MARCO ISLAND — Bart Bradshaw, Collections and Distribution Manager for Marco Island Utilities, contributed to the cover story of Utility Infrastructure Management Magazine's article on the three-phase sanitary sewer rehabilitation program being applied on Marco Island. The sewer project was used as an example of proactive wastewater collection system management.
The three phases are condition assessment, rehabilitation design and implementation. Based on the condition assessment, depreciation of the existing sanitary sewer system was predicted over the long-term. This analysis was used to determine the financial benefits of sanitary sewer rehabilitation in addition to preventing failures and reducing infiltration and inflow (I&I), thereby maintaining the desired level of service.
Portions of the cover story written by Bradshaw and V. Firat Sever, a consulting engineer at the Fort Myers office of AECOM Technology Corp. are below. The full article is also in City Manager Steve Thompson's Weekly Update of Oct. 9, in the related links.
The City of Marco Island purchased the water and wastewater utility from a private company in November 2003 and renamed it Marco Island Utilities. As the management became familiar with the utility’s capital needs, the projects were prioritized from most critical to least critical.
At the beginning of the fourth year of ownership, the utility director set aside funds to begin an assessment of the wastewater gravity system that was built between early and late 1960s. The existing central wastewater collection system consists of vitrified clay (VCP), built in the early 60s, and polyvinyl chloride (PVC) pipes, built in late 60s. Only 40 percent of the Island was served by centralized wastewater collection system at the time.
In coordination with the consultant, the city started an assessment/rehabilitation program for its sanitary sewers. The goals of this program were:
- Assess the structural integrity of the pipelines and manholes
- Identify sources of infiltration and inflow into the system
- Develop a capital improvement plan to prioritize and correct defects that are discovered.
Methods used for inspecting the existing system were closed-circuit television (CCTV) survey, visual inspection in field (manholes), and smoke testing of lateral sewers and selected manholes.
Existing System Inventory
Sanitary sewers on Marco Island consist of polyvinyl chloride (PVC) and vitrified clay pipe (VCP), with clay pipes being older than the PVC pipes. PVC pipes did not demonstrate any notable damage except a few discrete holes and cracks due to rock or miscellaneous object penetration.
The majority of the inspected VCP is in sound or moderate condition requiring point repairs or no repair at all in the foreseeable future. Sixteen percent of the existing clay pipes demonstrated extensive damage such as long and deep cracks and fractures in addition to loose/offset joints.
Approximately 40 percent of the existing pipes are made of clay and the remaining 60 percent are PVC.
Vitrified clay is known for its inertness against chemical attack. However, it is more brittle than PVC and tends to generate longitudinal or circumferential cracks due to external loads. In addition, VCP is less flexible than PVC and tends to separate at joints.
Marco Island is a barrier island located on the eastern section of the Gulf of Mexico in southwest Florida.
Marco Island’s groundwater table is on the average of 3 feet to 3.5 feet below the surface and is affected by tidal action. Because of the fluctuating water table and the constant movement of the sandy soil, the nonflexible clays pipes have sustained most of the damage. Additionally, traffic loads and installation deficiencies (e.g., poor bedding and backfill) may have played a significant role on service life of sanitary sewer pipes.
Extensive defects such as cracks, fractures, holes, loose joints on the pipelines and manholes will potentially allow I&I. Infiltration and inflow will increase the total flow rate and cost of wastewater treatment. Moreover, infiltration is a result of defects in the pipelines as well as manholes, which may be indicative of premature failure of the sewer lines that, in turn, may have costly consequences, such as sanitary sewer backups and damage to the road above the collapsed pipe segment.
Sanitary sewer failures also pose environmental hazards by wastewater leaks out of the pipeline (exfiltration).
The cleaning and televising program included the existing sewers, which are distributed over the Island through the northeast, northwest and southwest. These areas were divided into phases to prioritize and coordinate the project. The overall project included approximately 190,000 linear feet of sanitary sewer pipes ranging from 8-inch to 12-inch in diameter and 833 manholes, which serve 34 percent of the island.
Approximately 6 percent of the existing sewers were excluded from the rehabilitation program due to pipes/manholes being new or recent rehabilitation.
Based on the review of the CCTV records, the authors evaluated the condition of the sanitary sewer main pipes, which was indicated using color coded plans. Three levels of deterioration – sound, moderate and poor – were indicated by green, yellow and red.
Condition of the sewer main pipes was highly dependent on the location and material. Accordingly, minor or no damage was noted to the PVC sewers; whereas, the magnitude of damage to the clay pipes ranged from no damage to extensive. The most common types of defect were cracking, fracture and root intrusion (at joints) in clay, and sagging in PVC pipes. Pipe segments that have frequent fractures and cracks will allow infiltration or exfiltration, depending on the water table and the location of the pipe. In addition, structurally unsound pipe segments are prone to a collapse, which may cause sanitary sewer backups and damage to the road above the pipeline.
Based on the condition of the pipelines and flow rates into the system, upsizing any pipe segments was not deemed necessary by the project team; therefore, pipe replacement was not one of the methods included in the project. Cured-in-place-pipe (CIPP) was the method chosen for relining, which is an established method of structural pipeline rehabilitation.
A well installed CIPP with adequate resin will minimize, if not eliminate, the annular space between the host pipe and liner, thus reducing the risk of annular flow that may enter into the system through service lateral connections (after the liner is cut out at service laterals) and cracks/holes in the host pipe.
CCTV records indicated non-structural, localized damages and leaks in some of the sanitary sewers that are otherwise in sound condition. A complete relining of these pipe segments would not be economical; therefore the suggested method for fixing loose joints and circumferential cracks in the pipe segments that are otherwise in sound condition was chemical grout (acrylamide) injection.
CCTV inspection survey records did not provide a clear view of the manholes; and therefore, manholes were visually inspected by the project team.
Forty-six percent of the manholes included in the assessment program were included in the inspection (386 of 833). This selection was based on age and location of manholes (i.e. low lying areas such as swales and low elevations). Wet weather flow rates to the Marco Island Wastewater Treatment Plant are significantly higher than that of dry weather; whereas, based on the CCTV records review, it was understood that infiltration into the collection system was not substantial.
Hence, manhole inspection and assessment was aimed at reducing inflow through manholes and enhancing structural integrity of the deteriorated ones. Inspection was conducted using a standard form developed by the project team.
A majority of the manholes had defects on the upper section (i.e. cover, frame and chimney), whereas, some of them underwent corrosion due to hydrogen sulfide exposure, which damaged the cone and wall. Damage to the bench or trough was observed in only a few manholes, and this was mainly due to root intrusion. Like the sanitary sewer pipes, the condition of each manhole was depicted using a color code. Color coding identifies a manhole’s condition at three levels as sound (green), moderate (yellow), and poor (red).
Anticipated Benefits of Rehabilitation
The rehabilitation program is aimed at addressing all of the defects that have had or will have an impact on the operation of the system at the desired level of service, which is essentially minimized infiltration and inflow and risk of failure.
On the other hand, all of the yellow and red sanitary sewer pipes and manholes will be upgraded to green. In addition to minimized operational problems, this will increase the overall value of the wastewater collection system – an important capital asset of the city.
Conditions of the sanitary sewer pipes and manholes were predicted over the long-term if a rehabilitation program was not implemented. This prediction is based on a linear correlation between degradation observed in 40 years to date and the degradation that would occur without rehabilitation in 70 years.
Condition of the pipelines after 40 years (average age of the inspected pipes and manholes, which vary from 35 to 45 years) was broken down in accordance with the material (clay or PVC). Then it was assumed that the pipe degraded from green to yellow or red over this period in constant increments (i.e. linear degradation), which is a much greater value for clay pipes.
To illustrate, the rate at which clay pipes turned yellow from green was 40 percent; whereas, this rate was 3 percent for PVC pipes. It is more than likely that degradation of a sanitary sewer pipe or manhole is not linear over time as deterioration would be expected to occur at an accelerated pace when the pipes and manholes reach their design life.
However, there isn’t sufficient data to create a model for the deterioration of the pipes and manholes in long-term in the case of Marco Island, as the pipes were inspected once after 40 years. The nonlinearity in the condition of pipelines vs. time plot is a result of different rate of degradation between PVC and clay pipes.
All of the clay pipes are expected to sustain, at minimum, minor damage, and degrade to yellow in 40 years. In other words, there would be no green clay pipes in the system after 40 years without rehabilitation. This results in a decline in yellow pipe quantity as yellow clay pipes proceed to degrade and turn red; whereas, available green PVC pipes turn yellow then transition to red at a lower rate than clay pipes.
Anticipated benefits of rehabilitation in monetary terms were analyzed over the long-term (70 years). The value of a green pipe segment was taken equal to a new sanitary sewer pipe, which is based on new construction ongoing on the other parts of the island. The average unit price ($/linear foot for sanitary sewer pipes) for new construction was amplified by 25 percent to account for relatively low quantity and additional cost of replacing the existing pipes (which would include flow bypassing and extra labor).
On the other hand, there is a social cost associated with traffic interruption and nuisance to the public for new construction or open-trench replacement, which is very complex to define in mathematical terms and excluded from the value of a green pipe. A yellow or red pipe segment is the cost of a green pipe segment (per linear foot) less the cost of rehabilitation required to bring these pipes’ condition to green. The cost difference between green and red pipes is much greater for the red pipes that require replacement.
This study assumes that red pipes and manholes will require replacement in 70 years and therefore, they will be of negligible value at the end of this time period without rehabilitation. Accordingly, long-term depreciation of sanitary sewer mains and manholes is shown in Figure 1. It is anticipated that with the foregoing assumptions, the total value of 190,000 linear feet of sanitary sewer pipes and 386 manholes that were evaluated would decrease from $37.0 million to $32.9 million (present value) in 70 years without rehabilitation.
We anticipate that by implementing the sanitary sewer rehabilitation program, the City of Marco Island will eliminate the risk of failure of its sanitary sewers, reduce I&I and increase the value of an important capital asset. It is also expected that by reducing I&I, rehabilitation of defective sewer pipes will lower the cost of wastewater conveyance and treatment.
The lowest bid received for sanitary sewer main and manhole rehabilitation is approximately 20 percent of the anticipated depreciation without rehabilitation as present value. It should be noted that there will be some deterioration, hence depreciation, after rehabilitation.
However, the extent of deterioration after rehabilitation is extremely difficult to predict, since the rehabilitation techniques are relatively new, and the authors do not have any field data on the rehabilitated systems that have been in service for an extended period of time. On the other hand, deterioration/depreciation is expected to occur at a substantially lower rate than clay pipes based on the excellent performance of polymeric (PVC) pipes that have been in service for approximately 40 years.
The next step of the program is implementing rehabilitation, and then pre/post rehabilitation condition of the sanitary sewer system with data and pictures incorporated into the GIS database (smart maps) of the city.
More frequent inspection/assessment of rehabilitated or as-installed wastewater collection systems is needed to increase accuracy on the rate of deterioration/depreciation of these assets and a better evaluation of economic benefits of trenchless rehabilitation.
V. Firat Sever, Ph.D., P.E., is a consulting engineer at Fort Myers, Fla., office of AECOM Technology Corp. He has five years of industry in addition to two years of research experience in water and wastewater engineering.
Barton D. Bradshaw is the Collections and Distribution Manager for Marco Island Utilities, Marco Island. He has 16 years in the water and wastewater industry and 10 years in the ship building industry as a pipe and structural welder.