Life-Cycle Cost Considerations for Sewer Pipe

USA TODAY reported in 2009, ?The USA?s infrastructure is crumbling and threatens the nation?s economic well-being, according to a new report by a top engineering group.

?A lack of investment in roads, bridges, mass transit and aviation means the cost of repair over the next five years has skyrocketed to $2.2 trillion, according to the American Society of Civil Engineers (ASCE). In its last estimate four years ago, the cost was $1.6 trillion.?

Many policymakers are attempting to shape the future of America?s infrastructure systems through stimulus funds and state revolving funds. In addition to new funding sources, officials are strengthening environmental laws. A recent example is a bill that would restore federal protection to more remote waterways across the United States. It was introduced in the U.S. House of Representatives in April 2010 by Rep. Jim Oberstar (D-Minn.), Chairman of the Transportation and Infrastructure Committee, and is designed to clarify some of the provisions of the Clean Water Act of 1972 that were affected by court decisions.

Regardless of the source of funding, it is sorely needed, and investment in the nation?s infrastructure is as important as ever. To make the best use of our infrastructure dollars, projects that provide the lowest life-cycle cost make the most sense. This is as valid for above ground infrastructure as the often forgotten underground infrastructure.

Problems and Solutions

For years we have seen the phenomenon where many sanitary sewer flows become septic and develop hydrogen sulfide gas, which may eventually be converted to sulfuric acid. The acid accumulates in the crown of the pipes, often generating pH values of less than 1. This highly corrosive condition attacks many pipe materials including Portland cement concrete and mortar, steel and ductile iron.

Some pipe materials exist that are essentially inert to this environment. Centrifugally cast fiber reinforced polymer mortar (CCFRPM) is so resistant to septic sewer service that its life is nearly infinite.

CCFRPM pipes are constructed with polyester resins, glass fiber reinforcements and aggregates. These ingredients are precisely combined during the centrifugal casting manufacturing process to create a dense, void-free, polymer mortar wall composite structure that possesses inherent corrosion resistance. CCFRPM pipes have been in service around the world since 1960, and have exhibited a superior corrosion record in sanitary sewer applications.

Those agencies that plan to operate their own facilities cost-effectively must adopt a mindset that gives equal consideration to the operation, maintenance, repair and replacement expenses along with the installed cost. Long-term planning is the order of the day and life-cycle cost analysis is being adopted across the nation. Taxing authorities must emulate this approach that a private business would take in order to promote the best interests of their taxpayers.

Let?s look at these factors and find out how they must affect your design decisions and why installed cost alone is losing favor as a determinant while life-cycle cost is emerging as the primary criterion in bid selection.

To put this in context, here?s a scenario: your engineering assignment is the construction of two virtually identical, major sewer line extensions for medium-sized cities. One is going to be design-bid-build (DBB) and the other design-build-operate and maintain (DBOM). Will your specifications and evaluation be the same?
This is a typical question that municipalities face every day. They must decide whether to take a long-term or a short-term view. The traditional way of handling water and sewer installations or repairs has been DBB.

In order to win the bid, the contractor had to determine the least expensive way to meet the specifications.
After construction, the city or county took over and things worked well. Then, 10, 20 or more years later, when practically no one associated with the original project was still around, the taxpayers faced the consequences of short-term thinking that the DBB process encouraged. You may rest assured that 10 or 20 years is a short time in the potential life of a well-engineered water or sewer line.

The Future of Water and Sewer Systems

While DBOM still represents only a small percentage of the total, evidence of this gradual change from DBB to DBOM was presented in a paper from the Reason Public Policy Institute of Los Angeles (www.rppi.org/index.html). They researched the subject and published the results in Policy Study No. 272, issued in September 2003. The study points out that cities that own and operate their water and wastewater facilities mainly outsource only design, engineering and construction.

The same study also cited a survey in which public officials indicated that the important drivers of water and sewer improvements are growing demand, the age of existing installations and new environmental regulations.

The Factors of Change

Many factors are at work. Rapid population growth, urban sprawl, increased concern for the environment and government regulation referred to above all affect the way we must manage our sewer and water systems. These influences, in turn, make the economic factors more important than ever before. So, as standards are raised, installation, maintenance, rehabilitation and long-term integrity are a growing priority, which forces the old decision-making processes out and life-cycle cost evaluation in.

But, is the answer simple? Is the math straightforward? Are the records of previous performance available to facilitate historical comparisons? You know the answer – ?No.? Or in the best case, ?only to a limited extent.?

Background

The total life-cycle cost of any item includes all of the costs incurred during the study period, including: purchase price, installation, operation, maintenance, repair and replacement (if necessary). Proper life-cycle cost comparisons should be made by discounting future costs to present value using the time value of money concepts.

Key Pipe Characteristics

Key pipe characteristics that impact sewer operation, maintenance, repair and replacement are: corrosion resistance to the interior and exterior environment, leak-tightness and hydraulic characteristics. To avoid or delay many future costs, you must specify pipes that are inherently corrosion resistant to the burial and sewer environment, leak-free and hydraulically superior.

A limited number of tests are available to help determine projected service life. One is the American Society for Testing and Materials (ASTM) D3681-96 Test for Chemical Resistance of ?Fiberglass? (Glass-Fiber-Reinforced Thermosetting-Resin) Pipe in a Deflected Condition.

There is no single place where you can find all the information you need to compare the life-cycle cost of installations with various kinds of pipe. But, there are numerous resources for helpful information. Many of the pipe manufacturers have excellent literature and websites that will provide information on their performance. Some of these are more concise and to-the-point than the various pipe associations listed below.

But beware of comparing performance characteristics derived by an ASTM method with those posted by a manufacturer, which may rely on best-case results and be colored by marketing hyperbole. The ASTM standard for ?Fiberglass Pipes,? ASTM D3262, includes long-term tests for corrosion resistance (D3681) in sanitary sewers (normal sulfuric acid), which predicts longevity in this environment. New add-on linings and admixtures for materials, which are not inherently corrosion resistant, continue to surface. Will they stand the test of time? Certainly as new products and technologies continue to be developed, the designers and end users of these products will need to determine which technologies hold promise and which do not.

Local experience is a highly reliable indicator of future performance, so even if comprehensive long-term records are not available, the first-hand knowledge regarding life expectancy, leakage, flow, maintenance, etc., by someone who has been involved in the process for many years is of immense value.

Of course, veterans in the industry will also be familiar with materials and manufacturers through their reading of industry magazines, attendance at trade shows and other ways, such as direct mail and sales calls. But much of this information may be viewed in a whole new way when you decide (and convince the agencies you work for) that life-cycle cost is the only proper way to evaluate sewer and water pipe installation and rehabilitation.

The design engineer may find it impossible to extrapolate laboratory test data into projections under field conditions. Some of the factors that affect the longevity of sewer pipe are:

Temperature: is the installation in the North or the South? Ambient temperature will vary from Cleveland, Ohio to Cleveland, Texas.

Slope: is it in a hilly or flat terrain?

Surround: is it acidic or neutral, is it stable or subject to shifting?

Length of the flow: the more time the sewage spends in the pipe, the more septic it becomes.

Turbulence: often found at the intersection of laterals and at structures.

Pipe made of different materials have different properties and features that may affect life-cycle cost, including:

Corrosion resistance: how is the pipe affected by ground chemicals or sewer acids? This can vary from a few years to thousands of years.

Freedom from leaks: many pipe systems have a great deal of trouble meeting initial standards while others routinely test 100 percent leak-free.

Maintenance of hydraulic characteristics: measured by the Manning?s ?n? value – some pipes have been measured at 0.009 when new and as low 0.0105 after several years of sewer service and slime development. Others may start with an ?n? of 0.013 when new which rises to 0.018 or higher after a period of use.
Cleaning: Some systems require extensive and repeated cleaning or other maintenance to maintain flow capacity at a minimally tolerable level for the short term.

Discussion of Benefits

Superior long-term pipe performance avoids or delays many future costs. There is a wide variation in the performance of various pipe materials as measured by their life expectancy and ability to remain leak-free and retain a smooth interior surface that results in superior hydraulic characteristics (a low ?n? value). Superior hydraulics result in a higher flow capacity generating additional long-term cost savings, including:

Delayed relief line construction costs.

Higher flow velocities resulting in fewer deposits and reduced cleaning needs.
May allow use of a smaller diameter pipe.

Life-Cycle Cost Evaluation Comparison

As previously presented, the life-cycle cost is comprised of installed cost and costs for operation, maintenance, repair and replacement.

Performance

Based on results of recent acid strain corrosion tests, CCFRPM pipes? performance exceeds the ASTM D3262 requirements for chemical resistance by more than 35 percent. Because of this corrosion resistance, the ASTM analysis of the strain corrosion test data predicts a life of far in excess of 100 years for the average pipe installed with 5 percent deflection in continuous service of 1 N sulfuric acid. Since most buried CCFRPM pipes are deflected less than 2 percent (lower bending stress) and the acid exposure may not be continuous, the actual expected life in septic sanitary sewer service is even longer.

Benefits

The superior, inherent corrosion resistance of CCFRPM pipes provides many valuable benefits to the user:

Primary

  • Long, maintenance-free life
  • No costly add-on linings or coatings to damage, inspect, repair or maintain
  • Hydraulic characteristics are unchanged with time
  • No need for expensive cathodic protection or poly bags to install and monitor

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Secondary

  • No premature rehabilitation or replacement needed
  • Save money during construction and in operation
  • Preserves flow capacity so relief line needs are delayed
  • Reduced installation and maintenance costs

Recommendation

To realize cost long-term cost savings, pipes that have inherent corrosion resistance should be considered. Additionally, bidding contractors typically provide the installed cost during the tender process. These figures should be adjusted by the present value differences of the operations, maintenance, repair and replacement costs over the study period for the pipe materials being evaluated to determine the comparative life-cycle cost.

Example Calculation

Consider a 100-year evaluation for construction of a new, open-cut 48-inch sewer 5,000 feet long. Two products are specified: Product A with a 50-year life, a 100 inch-gallon leakage rate and a Manning?s ?n? of 0.013, and Product B with a 100-year life, leak-free and a Manning?s ?n? of 0.009. Inflation is assumed at 3 percent and the cost of money is taken at 8 percent.

Assumed operations, maintenance, repair and replacement differences between products A and B:

  • Product A must be rehabilitated after 50 years at a present cost of $200 per foot
  • 36-inch relief line at a present cost of $180 per foot needed in 40 years for product A, and in 60 years for product B
  • Product A causes street and utility repairs at years 15, 30 and 40 at a present cost of $100,000 each
  • Product A requires extra inspection and cleaning at a present cost of $10 per foot in years 10 and 30

Based on installed costs only, product A is 10 percent cheaper. However, when the present value of differential long-term costs is also considered, product B is now 10 percent cheaper without even considering many other potential savings, such as lower treatment costs, possible EPA fines, wastewater treatment plant expansions, etc.

As this demonstrates, the product with the lowest installed cost may not have the lowest life-cycle cost. In order to obtain a true cost comparison, the present value of differential costs incurred throughout the sewer design life must be considered along with the installed cost.

Conclusion

When we started our research for this paper, we envisioned being able to provide a straightforward formula into which you could enter easily determined variables and arrive at an empirical conclusion. But the subject is far too complex for that. Yes, the formula is here, but the variables are difficult to determine. Rather than oversimplify the problem, it seemed better to review the factors that should influence your decision. But one message is clear ? picking the products with the lowest initial price isn?t going to cut it any longer. If you?re not thinking in terms of life-cycle cost, you?re not making the best use of your infrastructure dollars.

Kimberly Paggioli, P.E., is Vice President ? Marketing and Quality Assurance, and Richard C. Turkopp, P.E., is Vice President ? Engineering, for HOBAS Pipe USA. Philip R. Snyder is a technical writer and Chairman of The Snyder Agency.

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