Sanitary sewer overflows, inflow & infiltration plague North Carolina paradise
Although named for the idyllic garden of the Old Testament, the city of Eden, N.C., has battled infrastructure challenges that have been anything but paradise. Formed in 1967 through the consolidation of three old mill towns and one sanitary sewerage district, the city inherited an aging wastewater collection system that has struggled with chronic sanitary sewer overflows (SSOs) and excessive inflow and infiltration (I&I) during wet weather.
Eden has approximately 161 miles of gravity sewer, 19 miles of sewer force main, and 19 pump stations. Most of the system is more than 50 years old, consisting of mostly vitrified clay gravity sewer mains. The pace of deterioration has resulted in chronic overflows in several basins during wet weather. The service area also lies in the Smith and Dan River Basins, making its aging system more susceptible to I&I.
Despite its best efforts to address and correct problems throughout the system, the city received an Administrative Order of Consent from the U.S. EPA in 2012. As cited in the order, the city experienced more than 150 individual SSOs from 2006 to 2011. These documented SSOs resulted in the release of more than 722,000 gal of untreated wastewater. Because the area encompassing Eden drains to the Dan and Smith Rivers, these overflows are of concern due to their surface water impact. In fact, the Dan River is designated as impaired and is listed on the North Carolina Deparment of Environment & Natural Resources 303(d) list.
Initially, the EPA levied the administrative order to canvas Eden’s entire wastewater collection system. However, WK Dickson worked with city staff to negotiate the scope of the order, which resulted in a reduction from all 18 of the city’s basins down to eight, which significantly reduced the scale of the investigation and the cost without compromising the overall objective of the order.
Step #1: Analysis & Planning
With no time to waste, the WK Dickson team started working with the city to develop a capacity, management, operation and maintenance (CMOM) program with the goal of eliminating all SSOs within the critical basins that are experiencing chronic overflows and were included in the administrative order. This work involved:
- Completing a pump station operations program;
- Establishing sewer overflow response plan requirements;
- Developing a sanitary sewer evaluation study work plan;
- Creating a capacity assessment work plan;
- Completing a rainfall and flow monitoring work plan; and
- Finishing a sanitary sewer evaluation study (SSES) report, which entailed a comprehensive evaluation of the physical condition of critical assets in the wastewater collection and transmission system.
A system evaluation and rehabilitation plan (SERP) also was prepared to evaluate and rehabilitate the critical basins and identify deficiencies within the critical basins that contributed excessive I&I. It also identified specific measures and schedules that would result in the elimination of the chronic overflows under current and future conditions once implemented.
Eden’s SERP is a multi-phased plan comprising dry and wet weather flow and rainfall monitoring, a sewer condition assessment, a SSES, a detailed capacity analysis through a dynamic hydraulic model, and a remediation plan. The work required more than 20 flow monitors and nine rainfall gauges; 75,000 ln ft of sewer condition assessment; 171,130 ln ft of smoke testing; 820 NASSCO manhole inspections; and development and calibration of a hydraulic model of the city’s major outfalls, critical collectors, main pumping stations and force mains.
Step #2: Improve Infrastructure
Once improvement recommendations were identified, prioritized and approved, design work to implement more than $33 million in infrastructure improvements began. Projects included replacement and rehabilitation of 54,000 ln ft of sewer; replacement of the existing siphon under the Smith River, rehabilitation of two regional pump stations; replacement of two small pump stations; and installation of permanent back-up generators at two other small pump stations; and several other collection system repairs.
Rehabilitation and replacement technologies included cured-in-place pipe (CIPP) lining—using both traditional felt CIPP and glass-reinforced polymer or GRP CIPP—point repairs, mainline replacements, and lateral and manhole repair.
Additionally, due to the departure of several large industries from the city, one large 24-in. force main became underutilized. To alleviate SSOs at two other pump stations, which utilize an 8-in. and 10-in. force main, the city initiated a program and received funding from the state to interconnect the 8- and 10-in. force mains to the large, underutilized pump station force main.
The interconnection allows the smaller pump stations to automatically pump into the 24-in. force main during wet weather while using their dedicated force mains during normal dry-weather flows. Through this interconnection, SSOs have been alleviated at these pump stations during rain events that would have caused SSOs in the past.
Several critical items were discovered as part of the assessment of the city’s system. One of which included two major outfalls—the Dan River Outfall and the Matrimony Creek Outfall—and manholes routinely submerged by floodwaters along numerous waterways.
Step #3: Maintenance
During the condition assessment of the city’s infrastructure, it was discovered that several outfalls were obstructed. The Smith River outfall, which consisted of 8,500 ln ft of pipe ranging in diameter from 16-in. to 36-in., had repetitive and chronic SSOs. Condition assessment technology, including HD CCTV and sonar inspection were strategically applied to the main outfall to provide Pipeline Assessment Certification Program-coded defects and debris volume estimates.
The sewer was significantly blocked by hardened debris and several sections of tuberculated cast iron pipe. A phased improvement plan was recommended to first thoroughly clean the pipes and then replace and rehabilitate the outfall based on a prioritized plan. As a result, the city removed more than 65 truckloads of debris from this outfall and the outfall has not experienced any SSOs from similar rain events that would have normally caused them. The whole system approach allowed the city to mitigate SSOs and allow the planned improvements to occur on a more palatable funding timeline.
The CMOM program is a critical tool that will help Eden maintain its wastewater infrastructure in the years to come. Now, with asset management, Eden has enhanced its decision-making for operations, maintenance and financial objectives. These programs integrated with capital planning allow Eden to shift its focus from a reactive approach to one that is proactive.
Step #4: Funding & Financing
$33 million in infrastructure improvements can be a hefty bill for any small municipality, but the city of Eden has been fortunate. In addition to coordination and cooperation with regulatory agencies, the availability of public funding through the Connect NC Bond has had a significant impact on the ability of the city to address the most critically needed projects in a way that is sustainable for its rate payers.
The Connect NC Bond Act was passed by the North Carolina General Assembly in 2015 and subsequently approved by North Carolina voters in March of 2016. This $2 billion comprehensive state-wide bond program plans to “…connect the state’s public facilities to the 21st century, enhance the state’s economic development efforts, and attract new and assist existing industry, business, technology, and tourism…” to North Carolina. With approximately 15% of the bond proceeds targeted for improvements to aging infrastructure across the state, Eden’s wastewater system rehabilitation project was a perfect match to leverage these funds through the North Carolina Department of Environmental Quality-Division of Water Infrastructure.
To date, improvements related to Eden’s SSOs have been funded by a $15.2 million state project reserve grant, a $15 million zero-interest state project reserve loan, a $534,811 state high unit cost grant from the water infrastructure fund, and approximately $3 million in local funds.
A Return to Paradise
The city of Eden has already made tremendous progress in eliminating its SSOs. For example, in the 2015 to 2016 timeframe, 418 I&I problems were responded to and repaired. By 2017 to 2018, that number was reduced to 42 I&I problems. Additionally, the volume of overflows also has decreased significantly with 294,889 gal in 2015 to 2016, compared to 34,500 gal in 2017 to 2018. There still is plenty of work to be done. Under review are plans and specifications for repair and rehabilitation of sewer lines, manholes, laterals, and pump stations in eight of the city’s 18 sewer basins. The city is on track to meet the administrative order Feb. 28, 2022 deadline for the construction and completion of these improvements.
Eden also is looking to move forward with additional work to further alleviate SSOs. This work includes the abandonment of a pump station and conversion to gravity sewer; the construction of a regional pump station to replace one or two existing pump stations, if feasible; the replacement of an aging force main; the relocation of a gravity sewer line along a stream which suffered wash-out during Hurricane Matthew; and the stabilization of a portion of the Dan River bank, which suffered from extreme erosion from multiple recent storms and has a 24-in. gravity sewer outfall and 16-in. transmission water line that needs protection from damage or even failure.
Eden may well soon live up to its “paradise” namesake, and that is certainly what everyone has been working towards. That, and perhaps a day of rest.