strategy, which was developed in parallel with the Sewer Rehabilitation Plan to ensure fiscal sustainability and affordability over time.

The Plan is guided by a “Holistic Approach” that views the collection system as an interconnected and fully integrated network. The primary goals include:

• Reducing existing sanitary sewer overflows (SSOs) by addressing known deficiencies
• Enhancing system reliability by proactively preventing future failures through planning and monitoring
• Accommodating future development by integrating capacity upgrades into system expansion planning

Ongoing performance monitoring and built-in flexibility are key components of the plan, enabling the City to adapt and refine its approach as new data becomes available or system conditions evolve. This dynamic framework ensures the rehabilitation program remains responsive, effective, and aligned with both regulatory requirements and community needs.

Fukunaga & Associates, Inc. (FAINC) served as the prime engineering consultant for the Long Range Sewer Rehabilitation Program. FAINC played a key role in supporting the City throughout its negotiations with the U.S. Environmental Protection Agency (EPA), helping to establish the terms of the Consent Decree that would govern the full duration of the program's development and implementation.

FAINC was responsible for the coordination, production, and timely submission of all project deliverables. The firm led a multidisciplinary team of subconsultants, including Brown & Caldwell of Pleasant Hill, California, which provided specialized technical support; and Belt Collins Hawaii, which managed computer systems and data processing. Additional field investigation and flow monitoring services were provided by ADS Environmental Services, Inc. and MGD Technologies, Inc., further enhancing the project’s comprehensive and data-driven approach.

One of the most complex aspects of the project was determining appropriate design flow levels for Honolulu’s wastewater collection system. Data collected through the extensive flow monitoring program revealed that the City’s existing Sewer Design Standards were insufficient for managing peak wet weather flows.

To address this, theoretical design flows were calculated for a range of storm recurrence intervals. A cost-performance analysis was then conducted to evaluate the relationship between system capacity and the expected frequency of sanitary sewer overflows (SSOs). The analysis demonstrated that designing the system to handle a two-year storm event would reduce wet weather SSOs by approximately 78%, striking an effective balance between performance and cost. Designing for more extreme events showed only marginal gains in overflow reduction, while significantly increasing implementation costs.

By substantiating this approach with real-world data and analysis, and persuading the EPA that the two-year design storm standard was both practical and cost-effective, the City was able to avoid unnecessary overbuilding — ultimately saving several hundred million dollars.

The comprehensive studies undertaken for this project highlighted the complexity and range of factors that must be considered in wastewater collection system planning and design. Each study contributed critical insight and justification for the methodologies and evaluation procedures used in developing the Long Range Sewer Rehabilitation Plan.

One of the most significant outcomes was the establishment of a new benchmark for determining wastewater flows, leading to the creation of a prototype hydraulic model specifically tailored for Honolulu’s sewer system. This model now serves as a foundational tool, promoting a more data-driven and rational approach to sizing sewer system components. As a result, the project has set a new standard for future system expansions and improvements — benefiting not only the City, but also contributing meaningful advancements to the engineering profession as a whole.

In developing Honolulu’s Long Range Sewer Rehabilitation Plan, the City had to strike a careful balance between protecting public health and managing financial impacts on the community. While the top priority was to reduce sanitary sewer overflows and safeguard the environment, the economic burden on residents had to be taken into account. As a result, cost-effectiveness and performance-based decision-making were central to the planning process. The final plan also incorporated built-in flexibility and a robust monitoring framework, enabling the City to adapt its strategies over time as conditions and priorities evolve.

The scope of assessing the entire wastewater collection system within a five-year window was immense. The project involved evaluating more than 1,800 miles of sewer pipelines, 67 wastewater pumping stations, and 8 treatment facilities. Throughout the study, multiple technical components were executed, including continuous flow and rainfall monitoring, hydrologic analysis, field assessments such as manhole entries and CCTV inspections, and chemical sampling and corrosion analysis. Comprehensive hydraulic modeling of the system supported the development of targeted solutions to address deficiencies. Given the Consent Decree requirements, all work was subject to close oversight and final approval by the U.S. Environmental Protection Agency, adding another layer of complexity and rigor to the effort.

The Sewer Rehabilitation Plan was successfully delivered on time and below the City’s original cost estimate. With a projected 20-year implementation budget of $890 million, the recommended improvements were determined to be financially sustainable under the City’s existing and planned sewer rate structures. Submitted to the EPA in December 1999 and formally approved in March 2000, the plan met every condition of the Consent Decree. This achievement was largely due to consistent, transparent communication with EPA Region IX, supported by the consultant team’s active participation in key negotiations. Ultimately, the plan earned recognition from EPA as a “model program” and stands as a benchmark for future wastewater infrastructure planning.