Sewer Pipe Life Cycle Cost
The most frequent method used to decide among various acceptable product alternatives for new sewers is an installed cost comparison. The results of such a restricted focus determination may be misleading since, the installed cost evaluation ignores many other costs which may occur during the lifetime of the sewer. A true cost comparison must also consider the costs incurred (or avoided) through out the design life of the sewer. The sum of all costs is called the Life Cycle Cost. When evaluated by a Life Cycle Cost comparison, HOBAS pipes are generally the easy winner.
The total cost (Life Cycle Cost) of any item includes costs experienced over the study period to:
- Purchase the item
- Install it
- Operate it
- Maintain & repair it
- Replace it (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 costs are:
- Corrosion resistance to the interior & exterior environment
- Hydraulic characteristics
Pipes, like HOBAS centrifugally cast fiberglass reinforced polymer mortar (CCFRPM), which are:
- Inherently corrosion resistant to the burial & sewer environment
- Hydraulically superior
..avoid or delay many future costs.
- Corrosion resistance - HOBAS CCFRPM pipes are virtually unaffected by ground chemicals or sewer acids. ASTM sewer pipe accelerated aging chemical tests project HOBAS pipe life in the many thousands of years!
- Leak-free - Installed HOBAS pipe systems not only regularly meet leakage standards, but routinely test leak-free!
- Hydraulically superior - The Manning's "n" value for HOBAS CCFRPM pipes has been measured at 0.009 (new) and 0.0105 even after several years of sewer service and slime development.
Discussion of Benefits
Superior long-term pipe performance avoids or delays many future costs.
Inherent corrosion resistance provides an extended, very long pipe life resulting in the following benefits:
- No liners or coatings to inspect, maintain, refurbish or replace
- No rehabilitation costs due to corrosion deterioration
- Hydraulic characteristics are substantially unchanged with time preserving the sewer flow capacity and thereby delaying relief line(s) construction costs
- Costs for premature replacement are avoided completely
- Leak-free performance reduces infiltration providing many cost saving benefits including:
- Reduced flows to WWTP's resulting in:
- Lower treatment costs
- Fewer EPA fines for by-passing treatment in high flows
- Delayed expenditure(s) for plant expansions
- Eliminates soil migration into the sewer which causes:
- Structural undermining of streets & other utilities
- Blockages requiring excessive cleaning
- Minimize wet weather overflows and associated costs for fines, clean-up, safety and disruption
- Maximize flow capacity utilization thereby delaying relief line(s) construction costs
Superior hydraulics result in a higher flow capacity generating additional long-term cost savings including:
- Delayed relief line(s) 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
- Costs for operation, maintenance, repair and replacement (O,M,R&R)
The installed cost is provided during the tender process by the bidding contractors. These figures must be adjusted by the present value differences of the O,M,R&R costs over the study period for the pipe materials being evaluated to determine the comparative Life Cycle Cost.
A 100 year evaluation for construction of a new open cut 48" sewer sewer 5000 ft. 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% and the cost of money is taken at 8%.
Assumed O,M,R&R differences between products A & B:
- Product A must be rehabilitated after 50 years at a present cost of $200 / ft.
- 36" relief line at a present cost of $180 / ft. 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 & 40 at a present cost of $100,000 each
- Product A requires extra inspection and cleaning at a present cost of $10 / ft. in years 10 & 30
Life Cycle Cost Comparison ($ / ft.)
O,M,R&R items present value:
|36" relief line||27.03||10.47|
|Street and utility repairs||16.65||0|
|Extra inspection & cleaning||8.64||0|
|Total Life Cycle Cost (comparative)||341.01||310.47|
Based on installed costs only, product A is 10% cheaper. However, when the present value of differential long-term costs is also considered, product B is now 10% cheaper without even considering many other potential savings such as lower treatment costs, EPA fines, WWTP expansions, etc.
As seen in the above example, 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 through out the sewer design life must also be considered along with the installed cost.
HOBAS CCFRPM pipes because of their inherent corrosion resistance (long life), leak-free performance and superior hydraulic characteristics (higher flow capacity), will generally have the lowest life cycle cost in most sewer applications. So, for your best value, specify HOBAS pipes and account for their many significant cost saving advantages in your evaluation!