Charlotte’s Paw Creek Basin: Significant I/I Reduction Achieved through Basin-Wide Sewer Rehabilitation

Charlotte Water (formerly the Charlotte-Mecklenburg Utility Department), located in Charlotte, N.C., provides water and sewer service to Charlotte and several surrounding smaller towns. Charlotte Water, the largest utility in the state, serves a population of more than 800,000 through over 4,200 miles of sewer pipe.

Over the last 20 years, Charlotte Water has been implementing a long-term sanitary sewer rehabilitation program to reduce infiltration and inflow (I/I) into its aging sewers and to correct historic maintenance problems. The utility has a yearly sewer rehabilitation budget of $11 million to $12 million. The rehabilitation program is part of Charlotte Water’s strategic efforts to maintain the performance of the sewer system, to provide superior sewer service to its customers, and to maintain compliance with regulatory requirements.

Paw Creek Basin

The Paw Creek basin in Charlotte Water’s system includes approximately 560,000 ft of sewer and 2,500 manholes. The sewers in the Paw Creek basin experienced increased wastewater flows during rain events due to significant I/I in the sewer system. High levels of inflow caused high peak wet-weather flows and sewer surcharging throughout the system. High levels of infiltration caused prolonged high wastewater flow volumes at the Paw Creek Pump Station after the rains ended.

Charlotte Water implemented a basin-wide sewer rehabilitation program to reduce I/I in the Paw Creek basin. Charlotte Water also used this rehabilitation work as a “demonstration” project to determine reasonable I/I reduction rates that can be achieved using various rehabilitation approaches and to evaluate techniques used for future planning of I/I reduction projects.

Rehabilitation Approach

Charlotte Water decided to perform basin-wide rehabilitation using both a comprehensive rehabilitation approach and a point repair approach. The primary focus of the rehabilitation work was to reduce the “quick-entry” type inflow, thus reducing the peak flow in the sewer system that was leading to surcharging.

Reduction of infiltration was also a focus in the Comprehensive Rehabilitation area listed below. Charlotte Water followed a “find-and-fix” construction approach to rehabilitate the sewers in the Paw Creek Basin. The find-and-fix approach works well for both comprehensive rehabilitation projects and point repair projects.

Comprehensive Rehabilitation: One area of the Paw Creek Basin had repeated backups and maintenance issues due to extensive roots, broken pipe, sags in the sewers, I/I leaks and other structural issues. Charlotte Water decided to comprehensively rehabilitate this portion of the Paw Creek Basin to repair the causes of the re-occurring problems and to reduce the inflow and infiltration that was entering the sewer system.

All of the sewers (approximately 55,000 ft) in the basin were lined with cured-in-place pipe (CIPP) from manhole-to-manhole or completely replaced if CIPP was not feasible; all visibly defective service laterals as seen from CCTV inspections were replaced from the main sewer to the edge of the easement or road right of way (210 laterals); and all of the brick manholes were coated with a special cementitious mortar product (295 manholes). No work was performed on private property. The comprehensive rehabilitation work totaled approximately $3.2 million.

Point Repair Rehabilitation Approach: The remainder of the Paw Creek basin (approximately 500,000 ft of sewer) was rehabilitated following a point repair approach. The primary objective of the point repair approach was to repair inflow defects. In general, infiltration defects were not addressed. All of the sewers were smoke tested, and all of the identified inflow defects were repaired. All of the manholes were inspected, and again, all of the identified inflow defects were repaired. Repairs to services on private property were performed if the inflow defect was deemed to be severe and a high priority. The point repair work totaled approximately $1.8 million.

The following summarizes the rehabilitation work that was completed under the comprehensive and point repair rehabilitation work. All work was performed using “find-and-fix” rehabilitation projects, which is Charlotte’s standard contractual method for rehabilitating their sewer system.

Comprehensive Rehabilitation Area:
CIPP =    52,000 ft
Sewer Replacement =    3,000 ft
Point Repairs prior to CIPP =     53
Replace Service Laterals =    210 laterals
Cementitious Mortar Coating =    295 manholes

Point Repair Areas:
CIPP =    7,000 ft
Point Repairs =    72
Cementitious Mortar Coating =    22 manholes
Repair/Replace Service Laterals =    31 laterals
New Manholes =    4
New Riser with Flat-top =    3
Inflow Dishes =    575
New Frame and Solid Cover =    53
New Frame and Watertight Cover =    364
Locate and Raise Manholes =    72
Raise Manholes to Grade =    63
Reset Existing Frame and Cover =    183
Rebuild Benches and Inverts =    95
Seal Active Wall Leaks =    26 manholes
New Cleanouts =    430

The total approximate cost of the above rehabilitation work is as follows:
Comprehensive Rehabilitation Area =     $3,200,000
Point Repair Areas =    $1,800,000
Total Rehabilitation Cost (approx.) =    $5,000,000

Pre- and Post-Rehab Flow Monitoring

Flow monitoring was performed prior to the rehabilitation work (pre-rehabilitation flow monitoring) and after the rehabilitation work was completed (post-rehabilitation flow monitoring). The pre- and post-rehabilitation flow monitoring were performed to determine the effectiveness of sewer rehabilitation on reducing I/I and to compare the I/I reduction results between a comprehensive approach and a point repair approach.

The flow meters were installed in the same manholes for the pre-rehabilitation and post-rehabilitation monitoring so that a direct comparison of flows could be made. In addition, several meters were installed outside of the Paw Creek Basin where no rehabilitation was performed to serve as “control” meters/basins. The control basins allow correlations to be made between the varying rain event sizes and durations and ground water conditions during the pre-rehab and post-rehab metering periods.

I/I Reduction Results

The post-rehabilitation flow monitoring documented the following reductions in I/I:

Comprehensive Rehab Area:
Peak Wet-Weather Flows (inflow) were reduced by approximately 66 percent

Wet-Weather Infiltration was reduced by approximately 47 percent
Point Repair Areas:  (note that peak flow reductions (inflow) were the main focus in the point repair areas)

Peak Wet-Weather Flows (inflow) were reduced by approximately 39 percent in the Point Repair areas.

Conclusions

1. Charlotte Water demonstrated that comprehensive sewer rehabilitation using trenchless technologies in its system can lead to substantial reductions in peak wet-weather flows (inflow) and wet-weather infiltration. The main sewer, service laterals and manholes should be included in the rehabilitation project for maximum reductions. The comprehensive approach can be expensive. In the Paw Creek Basin, the overall cost spread across the comprehensive area was about $60 per ft of sewer.

2. Charlotte Water demonstrated that a point repair rehabilitation approach in its system can lead to significant reductions in peak wet-weather flows (inflow). Reducing the peak flows reduces sewer system surcharging. Charlotte Water typically repairs the moderate to severe inflow defects, but every inflow defect that is identified should be repaired for maximum reduction in peak flows.

3. Charlotte Water is continuing to follow the model developed in the Paw Creek Basin. Comprehensive rehabilitation with trenchless technologies is being used where substantial I/I reduction is needed and where there are extensive historic maintenance problems due to extensive roots, broken pipe, sags in the sewers and other structural issues. The comprehensive approach is specifically utilized in smaller areas of sewer due to the higher cost of comprehensive rehabilitation. A point repair approach is being used throughout the remainder of Charlotte’s system to quickly and cost-effectively repair major inflow defects and significantly reduce inflow. With such a large sewer system (4,200 miles), Charlotte has found that a combination of approaches works best in its system to effectively address I/I and other sewer issues.

4. Charlotte Water completed the rehabilitation work in the Paw Creek Basin following a “find-and-fix” construction approach. Charlotte Water first started implementing find-and-fix contracts in 1999, well before the term “find-and-fix” began to be used in the Southeast. The find-and-fix approach works well for both comprehensive rehabilitation projects and point repair projects, as demonstrated by the successes in the Paw Creek Basin. Charlotte Water performs all of its rehabilitation construction with find-and-fix contracts. Charlotte Water estimates that its has saved more than $9 million in engineering and construction costs over the last 15 years following this construction approach.

Aaron Frazier is the founder of Frazier Engineering. Keith Shirley is with Charlotte Water.