Long Range Sewer Rehabilitation

In 1992, the City & County of Honolulu (City) was served with a notice of violation by the U.S. Environmental Protection Agency (EPA) for alleged violations of the Federal Clean Water Act, due to repeated spills from its extensive sewage collection system. The City

Sanitary Sewer Overflow (SSO) occurring on a city street

was mandated to control these overflows, which were deemed a severe threat to public health, safety and welfare. The City selected Fukunaga & Associates, Inc. (FAINC) to conduct a comprehensive assessment of the condition of the City’s aging sewer system and to formulate a proactive rehabilitation plan to improve the City’s sewers.

In 1995, a Consent Decree agreed to by the City, the Hawaii State Department of Health (DOH) and EPA, was filed in Federal court, requiring the City to prepare a comprehensive Sewer Rehabilitation Plan by December 31, 1999. FAINC and its subconsultants worked closely with the City to develop the Long Range Sewer Rehabilitation Plan, meeting all mandated deadlines and requirements established in the Consent Decree. This plan was approved by EPA and has been identified by EPA Region IX as a “model program”.

Unlike potable water distribution systems, sewage collection systems are not expected to be “water-tight”. It is common practice to design sewer systems to handle wastewater flow,

Infiltration occurring through cracked pipe

and also accommodate a nominal amount of extraneous water, known as infiltration and inflow (I/I). Infiltration occurs when water enters the sewers through defects such as leaky pipe joints, and cracked or corroded pipes and manholes. Inflow occurs through manhole covers, cleanout pipes or illegally connected area drains or roof downspouts.

When storms occur, this extraneous water, called “wet weather I/I”, can cause the total flow to exceed the hydraulic capacity of the collection system,

Infiltration through deteriorated sewer manhole wall

resulting in spills, referred to as sanitary sewage overflows (SSOs). SSOs threaten public health because they can expose people to disease-causing organisms called pathogens that are present in sewage.

The City’s task to develop a sewer rehabilitation plan addressing these spills was formidable because it was dealing with a large, complex and aging system. The City presently collects, treats and disposes over 120 million gallons of wastewater every day through 1800 miles of collection lines. The plan had to effectively address existing system deficiencies, factor in future development requirements, and program the necessary improvements over a 20-year time frame within a schedule that the City could reasonably afford.

The extensive monitoring network included flow monitors and recording rain gages

The City’s flow monitoring network included 65 permanent monitors, 50 temporary monitors and 25 recording rain gages. Over six years of continuous sewer flow and rainfall data were collected and used for the analyses. Characteristics of normal dry weather flows, as well as wet weather flows for each collection basin, were derived from analyses of the flow monitoring data. The analyses determined key factors that characterized the “leakiness” of each basin. These factors indicated the amount of I/I attributed to a given basin, how quickly the flows entered the system, and the duration of wet weather impacts.

The second component of the assessment effort addressed the structural condition of the sewer system. Evaluation of the sewer system inventory enabled the identification of the

Severely corroded pipe

structurally critical sewers. The critical sewers were assessed through manhole entry and closed circuit TV (CCTV) inspections. This included approximately 3000 manhole entry inspections and 130,000 feet of CCTV inspections, covering about 15 percent of the collection system. Recommended actions on the various defects were prioritized, addressing the most severe problem first and programming other problems based on defect severity. The most severe defects requiring immediate action were identified for emergency repair or inclusion on current CIP lists.

Severely corroded pipe viewed through CCTV inspection

Sewers with moderate defects were included in the Long Range Sewer Rehabilitation CIP. Minor defects and non-problem lines were identified for continued monitoring to track their condition over time. Non-critical sewers are to be addressed by the City’s on-going preventive maintenance program.

Results from all of the three condition assessment efforts were compiled and used to develop the City’s Long Range Sewer Rehabilitation Plan.

The City’s Long Range Sewer Rehabilitation Plan includes conceptual CIP projects formulated to address existing hydraulic and structural deficiencies identified in the assessment phase of the program. The Rehabilitation Program will systematically

20-Year Sewer Rehabilitation Capital Improvement Plan

address the deficiencies identified in the assessment effort over the 20-year (2000 – 2019) implementation period. It is estimated that the rehabilitation program will cost $890 million over the next 20 years. However, the cost of the program is considered affordable and is consistent with the City’s long range financial plan that was developed in parallel with the Sewer Rehabilitation Plan.

The Rehabilitation Plan was developed to apply a “Holistic Approach”, considering all aspects of the collection system as a complete (or “whole”) functioning system. The objectives are to minimize existing SSOs by addressing existing deficiencies, improve system reliability by preventing future deficiencies through proactive system planning and monitoring, and to integrate system expansions by planning system growth and providing necessary system upgrades. Part of the plan is to continue monitoring the impact of system improvements, and also to maintain program flexibility that will allow for adjustments to the Plan in the future.

FAINC served as the prime engineering consultant for the project, assisting the City in negotiations with EPA, establishing the terms of the consent decree, for the full term of the Rehabilitation Program development. FAINC was responsible for the production of all deliverables and meeting all deadlines. Subconsultants for the project included Brown & Caldwell of Pleasant Hill, California, providing technical support and Belt Collins Hawaii, of Honolulu, providing computer and data management services. ADS Environmental Services, Inc. and MGD Technologies, Inc. provided flow monitoring and field investigation support.

As stated earlier, one of the most significant challenges was the determination of reasonable design flows for Honolulu’s wastewater collection system. Real data obtained from the flow monitoring program revealed that the existing Sewer Design Standards used to design most of the existing sewer systems were inadequate. To address these deficiencies, theoretical design flows resulting from storms of various recurrence frequencies were calculated, and a cost performance analysis was conducted to estimate the projected occurrence of SSO’s under the various storm conditions. Based on this analysis, system capacities capable of handling the two-year design storm provided the optimal containment level, reducing wet weather SSOs by 78 percent. Increasing system capacity to accommodate higher magnitude events resulted in only a marginal improvement in SSO containment, while costing the City significantly more to implement. By convincing EPA that the selected design storm level was appropriate and truly optimized the system’s containment capacity, the City saved several hundred million dollars.

The many studies conducted as part of this project brought to the forefront the numerous factors and considerations involved in wastewater collection system planning and design. Each of the studies provided the background and rationale for the procedures and evaluation methodologies applied during the formulation of the Plan. A new “standard” for wastewater flow determination was established, including the development of a prototype sewer system hydraulic model for Honolulu. As a result, a more rational approach has been established to properly size system elements for all future extensions and improvements to the City’s sewer system.

The development of Honolulu’s Long Range Development Plan required a balance of social and economic considerations. The primary goal of the Plan was to protect public health and the environment. However, the level of protection had to be cognizant of the economic cost to the public. Therefore, cost-effectiveness and cost performance were keywords in the development of the plan. Flexibility and performance monitoring were also incorporated into the Final Plan, giving the City the ability to revise and modify the conceptual projects should future conditions warrant such changes.

The task of assessing the City’s entire wastewater collection system within the mandated five-year time frame appeared monumental at the onset of the project. The study encompassed over 1,800 miles of sewer lines, 67 wastewater pumping stations and 8 wastewater treatment plants. During the course of the project, the program included flow and rainfall monitoring, hydrologic analyses, field investigations (manhole entry and CCTV inspections), chemical sampling, corrosion modeling and analyses, hydraulic modeling of the entire collection system, and the development of alternative solutions to address structural, hydraulic and operational deficiencies. All work was closely monitored by EPA throughout the project duration; and the methods, results and recommendations were subject to their approval.

The Sewer Rehabilitation Plan was completed on time and under the original budget projected by the City. The improvements recommended in the Plan (estimated to cost $890 million over the next 20 years) are considered to be affordable, based on projected and approved sewer user fee schedules. The Plan was submitted in December 1999 and accepted by EPA in March 2000. This unprecedented action was made possible because of the open, on-going dialog with EPA. The consultant team was at the forefront of these discussions, accompanying City officials to EPA Region IX, headquarters in San Francisco on numerous occasions to negotiate the final requirements. In the end, Honolulu’s Sewer Rehabilitation Plan has been described by EPA Region IX as a “model program,” meeting all mandated deadlines and requirements set forth in the Consent Decree.