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The Problem:

Sewage collection and transport facilities are located out of sight, out of mind, and are generally subject to a very minimum of inspection and maintenance, usually and most often as a result of a failure. While a significant asset, the condition of the collection system is rarely known. Operating mainly under the influence of gravity, conduit deterioration is rarely noticed. This can lead to significant inflow and infiltration (I/I) of surface and subsurface water not of wastewater origin. Systems are regularly extended and flow loadings changed without much regard to impact on the overall system. These systems can fail to contain flow and can overflow untreated sewage. These occurrences are commonly known as Sanitary Sewer Overflows (SSO), and flow into the environment with unacceptable health impacts.

Wet weather SSO is a result of entry of surface and subsurface water into the system during periods of wet weather. Such overflows are a symptom of deterioration of the system and inadequate maintenance practices. It is usually a system-wide problem and the sources are very difficult to locate and correct. It has been well proven that attempts to address the symptoms rather than the causes of wet weather SSO most usually and unexplainably fail. Consequently, a more holistic system-wide approach is required to locate and correct the sources of the excessive water and/or to contain the flows within the system.

Goals and objectives:

the goal of the Rational Approach is to apply a holistic, system-wide methodology employing sound asset management techniques with a focus on environmental management requirements to provide a high level of service to all stakeholders, including the community, regulators and the system operators at a reasonable cost.

Identifying the Problem:

The traditional approach to solving SSO has been found to be expensive, while also being ineffective at meeting regulatory and community expectations. Typically this approach has failed to appreciate the power of hydrologic modeling along with hydraulic modeling to calibrate sewer flow response for a large range of rain events, and has not had the benefit of a sewer cost optimization model to select the most cost-effective solution sets. It is focused on sewer rehabilitation while ignoring storage and amplification solutions.

The Solution:

a Rational Approach. The Rational Approach is a systematic, logical plan designed to produce a negotiating tool to allow all stakeholders to reach agreement on the most appropriate course of action to address the particular issues of a sewerage system and to plan for the upgrade of the system to meet agreed performance standards, at a reasonable cost.

The best way to explain the Rational Approach is to understand that it is:

A logical method based on an understanding of current and long term system performance characteristics and risks that allows for realistic assessment of systemwide remedial options (rehabilitation, amplification, diversion, interception, storage, management, etc) to create a least-cost option to meet specific performance objectives, even, if required, zero discharge of overflow.

It involves the integration of asset management and environmental management principles and responsibilities that must be accepted by the collection system operating authorities in managing their stewardship over such important community assets. One can readily see the elements of CMOM in the approach.

In the early 1990's the Sydney Water Corporation's (SWC) Clean Waterways Programme identified that the deterioration of its sewage collection and transport system was the main cause of the waterway pollution by sewage in the Sydney region. To address this issue SWC developed a whole new concept, the Rational Approach, to investigate, plan, upgrade and rehabilitate its collection and transport systems to control overflow of sewage to a level that was affordable and acceptable to the community.

The process requires a major paradigm shift in sewerage system planning, design and management and moves away from conventional design theory and proactive initiatives in favor of measuring and predicting both dry and wet weather flows in the system. This approach was accepted by the New South Wales (NSW) EPA for the granting of sewerage system operating licenses to SWC and now has been adopted by the NSW EPA in its recently published “Licencing Guidelines for Sewerage Treatment Systems” as the basis for licensing all sewerage systems in NSW.

At about the same time, the Australian Accounting Standards Board updated AAS 27, Financial Reporting Requirements by Local Governments. The standard asserts the accounting and accountability requirements of local governments, including sewerage system operators, in their stewardship of public assets for the public good and requires strict asset management tools be applied in this process.

The Rational Approach has been used by a number of authorities throughout NSW, Victoria and now Queensland in Australia, and throughout New Zealand. It has been proposed that it become the standard approach for New Zealand. It ahs also been adopted in Singapore and attracted interest from the USEPA.


This paper will describe the Rational Approach process, including data collection, modeling, cost optimization modeling, and solution sets. It will include three case studies showing its application and results. The first will be the completion of the first stage of the implementation of the Project CARE (North Shore City, Auckland, NZ) upgrade strategy where complete success in reaching the required performance objectives ahs been obtained. The second will be the recent completion of a study and plan for Goulburn City Council, NSW, to protect the Sydney Stored Water Supply form pollution by sewer overflow. This project is the first of a number planned for towns located within this catchment area. The third project involves the town of Moss Vale in the Southern Highlands of NSW. All of these solutions have involved amplification and storage to a more significant degree than would have normally been expected rather than traditional comprehensive sewer rehabilitation approach which until recently has been the norm


Incorporating the concepts of asset management and environmental management, the Rational Approach provides a more enlightened, technically sound, cost effective methodology using modern asset management techniques with a focus on environmental management which has successfully addressed the problem of sewerage system overflows while providing a high level of service which is both affordable and acceptable to all stakeholders.
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Document Type: Research Article

Publication date: 2005-01-01

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